mdocod
Flashaholic
INTRODUCTION
I've decided to go ahead and put together a computability chart and guide that will help alleviate answering the same questions with lengthy complicated answers in separate threads over and over again. The daily question has become "what RCR123 for "xxx" and which one is best?" The answer is often different for different flashlights, and the answers are often far from simple. However, most of the questions can be answers through a search, but sometimes searching can generate a lot of inapplicable results and frustration.
Before I begin, I'd like to say that I am not intending on including every last flashlight in existence here, I don't think a list of every flashlight on KD/DX is necessary, but there are a number of them that are popular that I will try to add to the list in time. We'll try to focus on popular flashlights across the board, but primarily LED flashlights. I have already covered just about every possible configuration of lamp and body and li-ion cell for tactical incandescent flashlights over here:
MDs Lithium-Ion>Incandecent guide, +compatability/comparison chart
I'd like to do the same thing here, but with focus diverted from modifications, sticking to stock configurations instead. I'll discuss benefits and tradeoffs of different rechargeable chemistries, different types of common regulation found in common LED flashlight, how voltage and capacity effect performance and runtime, then list the behavior of the lights in the compatibility chart on different rechargeable options applicable to the flashlight in question.
This Guide will be a work in progress for as long as I can devote time to it. This thread can be re-located easily as it has been "sticky'd" (Thank You DM51!) to the "threads of interest" for the batteries/electronics section of this great forum
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COMMON RECHARGEABLE CHEMISTRIES
NiCD [Nickel Cadmium]:
General Information:
1.2V Nominal voltage, direct replacement for most consumer devices calling for 1.5V alkaline or 1.2V NiMH cells, available in common consumer cell sizes from AAA to D, also available in non-standard cell sizes for special applications, like Sub-C, A, 2/3A, F etc.
Advantages:
A NiCD cell has the advantage of being extremely robust. They can be overcharged, over-discharged, over-heated, abused with high currents in both directions and have respectable cycle life. (often several hundred cycles or more).
Disadvantages:
The disadvantages of NiCD are it's relatively low energy density, high self-discharge rates and relatively toxic (environmentally unfriendly) contents when compared to more modern chemistries.
Charging:
Charging can be done individually or in series configuration of like capacity cells in similar state of charge, charge termination methods vary widely. Smart chargers will usually use negative delta V style termination, less smart chargers either use a timer to terminate the charge or look for temperature rise. dumb chargers just continue to trickle charge indefinitely until the user removes the cell or device from the charger. the excess charge is converted to heat. Excess charge at slow rates is considered acceptable in most applications but does reduce cell life. Smart chargers will extend cycle life and maintain higher useful capacity through aging than dumb chargers. Pay more for a charger, and buy less cells over the years.
Discharging:
There are a wide variety of NiCD cells on the market targeted for different applications. Ordinarily speaking, NiCD can be discharged rapidly, but some application specific cells will perform better than others under high loads. Discharge should ordinarily be terminated at around 0.9-1.0V, then recharged. Occasionally discharging below this level isn't a major concern. Discharging all the way to 0V can be done, I have heard it suggested that this can be done to help improve performance in high drain applications, at the sacrifice of cycle life and capacity.
Safety Concerns:
NiCD cells have a good track record of safety, they are not likely to explode or flame under normal circumstances. Repeated abuse usually just leads to a dead cell. NiCD can violently vent hot gas or leak it's guts if it is overcharged at a very fast charge rate for way too long (or heated up way to much by other means, like repeated charging/discharging at high rate without time to cool.)
Myths:
"Memory effect:" yes, this is a myth. NiCD cells used in standard NiCD chargers and put through typical rigors of various discharge depths will never experience memory effect.
"1.2V isn't enough to power my 1.5V device:" Total myth in 99% of situations. In reality, under a load, a "1.5V" alkaline cell will fall below 1.2V very quickly, long before the cell is expelled. If you compare a discharge graph of a 1.2V NiCD to an Alkaline cell in most consumer electronics with normal loads (0.5-1A), you'll find that the NiCD will often spend much more of the discharge time above the voltage of the alkaline.
NiMH [Nickel Metal Hydride]:
General Information:
1.2V Nominal voltage, direct replacement for most consumer devices calling for 1.5V alkaline or 1.2V NiCD cells (in some cases), available in common consumer cell sizes from AAA to D, also available in non-standard cell sizes for special applications, like Sub-C, A, 2/3A, F etc.
Advantages:
NiMH is a reasonably safe chemistry that is far less toxic than NiCD, has about double the energy density compared with NiCD, and is available in a wide range of application specific "styles," like "high current capable" "maximum capacity" or "low self discharge." Picking the appropriate NiMH cell for the application is important, with the right type of cell selected, using rechargeable cells becomes a rewarding money saving experience. If you aren't sure which cell is best for your needs, 90% of the time, a Low-Self-Discharge (LSD) is the best option.
Disadvantages:
While still tolerant to some abuse, NiMH cells are not generally as robust as NiCD cells. By picking the right cell for the application, and using higher quality charging methods, most of the disadvantages can be overcome. These cells are a bit more sensitive to the extremes like overcharging, over-discharging, etc. While being more energy dense than NiCD, they are still not quite as energy dense as modern Lithium chemistry cells. Some NiMH cells suffer from extremely high rates of self-discharge and are not suitable for long term energy storage. The highest capacity AA/AAA cells are usually the most susceptible to unacceptable rates of self-discharge, which can develop within the first few dozen cycles, making their useful cycle life much lower than suggested by the packaging.
Charging:
NiMH cells are best charged by smart negative delta V termination chargers at a reasonable charging rate for the cell in question. They can be charged individually or in series packs containing like capacity cells in a similar state of charge. Charging in the range of 1-6 hours is a good target for charging speed for most cells. Faster charging rates will result in more heat, which can be damaging, but slower charging rates can cause the charger to miss termination signals, which can lead to overcharge. A combination of negative delta V and temperature termination backup can be found on some nice chargers. NIMH cells can be charged by "less smart" and "dumb" chargers just like a NiCD (timer, temperature, or user terminated trickle) but it is less than ideal as these charging techniques will wear out NiMH cells more rapidly. NiMH can expel excess charge as heat and can safely be overcharged at slow rates for long periods of time without danger. Higher quality more expensive chargers will allow cells to live longer healthier lives, buy a quality charger, get more use and a better experience out of your cells.
Discharging:
Discharge rates that are acceptable will vary depending on the specific cell. Some can handle very high discharge rates, others perform poorly at high discharge rates. The faster the discharge, the more heat, and the harder on the cell. High current specific cells have lower internal resistance and can accomplish massive discharge rates with less heating. Discharge should be stopped at 0.9V, as far as I know, there is no practical reason to discharge below 0.9V with a NiMH cell, but doing so will not cause any catastrophic problems.
Safety Concerns:
NiMH has a good track record of safety, severe overcharging at high rates or repeated charging and discharging at high rates could cause enough heat build-up to make the cell vent hot gas or leak chemical residue. Explosions and fire are highly unlikely in normal use.
Myths:
"Memory effect:" yes, this is a myth. NiMH cells used in standard NiMH chargers and put through typical rigors of various discharge depths will never experience memory effect.
"1.2V isn't enough to power my 1.5V device:" Total myth in 99% of situations. In reality, under a load, a "1.5V" alkaline cell will fall below 1.2V very quickly, long before the cell is expelled. If you compare a discharge graph of a 1.2V NiMH to an Alkaline cell in most consumer electronics with normal loads (0.2-1A), you'll find that the NiMH will often spend much more of the discharge time above the voltage of the alkaline. And usually last much longer too
LiCo02 [Lithium Cobalt Oxide]:
General Information:
3.7V cells ordinarily found in consumer devices like cell phone, laptops, MP3 players, PDAs, etc. These energy dense cells have found their way into flashlights in recent years as the demand for more compact, lightweight, rechargeable power solutions has gone up. When Someone says "lithium-ion" without stating a specific lithium chemistry this is almost exclusively the chemistry being discussed. LiCo02 cells are available in a variety of sizes including AAA and AA and CR123 size, but they use a different naming structure for size labeling. The size of the cell is described in a string of numbers that define the dimensions in millimeters. So a AAA li-ion cell is a 10440. (10mm x 44mm x "cylindrical"). AA = 14500. CR123 = 16340. other common sizes: 17500, 18500, 17670, 18650. These are NOT USUALLY compatible with devices that call for a 1.5V or 1.2V alkaline/NiCD/NiMH, however, there are SOME flashlights on the market that ARE compatible with the higher voltage. Most flashlights that are compatible with them, will indicate so in the product details. These should never be considered direct replacements for 3V CR123 primary cells as they have substantially higher operating voltage. Always check for compatibility with the higher voltage on devices before using them. There are a large number of LED flashlights that are compatible, and a number that are not.
Advantages:
Highest available energy density commonly found in rechargeable cells. Especially in the larger sizes, 17500 and up. Very efficient charging and discharging with the least amount of energy expelled as heat. Higher per-cell voltage means less cells are needed to achieve certain voltage requirements. When treated properly, these have exceptional cycle life (hundreds+). Li-Ion also has exceptionally low self discharge.
Disadvantages:
Smaller size Li-Ion cells, like 14500, RCR123 (16340), and 10440, do not generally live up to their label capacity claims and usually have lower energy density than alternative chemistries in the same size. LiCo02 is not tolerant to abuse, these cells must be used within the bounds as listed by the manufacture. Rapid charging (faster than 1 hour) and rapid discharging (faster than 30 minutes) is not possible with these, so they are not necessarily as flexible as Nickel chemistry cells. In order for loose li-ion cells to be used in devices like flashlights, they need to have protection circuits installed for safety reasons, which adds a layer of potential failure to the device. Li-Ion is more prone to vent-with-flame/explode than Nickel chemistry cells if abused. LiCo02 also suffers from the effects of aging whether it is being used or not, though in recent times, this has becomes less and less of a factor with li-ion cells. Used to be that they would be considered "dead" after a few years from production whether they were used or not. Now they seem to be lasting 7-10 years without much trouble.
Charging:
The proper charging technique for LiCo02 must be followed to tight specification for maximum safety. The cell should be charged at a 1C or slower rate at a constant current until the cell reaches 4.20V, at which point the charger should hold 4.20V (constant voltage) until the charge current drops to some fraction of the original charge current (usually around 0.05C give or take) (varies from charger to charger, but there is probably an ideal termination current based on cell capacity that would be impossible to have perfect on a charger designed for multiple cell sizes). Charging in series packs can only be done properly with balance taps on the pack and a balance charger. Li-Ion cells in a similar state of charge can be charged in parallel as if they were a single cell. Charging above 4.20V will cause increased rate of internal oxidation, reducing effective cycle life and capacity, while simultaneously increasing the risk of explosion/fire. 4.30V will not usually cause an immediate danger, this is where most protection circuits will kick in. Use a high quality charger to perform charging if possible. Most cheap chargers do not follow the proper charging requirements. The Pila ICB is most often recommended and is worth the $40 or so.
Discharging:
LiCo02 cells should not be discharged below ~3.0V under a load, (varies by manufacture). A good rule of thumb is that when the cell reaches ~3.5V open circuit, it is dead and should be recharged. Over-discharging a cell will increase the rate of internal oxidation leading to reduced capacity, reduced cycle life, and increased likelihood of explosion/fire. Different cells are rated for different maximum discharge rates, usually specified between 1.5 and 2C. (C ratings are having to do with time, a 2C rating, means 30 minutes, 1C means 1 hour, 4C means 15 minutes, 0.5C means 2 hours, etc etc, bigger C). Check to see what your cells are rated at and use them in an application that is within the bounds of the maximum discharge rate.
Safety Concerns:
Abusing these cells by overcharging, over-discharging, discharging too quickly or charging too quickly, or causing physical damage of sorts can increase the risk of fire/explosion. These cells need to be treated with a higher level of respect and care than NiMH or NiCD. Use protected cells whenever possible to reduce the risk of an incident. Keep in mind that li-ion is most apt to flame/explode while charging, not while discharging, so to maximize the safety of a questionable cell, charging in a fireproof box in a well ventilated area is recommended. A flaming/exploding LiCo02 cell releases Hydrofluoric acid. Breathing the gas or coming into direct contact with the remnants of a LiCo02 fire can cause severe poisoning that can cause major illness or death.
Myths:
"I have a protection circuit, so don't have to worry about over-charging or over-discharging." This is the most common misunderstanding. The protection circuit is set to prevent dangerous events from occurring, it does not prevent smaller scale overcharging and over-discharging. They are often set at ~2.5V and ~4.3V whcih would not be healthy termination points for normal cycles.
"My cell is rated at 900mAH and 2C, so it can handle a 1.8 amp discharge." (I was guilty!)
The C ratings assigned are based on time, not label capacity. In reality, there are many 900mAH RCR123 size cells out there that are actually only good for 500mAH capacity or less at 2C, which means their maximum discharge rate is only 1 amp.
LiFeP04 [Lithium Iron Phosphate]:
General Information:
Often sold as 3.0V rechargeable cells, these are technically 3.2V li-ion cells based on a new cathode material that is inherently safe. These cells can *often* be used as a direct replacement for CR123 primary cells in lights that can tolerate the slight voltage difference compared with primary cells. (keep in mind, that primary CR123s actually operate below 3V when under a load, more like 2.5V). For the most part, LED lights that normally run CR123s can run these no problem, incans usually can not unless regulated (rare). LiFeP04 cells are currently available in only a few sizes, including "RCR123" (16340) and 18650 and few others we won't discuss at this time. I lean towards recommending these over the 3.0V regulated cells discussed below.
Advantages:
Safe chemistry won't explode or flame, can tolerate some abuse without too much issue, does not need protection circuit like LiCo02 to be used in consumer devices, so less failure points. Higher voltage per cell than NiMH/NiCD means less cells are required to achieve voltage desired, can often be used where 3.7V cell is not advisable. Offers a safer more reliable alternative to 3.0V voltage regulated LiCoO2 cells.
Disadvantages:
Much lower energy density compared to LiCo02, generally speaking, around 50% less stored energy per volume. Needs special LiFeP04 charger, one more device to have floating around. Label capacities are generally way overstated on smaller cells. Expect 200-400mAH from 16340 size cells depending on load. For comparison purposes, a CR123 primary has between 1200 and 1500mAH capacity. So these will really hurt runtime.
Charging:
Charging rate is fairly flexible on these, most small RCR123s in this chemistry are sold with matching charger that charges in an hour or a few hours. Charge is usually just constant current until voltage reaches about 3.6-3.8V (varies by manufacture) followed by some constant voltage until the current drops to around 0.05C give or take. (when charged CC to 3.8V it's probably pretty close to full, when terminating at 3.6V, some CV is probably required to finish the charge) overcharging won't cause too much damage provided it isn't done too rapidly or for too long. A LiFeP04 cell can be charged in a "4.2V" LiCo02 charger in a pinch, but you would want to pull the cell manually sometime around 3.8V if possible(use volt-meter to check). As far as I understand, these can be charged in series or parallel most of the time, but should be isolated on occasional charges to balance them out. (I could be wrong on this)
Discharging:
Discharge capabilities vary by cell manufacture and size. Larger scale LiFeP04 cells were originally intended for use in high drain applications like power tools and electric cars. Small scale LiFeP04 cells aren't quite as tolerant to high discharge rates and tend to "fall on their face" at discharge rates higher than ~2-3C. But Discharging even the small cells at higher than recommended rates is still not really dangerous, just wears out the cells more quickly. Discharge should be terminated at 2.0V whenever possible. Discharging below 2V will degrade the cell more rapidly, some cells seem to be more tolerant to over-discharging than others.
Safety Concerns:
Very few issues of safety, I would classify them as similar in safety to NiCD/NiMH cells, major heating from constant abuse might cause a hot gaseous venting or leak, but this chemistry does not typically ignite.
Myths:
"It's a 3V cell so will work in any device designed for CR123 primaries."
They will work in most devices, but any direct drive incandescent will likely blow it's bulb on these cells.
3.0V RCR123s not labeled LiFeP04:
General Information:
These are usually 3.7V LiCo02 RCR123 cells that have a voltage regulator installed to shunt the operating voltage down to around 3.0V to make them more compatible with voltage sensitive devices. These are often the alternative to the LiFeP04 cell, or you could say, that the LiFeP04 cell is the alternative to this. Most of these cells are sold as protected cells, but I just found one the other day online that is voltage regulated but NOT protected. I highly recomend picking protected versions of this type of cell if you decide to use them. Overall I lean towards recommending the LiFeP04 cells for applications where these are often specified.
Advantages:
Can often work where 3.7V cells would not. Usually has slightly better capacity compared to LiFeP04 RCR123s.
Disadvantages:
More components to fail. The voltage regulating component of these generates heat right next to the cell, which is less than desirable for cell longevity. The cell itself has to be smaller to make room for additional components, or the cell ends up being too long for some devices. Accidentally putting a 3.7V cell into a charger designed for these would probably cause an explosion. Not a good charger to have floating around in a collection of various cells and chargers and devices.
Charging:
Charging must be done on the charger that is sold with the cells or recommended for the cells as these things vary from one manufacture to the next on their recommended charge voltage termination from 4.4-4.5V. The cell itself still needs to be terminated at 4.20V, but the charger has to overcome the voltage regulation device "backwards" through the circuit, so to speak, (if that makes any sense). Do NOT use one of these chargers on any cell other than the cells it is sold with!!!
Discharging:
Often limited by the voltage regulating device to around 1-2C, discharging continuously above 1C IMO could cause overheating of the cell or failure of the voltage regulator. Discharge should be terminated at around 2.0-2.5V give or take (follow manufacture recommendations).
Safety Concerns:
Same as LiCo02 cells above. Abuse can lead to vent with flame, these are IMO more susceptible because of that heat making deice attached to the cell.
Myths:
"It's a 3V cell so will work in any device designed for CR123 primaries."
They will work in most devices, but any direct drive incandescent will likely blow it's bulb on these cells.
LiMnO2 / LiMn2O4 / "IMR" / LiNiMnCoO2 [various Lithium Manganese Oxide type cells]:
General Information:
3.6-3.8V cells ordinarily found in Power tools. These cells have found their way into flashlight applications for various reasons. Their properties are similar to LiCoO2 cells in many ways, but with a few key differences. These cells are not available with protection circuits on individual cells, but are considered "safe" chemistry cells, similar in safety to a LiFePO4 or NIMH cell.
Advantages:
Excellent balance of energy density and power density. Capable of driving loads that LiCoO2 cells can not safely handle. Safe chemistry means they will not fuel their own fire in the event of a catastrophic failure with oxygen.
Disadvantages:
Lower Energy Density than LiCoO2 cells.
Charging:
Charging requirements are basically the same as most LiCoO2 cells, 4.20V termination for most LiMnO2 cells is common. There are a few LiMnO2 chemistry cells found in power tools that need to have their charge terminated at 4.10V rather than 4.20V, but at the time of writing this, the only "consumer oriented" loose cells available are from AW, and they will charge fine in MOST 3.7V li-ion chargers. Over-charging will dramatically reduce cycle life and severe over-charging could cause the cell to "pop." These cells can usually handle faster charging rates than LiCoO2 cells, check the specifications on your specific cells for more clarity on this issue.
Discharging:
The minimum discharge voltage and maximum recommended discharge rates, continuous and pulse, vary by manufacture. Generally speaking these cells can handle very aggressive discharge rates without much trouble, 5-10C range is common for the upper end limit. Discharge should be terminated at ~2.5-3V under a load, shallow discharges are healthier for these cells just like for LiCoO2 cells.
Safety Concerns:
Nothing too serious, similar to NIMH/NiCD/LiFePO4 safety concerns. When overheated or overcharged or repeatedly abused they could pop and/or vent gas. Ignition is unlikely but theoretically possible under the right circumstances, but the fire would not be fueled by a chemical reaction from the cell like in the case of a LiCo cell, so it would be a far less aggressive failure.
Myths:
"These are safe chemistry cells, so I can use them in anything!"
Flashlights that contain regulation circuits can over-discharge these cells severely. While over-discharging may not create any immediate danger, it will severely reduce the useful life of the cell. There are exceptions to every rule, some LED lights would be perfectly fine with LiMnO2 cells, some configurations becoming available will actually require them to handle the current demands safely, but each situations should be looked at individually.
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Common types of "regulation" circuits in LED lights
My hope is that by having a quick summery of each type of common regulation (or lack there-of) found in LED lights, readers can better understand the reasons behind the behaviors of their lights on the various rechargeable options.
direct drive:
Ok, so this isn't a regulation type, it is a lack of regulation type. I personally classify any light that lacks a regulator, but may have a simple resistor, a direct drive light. In a direct drive light, output will decline through the run. The resistor, or the Vf of the LED is selected such that the maximum current the LED sees is at the beginning of the run on fresh cells, and is set to something reasonable for the LED to handle, it's all down hill from there. Generally speaking, a direct drive light that was intended for use with alkaline cells (almost always 3 cells) will work fine on NiMH or NiCD, the rechargeable chemistry will actually help stabilize the output through the run, making the discharge curve more useful. Some direct drive lights run on a pair of CR123s, this is pretty uncommon but I have seen a number of them on cheapo lights from places like DX/KD. On these lights, the only way to switch to a rechargeable cell would be to switch out the resistor to a different value. Sorry, there is no direct drop-in solution to making a direct drive 2xCR123 light rechargeable.
buck:
Buck style regulation takes a voltage input higher than the Vf of the LED, and bucks it down to match the LED and hold a set current across it. Often times these lights are sold as 2xCR123 lights, but are often compatible with higher voltage input from a pair of 3.7V cells without any problem. 3.0V rechargeable cells could also be used, but the 3.7V protected cell option will provide the best runtime and performance. Buck style regulated lights drop out of regulation when the voltage of the battery/s drops down close to the Vf of the LED. With buck regulation, it's usually better to move up in source voltage rather than down when seeking out rechargeable solutions as they can ordinarily handle it. (always check compatibility before attempting) 2xCR123 powered lights with buck regulation can often take a single 3.7V 17670 (or 18650, if there's room) as a power source, but the light will operate as a direct drive light through most of the discharge. This extends runtime at the cost of flat regulation. In many cases, running a buck-regulated light on a single li-ion cell will result in both lower beginning brightness, and diminishing output through the run, it depends on the specific nature of the regulator and the Vf of the LED.
boost:
Boost regulation takes a voltage below the Vf of the LED, and pulls extra current from the battery and converts it to a higher voltage to drive the LED. This type of regulation is found in most 1xAA, 2xAA, and 1xCR123 lights. When the input voltage rises above the Vf of the LED, the regulator is essentially bypassed with minimal resistance and the battery direct drives the LED. So most boost regulated 1xAA and 1xCR123 lights are NOT entirely compatible with a 3.7V li-ion cell as it will remove all lower operating modes and overdrive the LED. How much the LED is overdriven depends on the Vf of the LED, this is luck of the draw so one persons light may survive a 3.7V cell, while the next persons will not. In order to maintain access to all modes, and not risk damaging the flashlight, a 3.0V rechargeable cell (LiFeP04 or V-regulated) is recommended for 1xCR123 lights, or a NIMH or NiCD cell/cells for AA lights. Putting a pair of 14500 3.7V cells in a 2xAA light will instantly blow the LED, don't bother.
buck/boost:
Some lights have the advantage of being capable of either boosting or bucking voltage within a limited range. This can be a good and bad thing depending on the situation. Usually these types of regulators have a narrower band of voltage input compatibility than buck regulators, but make ideal solutions for power sources that will be above or below the Vf of an LED depending on state of charge or power source selected. These come in 3 dominant types. Found in 1xAA and 1xCR123 lights the voltage input range is usually ~0.9V-~4.2V, which allows the light to run on any single alkaline, NiCD, NiMH, CR123 or RCR123 of any available voltage (1.2V up to 3.7V cells). another style is typically compatible with ~2.5V-6V input, so is usually found in 2xCR123 length lights, but will work well on a single 3.7V cell of the same length as the 2 CR123s, or 2 3V CR123s (a 17670 or 18650, if bored to accept 18650 that is the preferred choice), these type of regulars are often picky about voltage above 6V and will not work properly on a pair of rechargeable 3.0V cells as their operating voltage is usually higher than 3V per cell when fresh from the charger. Another common buck/boost is designed for ~2.7-4.2V operation, and is designed specifically for use with a 3.7V cell, these are almost exclusively found in 1x18650 powered lights and should only be fed a 3.7V 18650 as a power source.
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Voltage, Amp hours, etc...
A quick overview of how common electrical characteristics translate to their behavior in flashlights should improve the overall experience here. Most of this is covered in the Welcome Mat and probably in a billion others threads here and a gazillion websites on the Internet, but, we can never be too sure, so here we go
The best analogy I can think of to explain voltage and current as it pertains to a battery is a water tower. The higher up in the air the water tower is, the more pressure there will be available at the base of the tower at the pipes. The more pressure, the more water can be forced through a given size pipe of fixture. The pipes that the water is forced through are like the resistive load that a battery pushes electrons through.
Think of mAH (mili amp hours, amp hours etc) as the size of the tank on top of the tower. A bigger tank will supply water for a longer period of time. More mAH means the flashlight runs longer.
When you add cells in series, it's like taking the poles that are holding up the tower, and stacking all the poles on top of each-other, and putting the tank on top of all of those poles, the tank stays the same size, but is raised to a higher elevation, more pressure. More voltage. Two 3V 1300mAH cells in series makes 6V battery with the same 1300mAH mAH capacity.
When you add cells in parallel, it's like putting more tanks in the air, but at the same elevation as the original tank, so the new battery has more storage capacity, but the same amount of pressure available. Two 3V 1300mAH cells in parallel makes a 3V battery with 2600mAH capacity.
We can represent the combination of pressure and tank size as watt-hours by multiplying voltage by amp-hours. A 3V cell with 1300mAH capacity can be said to have 3900mWH (mili watt hours), or, 3.9 Watt Hours (WH). This is useful for comparing total energy storage of different arrangements of different types of cells.
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On a side note, It's important to note that label ratings on cells should not always be trusted, just because one brand claims more mAH than another does not mean it is always true. Since many of the flashlights in the compatibility chart will be dealing with RCR123 size cells, I think it's important to note that these are pretty much all 500-650mAH true capacity regardless of what the label says. Before making a purchase decision based on manufacture claims, it would be wise to take a moment to look over many of the tests that have been run no various cells by some of our own beloved CPF members. Check out the "threads of interest" here in the Battery's/Electronics section for a number of links to these tests. Also keep in mind that not all RCR123s are the same size as a CR123 cell, some are a little longer or fatter, so they might have slightly better capacity ratings, but might not fit in some lights.
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That's enough for the guide, for now, I'm sure I'll add more and revise this many times in the future so stay tuned
Eric
I've decided to go ahead and put together a computability chart and guide that will help alleviate answering the same questions with lengthy complicated answers in separate threads over and over again. The daily question has become "what RCR123 for "xxx" and which one is best?" The answer is often different for different flashlights, and the answers are often far from simple. However, most of the questions can be answers through a search, but sometimes searching can generate a lot of inapplicable results and frustration.
Before I begin, I'd like to say that I am not intending on including every last flashlight in existence here, I don't think a list of every flashlight on KD/DX is necessary, but there are a number of them that are popular that I will try to add to the list in time. We'll try to focus on popular flashlights across the board, but primarily LED flashlights. I have already covered just about every possible configuration of lamp and body and li-ion cell for tactical incandescent flashlights over here:
MDs Lithium-Ion>Incandecent guide, +compatability/comparison chart
I'd like to do the same thing here, but with focus diverted from modifications, sticking to stock configurations instead. I'll discuss benefits and tradeoffs of different rechargeable chemistries, different types of common regulation found in common LED flashlight, how voltage and capacity effect performance and runtime, then list the behavior of the lights in the compatibility chart on different rechargeable options applicable to the flashlight in question.
This Guide will be a work in progress for as long as I can devote time to it. This thread can be re-located easily as it has been "sticky'd" (Thank You DM51!) to the "threads of interest" for the batteries/electronics section of this great forum
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COMMON RECHARGEABLE CHEMISTRIES
NiCD [Nickel Cadmium]:
General Information:
1.2V Nominal voltage, direct replacement for most consumer devices calling for 1.5V alkaline or 1.2V NiMH cells, available in common consumer cell sizes from AAA to D, also available in non-standard cell sizes for special applications, like Sub-C, A, 2/3A, F etc.
Advantages:
A NiCD cell has the advantage of being extremely robust. They can be overcharged, over-discharged, over-heated, abused with high currents in both directions and have respectable cycle life. (often several hundred cycles or more).
Disadvantages:
The disadvantages of NiCD are it's relatively low energy density, high self-discharge rates and relatively toxic (environmentally unfriendly) contents when compared to more modern chemistries.
Charging:
Charging can be done individually or in series configuration of like capacity cells in similar state of charge, charge termination methods vary widely. Smart chargers will usually use negative delta V style termination, less smart chargers either use a timer to terminate the charge or look for temperature rise. dumb chargers just continue to trickle charge indefinitely until the user removes the cell or device from the charger. the excess charge is converted to heat. Excess charge at slow rates is considered acceptable in most applications but does reduce cell life. Smart chargers will extend cycle life and maintain higher useful capacity through aging than dumb chargers. Pay more for a charger, and buy less cells over the years.
Discharging:
There are a wide variety of NiCD cells on the market targeted for different applications. Ordinarily speaking, NiCD can be discharged rapidly, but some application specific cells will perform better than others under high loads. Discharge should ordinarily be terminated at around 0.9-1.0V, then recharged. Occasionally discharging below this level isn't a major concern. Discharging all the way to 0V can be done, I have heard it suggested that this can be done to help improve performance in high drain applications, at the sacrifice of cycle life and capacity.
Safety Concerns:
NiCD cells have a good track record of safety, they are not likely to explode or flame under normal circumstances. Repeated abuse usually just leads to a dead cell. NiCD can violently vent hot gas or leak it's guts if it is overcharged at a very fast charge rate for way too long (or heated up way to much by other means, like repeated charging/discharging at high rate without time to cool.)
Myths:
"Memory effect:" yes, this is a myth. NiCD cells used in standard NiCD chargers and put through typical rigors of various discharge depths will never experience memory effect.
"1.2V isn't enough to power my 1.5V device:" Total myth in 99% of situations. In reality, under a load, a "1.5V" alkaline cell will fall below 1.2V very quickly, long before the cell is expelled. If you compare a discharge graph of a 1.2V NiCD to an Alkaline cell in most consumer electronics with normal loads (0.5-1A), you'll find that the NiCD will often spend much more of the discharge time above the voltage of the alkaline.
NiMH [Nickel Metal Hydride]:
General Information:
1.2V Nominal voltage, direct replacement for most consumer devices calling for 1.5V alkaline or 1.2V NiCD cells (in some cases), available in common consumer cell sizes from AAA to D, also available in non-standard cell sizes for special applications, like Sub-C, A, 2/3A, F etc.
Advantages:
NiMH is a reasonably safe chemistry that is far less toxic than NiCD, has about double the energy density compared with NiCD, and is available in a wide range of application specific "styles," like "high current capable" "maximum capacity" or "low self discharge." Picking the appropriate NiMH cell for the application is important, with the right type of cell selected, using rechargeable cells becomes a rewarding money saving experience. If you aren't sure which cell is best for your needs, 90% of the time, a Low-Self-Discharge (LSD) is the best option.
Disadvantages:
While still tolerant to some abuse, NiMH cells are not generally as robust as NiCD cells. By picking the right cell for the application, and using higher quality charging methods, most of the disadvantages can be overcome. These cells are a bit more sensitive to the extremes like overcharging, over-discharging, etc. While being more energy dense than NiCD, they are still not quite as energy dense as modern Lithium chemistry cells. Some NiMH cells suffer from extremely high rates of self-discharge and are not suitable for long term energy storage. The highest capacity AA/AAA cells are usually the most susceptible to unacceptable rates of self-discharge, which can develop within the first few dozen cycles, making their useful cycle life much lower than suggested by the packaging.
Charging:
NiMH cells are best charged by smart negative delta V termination chargers at a reasonable charging rate for the cell in question. They can be charged individually or in series packs containing like capacity cells in a similar state of charge. Charging in the range of 1-6 hours is a good target for charging speed for most cells. Faster charging rates will result in more heat, which can be damaging, but slower charging rates can cause the charger to miss termination signals, which can lead to overcharge. A combination of negative delta V and temperature termination backup can be found on some nice chargers. NIMH cells can be charged by "less smart" and "dumb" chargers just like a NiCD (timer, temperature, or user terminated trickle) but it is less than ideal as these charging techniques will wear out NiMH cells more rapidly. NiMH can expel excess charge as heat and can safely be overcharged at slow rates for long periods of time without danger. Higher quality more expensive chargers will allow cells to live longer healthier lives, buy a quality charger, get more use and a better experience out of your cells.
Discharging:
Discharge rates that are acceptable will vary depending on the specific cell. Some can handle very high discharge rates, others perform poorly at high discharge rates. The faster the discharge, the more heat, and the harder on the cell. High current specific cells have lower internal resistance and can accomplish massive discharge rates with less heating. Discharge should be stopped at 0.9V, as far as I know, there is no practical reason to discharge below 0.9V with a NiMH cell, but doing so will not cause any catastrophic problems.
Safety Concerns:
NiMH has a good track record of safety, severe overcharging at high rates or repeated charging and discharging at high rates could cause enough heat build-up to make the cell vent hot gas or leak chemical residue. Explosions and fire are highly unlikely in normal use.
Myths:
"Memory effect:" yes, this is a myth. NiMH cells used in standard NiMH chargers and put through typical rigors of various discharge depths will never experience memory effect.
"1.2V isn't enough to power my 1.5V device:" Total myth in 99% of situations. In reality, under a load, a "1.5V" alkaline cell will fall below 1.2V very quickly, long before the cell is expelled. If you compare a discharge graph of a 1.2V NiMH to an Alkaline cell in most consumer electronics with normal loads (0.2-1A), you'll find that the NiMH will often spend much more of the discharge time above the voltage of the alkaline. And usually last much longer too
LiCo02 [Lithium Cobalt Oxide]:
General Information:
3.7V cells ordinarily found in consumer devices like cell phone, laptops, MP3 players, PDAs, etc. These energy dense cells have found their way into flashlights in recent years as the demand for more compact, lightweight, rechargeable power solutions has gone up. When Someone says "lithium-ion" without stating a specific lithium chemistry this is almost exclusively the chemistry being discussed. LiCo02 cells are available in a variety of sizes including AAA and AA and CR123 size, but they use a different naming structure for size labeling. The size of the cell is described in a string of numbers that define the dimensions in millimeters. So a AAA li-ion cell is a 10440. (10mm x 44mm x "cylindrical"). AA = 14500. CR123 = 16340. other common sizes: 17500, 18500, 17670, 18650. These are NOT USUALLY compatible with devices that call for a 1.5V or 1.2V alkaline/NiCD/NiMH, however, there are SOME flashlights on the market that ARE compatible with the higher voltage. Most flashlights that are compatible with them, will indicate so in the product details. These should never be considered direct replacements for 3V CR123 primary cells as they have substantially higher operating voltage. Always check for compatibility with the higher voltage on devices before using them. There are a large number of LED flashlights that are compatible, and a number that are not.
Advantages:
Highest available energy density commonly found in rechargeable cells. Especially in the larger sizes, 17500 and up. Very efficient charging and discharging with the least amount of energy expelled as heat. Higher per-cell voltage means less cells are needed to achieve certain voltage requirements. When treated properly, these have exceptional cycle life (hundreds+). Li-Ion also has exceptionally low self discharge.
Disadvantages:
Smaller size Li-Ion cells, like 14500, RCR123 (16340), and 10440, do not generally live up to their label capacity claims and usually have lower energy density than alternative chemistries in the same size. LiCo02 is not tolerant to abuse, these cells must be used within the bounds as listed by the manufacture. Rapid charging (faster than 1 hour) and rapid discharging (faster than 30 minutes) is not possible with these, so they are not necessarily as flexible as Nickel chemistry cells. In order for loose li-ion cells to be used in devices like flashlights, they need to have protection circuits installed for safety reasons, which adds a layer of potential failure to the device. Li-Ion is more prone to vent-with-flame/explode than Nickel chemistry cells if abused. LiCo02 also suffers from the effects of aging whether it is being used or not, though in recent times, this has becomes less and less of a factor with li-ion cells. Used to be that they would be considered "dead" after a few years from production whether they were used or not. Now they seem to be lasting 7-10 years without much trouble.
Charging:
The proper charging technique for LiCo02 must be followed to tight specification for maximum safety. The cell should be charged at a 1C or slower rate at a constant current until the cell reaches 4.20V, at which point the charger should hold 4.20V (constant voltage) until the charge current drops to some fraction of the original charge current (usually around 0.05C give or take) (varies from charger to charger, but there is probably an ideal termination current based on cell capacity that would be impossible to have perfect on a charger designed for multiple cell sizes). Charging in series packs can only be done properly with balance taps on the pack and a balance charger. Li-Ion cells in a similar state of charge can be charged in parallel as if they were a single cell. Charging above 4.20V will cause increased rate of internal oxidation, reducing effective cycle life and capacity, while simultaneously increasing the risk of explosion/fire. 4.30V will not usually cause an immediate danger, this is where most protection circuits will kick in. Use a high quality charger to perform charging if possible. Most cheap chargers do not follow the proper charging requirements. The Pila ICB is most often recommended and is worth the $40 or so.
Discharging:
LiCo02 cells should not be discharged below ~3.0V under a load, (varies by manufacture). A good rule of thumb is that when the cell reaches ~3.5V open circuit, it is dead and should be recharged. Over-discharging a cell will increase the rate of internal oxidation leading to reduced capacity, reduced cycle life, and increased likelihood of explosion/fire. Different cells are rated for different maximum discharge rates, usually specified between 1.5 and 2C. (C ratings are having to do with time, a 2C rating, means 30 minutes, 1C means 1 hour, 4C means 15 minutes, 0.5C means 2 hours, etc etc, bigger C). Check to see what your cells are rated at and use them in an application that is within the bounds of the maximum discharge rate.
Safety Concerns:
Abusing these cells by overcharging, over-discharging, discharging too quickly or charging too quickly, or causing physical damage of sorts can increase the risk of fire/explosion. These cells need to be treated with a higher level of respect and care than NiMH or NiCD. Use protected cells whenever possible to reduce the risk of an incident. Keep in mind that li-ion is most apt to flame/explode while charging, not while discharging, so to maximize the safety of a questionable cell, charging in a fireproof box in a well ventilated area is recommended. A flaming/exploding LiCo02 cell releases Hydrofluoric acid. Breathing the gas or coming into direct contact with the remnants of a LiCo02 fire can cause severe poisoning that can cause major illness or death.
Myths:
"I have a protection circuit, so don't have to worry about over-charging or over-discharging." This is the most common misunderstanding. The protection circuit is set to prevent dangerous events from occurring, it does not prevent smaller scale overcharging and over-discharging. They are often set at ~2.5V and ~4.3V whcih would not be healthy termination points for normal cycles.
"My cell is rated at 900mAH and 2C, so it can handle a 1.8 amp discharge." (I was guilty!)
The C ratings assigned are based on time, not label capacity. In reality, there are many 900mAH RCR123 size cells out there that are actually only good for 500mAH capacity or less at 2C, which means their maximum discharge rate is only 1 amp.
LiFeP04 [Lithium Iron Phosphate]:
General Information:
Often sold as 3.0V rechargeable cells, these are technically 3.2V li-ion cells based on a new cathode material that is inherently safe. These cells can *often* be used as a direct replacement for CR123 primary cells in lights that can tolerate the slight voltage difference compared with primary cells. (keep in mind, that primary CR123s actually operate below 3V when under a load, more like 2.5V). For the most part, LED lights that normally run CR123s can run these no problem, incans usually can not unless regulated (rare). LiFeP04 cells are currently available in only a few sizes, including "RCR123" (16340) and 18650 and few others we won't discuss at this time. I lean towards recommending these over the 3.0V regulated cells discussed below.
Advantages:
Safe chemistry won't explode or flame, can tolerate some abuse without too much issue, does not need protection circuit like LiCo02 to be used in consumer devices, so less failure points. Higher voltage per cell than NiMH/NiCD means less cells are required to achieve voltage desired, can often be used where 3.7V cell is not advisable. Offers a safer more reliable alternative to 3.0V voltage regulated LiCoO2 cells.
Disadvantages:
Much lower energy density compared to LiCo02, generally speaking, around 50% less stored energy per volume. Needs special LiFeP04 charger, one more device to have floating around. Label capacities are generally way overstated on smaller cells. Expect 200-400mAH from 16340 size cells depending on load. For comparison purposes, a CR123 primary has between 1200 and 1500mAH capacity. So these will really hurt runtime.
Charging:
Charging rate is fairly flexible on these, most small RCR123s in this chemistry are sold with matching charger that charges in an hour or a few hours. Charge is usually just constant current until voltage reaches about 3.6-3.8V (varies by manufacture) followed by some constant voltage until the current drops to around 0.05C give or take. (when charged CC to 3.8V it's probably pretty close to full, when terminating at 3.6V, some CV is probably required to finish the charge) overcharging won't cause too much damage provided it isn't done too rapidly or for too long. A LiFeP04 cell can be charged in a "4.2V" LiCo02 charger in a pinch, but you would want to pull the cell manually sometime around 3.8V if possible(use volt-meter to check). As far as I understand, these can be charged in series or parallel most of the time, but should be isolated on occasional charges to balance them out. (I could be wrong on this)
Discharging:
Discharge capabilities vary by cell manufacture and size. Larger scale LiFeP04 cells were originally intended for use in high drain applications like power tools and electric cars. Small scale LiFeP04 cells aren't quite as tolerant to high discharge rates and tend to "fall on their face" at discharge rates higher than ~2-3C. But Discharging even the small cells at higher than recommended rates is still not really dangerous, just wears out the cells more quickly. Discharge should be terminated at 2.0V whenever possible. Discharging below 2V will degrade the cell more rapidly, some cells seem to be more tolerant to over-discharging than others.
Safety Concerns:
Very few issues of safety, I would classify them as similar in safety to NiCD/NiMH cells, major heating from constant abuse might cause a hot gaseous venting or leak, but this chemistry does not typically ignite.
Myths:
"It's a 3V cell so will work in any device designed for CR123 primaries."
They will work in most devices, but any direct drive incandescent will likely blow it's bulb on these cells.
3.0V RCR123s not labeled LiFeP04:
General Information:
These are usually 3.7V LiCo02 RCR123 cells that have a voltage regulator installed to shunt the operating voltage down to around 3.0V to make them more compatible with voltage sensitive devices. These are often the alternative to the LiFeP04 cell, or you could say, that the LiFeP04 cell is the alternative to this. Most of these cells are sold as protected cells, but I just found one the other day online that is voltage regulated but NOT protected. I highly recomend picking protected versions of this type of cell if you decide to use them. Overall I lean towards recommending the LiFeP04 cells for applications where these are often specified.
Advantages:
Can often work where 3.7V cells would not. Usually has slightly better capacity compared to LiFeP04 RCR123s.
Disadvantages:
More components to fail. The voltage regulating component of these generates heat right next to the cell, which is less than desirable for cell longevity. The cell itself has to be smaller to make room for additional components, or the cell ends up being too long for some devices. Accidentally putting a 3.7V cell into a charger designed for these would probably cause an explosion. Not a good charger to have floating around in a collection of various cells and chargers and devices.
Charging:
Charging must be done on the charger that is sold with the cells or recommended for the cells as these things vary from one manufacture to the next on their recommended charge voltage termination from 4.4-4.5V. The cell itself still needs to be terminated at 4.20V, but the charger has to overcome the voltage regulation device "backwards" through the circuit, so to speak, (if that makes any sense). Do NOT use one of these chargers on any cell other than the cells it is sold with!!!
Discharging:
Often limited by the voltage regulating device to around 1-2C, discharging continuously above 1C IMO could cause overheating of the cell or failure of the voltage regulator. Discharge should be terminated at around 2.0-2.5V give or take (follow manufacture recommendations).
Safety Concerns:
Same as LiCo02 cells above. Abuse can lead to vent with flame, these are IMO more susceptible because of that heat making deice attached to the cell.
Myths:
"It's a 3V cell so will work in any device designed for CR123 primaries."
They will work in most devices, but any direct drive incandescent will likely blow it's bulb on these cells.
LiMnO2 / LiMn2O4 / "IMR" / LiNiMnCoO2 [various Lithium Manganese Oxide type cells]:
General Information:
3.6-3.8V cells ordinarily found in Power tools. These cells have found their way into flashlight applications for various reasons. Their properties are similar to LiCoO2 cells in many ways, but with a few key differences. These cells are not available with protection circuits on individual cells, but are considered "safe" chemistry cells, similar in safety to a LiFePO4 or NIMH cell.
Advantages:
Excellent balance of energy density and power density. Capable of driving loads that LiCoO2 cells can not safely handle. Safe chemistry means they will not fuel their own fire in the event of a catastrophic failure with oxygen.
Disadvantages:
Lower Energy Density than LiCoO2 cells.
Charging:
Charging requirements are basically the same as most LiCoO2 cells, 4.20V termination for most LiMnO2 cells is common. There are a few LiMnO2 chemistry cells found in power tools that need to have their charge terminated at 4.10V rather than 4.20V, but at the time of writing this, the only "consumer oriented" loose cells available are from AW, and they will charge fine in MOST 3.7V li-ion chargers. Over-charging will dramatically reduce cycle life and severe over-charging could cause the cell to "pop." These cells can usually handle faster charging rates than LiCoO2 cells, check the specifications on your specific cells for more clarity on this issue.
Discharging:
The minimum discharge voltage and maximum recommended discharge rates, continuous and pulse, vary by manufacture. Generally speaking these cells can handle very aggressive discharge rates without much trouble, 5-10C range is common for the upper end limit. Discharge should be terminated at ~2.5-3V under a load, shallow discharges are healthier for these cells just like for LiCoO2 cells.
Safety Concerns:
Nothing too serious, similar to NIMH/NiCD/LiFePO4 safety concerns. When overheated or overcharged or repeatedly abused they could pop and/or vent gas. Ignition is unlikely but theoretically possible under the right circumstances, but the fire would not be fueled by a chemical reaction from the cell like in the case of a LiCo cell, so it would be a far less aggressive failure.
Myths:
"These are safe chemistry cells, so I can use them in anything!"
Flashlights that contain regulation circuits can over-discharge these cells severely. While over-discharging may not create any immediate danger, it will severely reduce the useful life of the cell. There are exceptions to every rule, some LED lights would be perfectly fine with LiMnO2 cells, some configurations becoming available will actually require them to handle the current demands safely, but each situations should be looked at individually.
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Common types of "regulation" circuits in LED lights
My hope is that by having a quick summery of each type of common regulation (or lack there-of) found in LED lights, readers can better understand the reasons behind the behaviors of their lights on the various rechargeable options.
direct drive:
Ok, so this isn't a regulation type, it is a lack of regulation type. I personally classify any light that lacks a regulator, but may have a simple resistor, a direct drive light. In a direct drive light, output will decline through the run. The resistor, or the Vf of the LED is selected such that the maximum current the LED sees is at the beginning of the run on fresh cells, and is set to something reasonable for the LED to handle, it's all down hill from there. Generally speaking, a direct drive light that was intended for use with alkaline cells (almost always 3 cells) will work fine on NiMH or NiCD, the rechargeable chemistry will actually help stabilize the output through the run, making the discharge curve more useful. Some direct drive lights run on a pair of CR123s, this is pretty uncommon but I have seen a number of them on cheapo lights from places like DX/KD. On these lights, the only way to switch to a rechargeable cell would be to switch out the resistor to a different value. Sorry, there is no direct drop-in solution to making a direct drive 2xCR123 light rechargeable.
buck:
Buck style regulation takes a voltage input higher than the Vf of the LED, and bucks it down to match the LED and hold a set current across it. Often times these lights are sold as 2xCR123 lights, but are often compatible with higher voltage input from a pair of 3.7V cells without any problem. 3.0V rechargeable cells could also be used, but the 3.7V protected cell option will provide the best runtime and performance. Buck style regulated lights drop out of regulation when the voltage of the battery/s drops down close to the Vf of the LED. With buck regulation, it's usually better to move up in source voltage rather than down when seeking out rechargeable solutions as they can ordinarily handle it. (always check compatibility before attempting) 2xCR123 powered lights with buck regulation can often take a single 3.7V 17670 (or 18650, if there's room) as a power source, but the light will operate as a direct drive light through most of the discharge. This extends runtime at the cost of flat regulation. In many cases, running a buck-regulated light on a single li-ion cell will result in both lower beginning brightness, and diminishing output through the run, it depends on the specific nature of the regulator and the Vf of the LED.
boost:
Boost regulation takes a voltage below the Vf of the LED, and pulls extra current from the battery and converts it to a higher voltage to drive the LED. This type of regulation is found in most 1xAA, 2xAA, and 1xCR123 lights. When the input voltage rises above the Vf of the LED, the regulator is essentially bypassed with minimal resistance and the battery direct drives the LED. So most boost regulated 1xAA and 1xCR123 lights are NOT entirely compatible with a 3.7V li-ion cell as it will remove all lower operating modes and overdrive the LED. How much the LED is overdriven depends on the Vf of the LED, this is luck of the draw so one persons light may survive a 3.7V cell, while the next persons will not. In order to maintain access to all modes, and not risk damaging the flashlight, a 3.0V rechargeable cell (LiFeP04 or V-regulated) is recommended for 1xCR123 lights, or a NIMH or NiCD cell/cells for AA lights. Putting a pair of 14500 3.7V cells in a 2xAA light will instantly blow the LED, don't bother.
buck/boost:
Some lights have the advantage of being capable of either boosting or bucking voltage within a limited range. This can be a good and bad thing depending on the situation. Usually these types of regulators have a narrower band of voltage input compatibility than buck regulators, but make ideal solutions for power sources that will be above or below the Vf of an LED depending on state of charge or power source selected. These come in 3 dominant types. Found in 1xAA and 1xCR123 lights the voltage input range is usually ~0.9V-~4.2V, which allows the light to run on any single alkaline, NiCD, NiMH, CR123 or RCR123 of any available voltage (1.2V up to 3.7V cells). another style is typically compatible with ~2.5V-6V input, so is usually found in 2xCR123 length lights, but will work well on a single 3.7V cell of the same length as the 2 CR123s, or 2 3V CR123s (a 17670 or 18650, if bored to accept 18650 that is the preferred choice), these type of regulars are often picky about voltage above 6V and will not work properly on a pair of rechargeable 3.0V cells as their operating voltage is usually higher than 3V per cell when fresh from the charger. Another common buck/boost is designed for ~2.7-4.2V operation, and is designed specifically for use with a 3.7V cell, these are almost exclusively found in 1x18650 powered lights and should only be fed a 3.7V 18650 as a power source.
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Voltage, Amp hours, etc...
A quick overview of how common electrical characteristics translate to their behavior in flashlights should improve the overall experience here. Most of this is covered in the Welcome Mat and probably in a billion others threads here and a gazillion websites on the Internet, but, we can never be too sure, so here we go
The best analogy I can think of to explain voltage and current as it pertains to a battery is a water tower. The higher up in the air the water tower is, the more pressure there will be available at the base of the tower at the pipes. The more pressure, the more water can be forced through a given size pipe of fixture. The pipes that the water is forced through are like the resistive load that a battery pushes electrons through.
Think of mAH (mili amp hours, amp hours etc) as the size of the tank on top of the tower. A bigger tank will supply water for a longer period of time. More mAH means the flashlight runs longer.
When you add cells in series, it's like taking the poles that are holding up the tower, and stacking all the poles on top of each-other, and putting the tank on top of all of those poles, the tank stays the same size, but is raised to a higher elevation, more pressure. More voltage. Two 3V 1300mAH cells in series makes 6V battery with the same 1300mAH mAH capacity.
When you add cells in parallel, it's like putting more tanks in the air, but at the same elevation as the original tank, so the new battery has more storage capacity, but the same amount of pressure available. Two 3V 1300mAH cells in parallel makes a 3V battery with 2600mAH capacity.
We can represent the combination of pressure and tank size as watt-hours by multiplying voltage by amp-hours. A 3V cell with 1300mAH capacity can be said to have 3900mWH (mili watt hours), or, 3.9 Watt Hours (WH). This is useful for comparing total energy storage of different arrangements of different types of cells.
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On a side note, It's important to note that label ratings on cells should not always be trusted, just because one brand claims more mAH than another does not mean it is always true. Since many of the flashlights in the compatibility chart will be dealing with RCR123 size cells, I think it's important to note that these are pretty much all 500-650mAH true capacity regardless of what the label says. Before making a purchase decision based on manufacture claims, it would be wise to take a moment to look over many of the tests that have been run no various cells by some of our own beloved CPF members. Check out the "threads of interest" here in the Battery's/Electronics section for a number of links to these tests. Also keep in mind that not all RCR123s are the same size as a CR123 cell, some are a little longer or fatter, so they might have slightly better capacity ratings, but might not fit in some lights.
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That's enough for the guide, for now, I'm sure I'll add more and revise this many times in the future so stay tuned
Eric
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