I created this guide because I felt we lacked one single comprehensive guide which tells newbies how to use Li-ion cells in a safe manner in their LED flashlights. Now I know why: At least the "comprehensive" part is impossible.
I'm not an expert myself, but I have picked up quite a bit from CPF and other sources. If you see mistakes, typing errors, language errors, dangerous things or do not agree with anything in this guide, tell me so. If you see something which is really dangerous and you have moderator powers, feel free to correct it before somebody who doesn't know better does it and hurts himself. In that case, please be so kind to inform me of what you corrected. I'm not a native English speaker, so feel free to inform me about any spelling / grammar errors you find. It would probably be best to correct spelling / grammar by sending me a PM so we don't clutter this thread up with the fine nuances of the English language.
Using Li-ion cells in LED flashlights safely for newbies
This guide is intended for Li-ion newbies (not the member Newbie, who is one of the battery safety experts here on CPF and has forgotten more about cells than I will ever know). You’ve come to CPF and you discovered Li-ion cells and LED lights which use them and you want to learn how to use them safely. This guide is specifically aimed at LED lights, using Li-ions in incandescent lights requires more care and has its own thread in the incandescent forum. If you follow this guide you will be safe. That is not a legal guarantee. By reading past this point you agree to these things: You will not sue me no matter what happens when you follow the advice from this guide and everything you do is completely your own responsibility.
Take the time to read this all, it’s a lot, more than I expected before I started typing, but you need to know what you’re doing in order to do it in a safe manner.
1 What Li-ion cells are we talking about?
There are several kinds of Li-ion cells. In this guide I will only talk about what most people call Li-ion cells: Lithium Cobalt Oxyde cells, which have an average voltage of 3.7 volts.
Three reasons: 1) These are the most commonly used cells in LED flashlights. 2) By the time you need to use other chemistries you will probably have some experience with Li-ion cells and should start to do your own research. 3) I don’t know enough about the other Li-ion chemistries. So, from here on, when I’m talking about Li-ion cells in this guide, I am talking about Lithium Cobalt Oxyde cells, the kind which is in you cellphone, laptop, MP3-player, GPS, cordless drill etc. and which carries the most risk as far as I know.
2 Risks? What risks? Should I worry?
Explosion. Fire. Horrible burns, lost fingers and fleshwounds. That’s what we all want to avoid.
When you seriously abuse a Li-ion cell it gets angry. When a Li-ion cell gets angry it becomes a terrorist cell and it will “vent with flame” (AKA explode). (Read the very entertaining story of Shorty and Longman for the origins of “terrorist cell”, although the one in the story is a primary lithium cell. Don’t confuse primary lithium cells with rechargeable Li-ion cells; they are 2 completely different things.)
Remember the fuss about exploding laptop and cellphone batteries, the huge product recalls, the lawsuits, the martian invasion to rid us of unsafe cells? No? Well, I might be imagining part of that. But rest assured, today’s cellphones and laptops are completely safe. The manufacturers have started using the exact same cells they were using in the exploding products. Hey, what’s that? That’s right, the exact same cells. The problem lies not with the cells, it lies with the way you treat ‘m. Treat ‘m nice and you won’t have a problem. That’s what this guide is about. Li-ion cells are only dangerous if you don’t use them the way you’re supposed to use them. Thanks to the lawsuits and the martian invasion a lot of money has been spent to find out what exactly turns a Li-ion cell in a terrorist cell and how to prevent that from happening. Preventive actions are what this article is about. If you are sitting next to a burning Li-ion cell, quit reading, get outside, call the fire department or your local emergency number (US: 911, AU:000, UK and most former colonies: 999 Most of Europe: 112) and tell them your name, location and don’t forget to mention the fact that there’s a lithium fire burning.
3 How to prevent the creation of a terrorist cell
This is the core of the information. The CIA, the NSA, the MI6, the Mossad and the KGB couldn’t figure this one out, but here is the only real and effective method to prevent terrorist cells: Treat every single one of ‘m nice and there won’t be no terrorist cells.
3.1 During charging
Don’t overcharge your cells, don’t charge ‘m too fast and don’t leave ‘m alone.
No overcharging: You should stop charging when the cell has reached 4.2 volts. Just about any Li-ion charger I have ever heard off shows you when your cells have reached 4.2 volts (some lower, that’s no problem unless you want absolute full capacity, some higher, those are dangerous and should be disposed off). Some flash a light, some turn on a green light, some wave a flag. Here’s the kicker: most cheap chargers don’t quit charging at that moment. Why? Poor engineering, that’s why. There’s no excuse in my opinion. It’s plain and simple poor engineering. More on chargers later on in this guide.
So you need to take your cells off the charger as soon as the green light goes on / he starts waving the flag. 15 – 20 minutes overcharging won’t kill your cells, you don’t need to put a three man watch on your charger and hysterically pull the cells out when the light turns green, but if the flag has been waving for half an hour and the cells are still in the charger you’re doing things wrong. You need to do this with all your cells; no matter whether you use a cheap or an expensive charger. Because you never know, that’s why.
Also, if you don’t have one, buy a multimeter. A cheap one is good enough if you don’t plan on becoming heavily involved with electronics. Check the voltage of your cells when they come off the charger. Here's a guide on using a multimeter if you don't know how. Above 4.2 volts? Get rid of the charger and the cell. Recycle them both, don’t just throw them in the bin with the regular trash. Think about your children who will live in the mess you’re creating.
No charging too fast: The dangerous charge current for a Li-ion cell is 2C or higher. This means 2 times the capacity of the cell in one hour. If your cell has a capacity of 2200 mAh 2C is 4400 mA or 4.4 A. This current should never, ever be exceeded, not even for a second. Safe charging is done around 1C. Less is not necessarily better; Li-ion cells like to be exercised a bit during charging. Charging too slow won’t hurt the cell, but it will reduce the capacity faster over time.
If your cells are being charged too fast they will get hot to the touch. It’s normal for your cells to get warm during charging, but not hot. I think you know the difference between warm and hot. When a cell reaches 120°C it has officially become a terrorist cell and there is nothing you can do about it. Luckily, us mere humans can’t touch anything hotter than 60°C without serious discomfort, so as long as you can touch your cells you don’t need to run like hell. However, you do need to quit charging them in that charger if they feel hot, even when you can comfortably touch them.
Don’t leave ‘m alone: There is a bit of discussion about this one. Some people say you should never leave charging Li-ion cells alone, not even to go to the bathroom or answer the door. In my opinion, when you are charging one or two protected Li-ion cells in a decent charger that is overreacting. (More about decent chargers and protected cells later) You can leave your charging cells alone for 15 minutes maximum, not longer. After 15 minutes you need to check on your charging cells: are they hot to the touch, do they smell funny, is the green light on? Any of these means you need to quit charging. If you’re charging packs of multiple cells for RC vehicles, hotwires etc. you need to do your own research and decide what you think is the fine line between cautious and overcautious behaviour.
Let me also add this here: Li-ion cells should be charged on a fireproof surface. Don’t charge ‘m on a wooden table, don’t charge ‘m on paper, on a sofa, on a chair … Even if your cells don’t “vent with flame”, the charger and the cells can still get pretty hot (which usually means something is wrong), so don’t leave ‘m on a surface that can start a fire.
3.2 During discharging
Don’t over discharge, don’t discharge too fast, don’t leave ‘m alone. Sounds familiar, doesn’t it? That’s the reason this section is shorter than the “charging” section.
No over discharging: Li-ion cells should not be discharged below 3.0 volts. Take that multimeter you used after charging and check your cells when they’re discharged. Below 3.0 volts means you over discharged them. Be careful it doesn’t happen again. If the cell is discharged below 2.9 volts, recycle it. Discharging your cells too deep is not very dangerous in itself, but it is not safe to recharge them again. To prevent this from happening, quit using the same cell when your light gets noticeably dimmer (carry spare cells / lights if necessary) and recharge your cells often. Li-ion cells can be recharged without any problems even if discharged only a little bit so you can charge your cells when you know you've used them a bit and you will have full runtime again.
No discharging too fast: The li-ion cells we’re talking about in this guide should never be discharged faster than 2C. A proper Li-ion compatible LED flashlight won’t discharge the correct cell too fast, so this should not be a problem.
Don’t leave ‘m alone: Make sure you are there to turn the light off if it gets too hot. If a hissing sound comes out of it or it starts to smell funny you have a terrorist cell in a (probably) metal container in your hand. Throw it somewhere it won’t hurt anybody when it explodes IMMEDIATELY. This is a thing which will not happen if you use proper cells and treat ‘m well, but I’m mentioning it just in case.
3.3 The cells themselves
Let’s do some good old racial profiling. The cells most likely to turn into terrorist cells are cheap Chinese cells. The cells least likely to turn into terrorist cells are expensive Chinese cells. Gee, maybe race hasn’t got anything at all to do with it?
Only buy name brand cells. Be a snob. It’s for your own safety. AW sells Li-ion cells here in the CPFmarketplace and his cells are generally regarded to be the best cells you can get because they have the best protective measures. Other decent brands include, but are not limited to: Sony, Sanyo and Panasonic. I used to include some low-end Chinese cells here, but the more I read and try out myself, the more I become convinced that there is no point in trying to save money by buying low-end Chinese protected cells. The price difference with high-end cells is not that big and the low-end cell's performance and safety is always worse. If you have cells from a different brand, check their reputation here on CPF or on RC forums or with other people who use Li-ion cells extensively. Do not trust the reviews on the sites you buy them from. The seller can add or remove reviews at his own will and anybody who can use a computer can call himself an expert reviewer.
Only buy protected cells. Protection circuits are a last measure to prevent your cells from turning into terrorist cells. A protection circuit will prevent current from flowing when a cell is being overcharged, charged too fast, over discharged, discharged too fast or gets too hot. This is a last measure. Don’t discharge until the protection circuit kicks in; don’t charge until the protection circuit kicks in. That’s dangerous behaviour. Protection circuits are designed and fabricated by humans, so they can never be 100% flawless, just like us mere humans. When you’re treating a protected cell properly two things have to fail before a serious accident occurs: you and the protection circuit. When you rely on the protection circuit to tell you when to quit charging / discharging, only one thing needs to fail for things to go seriously wrong.
If you want to use non-protected cells for any reason at all you need to do your own research and determine what safe and unsafe behaviour is. Some lights cannot use protected Li-ion cells because there are no protected Li-ion cells that fit them. Don’t buy those lights unless you are certain you know enough about the cells, the charger and the light you’re planning on using to avoid dangerous situations.
4 Cells: Sizes, numbers, names, compatibility, confusion
Size and voltage are the two main factors which determine whether any given cell can be used in any given light. Both size and voltage have to be right, it’s not because the cell will fit that it will work without ruining your light, your cells, your hand, your environment or any possible combination of these four.
Let’s be honest: size matters. Your cells have to physically fit in your light or they won’t be compatible, that’s for sure. Li-ion cells are named by using a numerical code which determines the size. The first pair of numbers is the diameter in millimetres, the second pair of numbers is the length in millimetres. 18650 cells have a diameter of 18 mm and a length of 65 mm. The last zero means it's a cylindrical cell.
These are some common Li-ion sizes:
14500: The size of a AA cell.
10440: The size of a AAA cell. Never protected. Do not use these without further research.
16340 AKA RCR123: The size of one CR123 cell.
17670: The size of two stacked CR123 cells.
17500: The size of one and a half stacked CR123 cells.
18650: These are the cells used in laptop battery packs. That’s why this size cells is most researched and most developed and these have the biggest capacity / volume ratio.
The Li-ion cells we’re talking about in this guide deliver on average 3.7 volts under load and can deliver as much as 4.2 volts fresh off the charger. No primary cells do this, so most lights not designed to function with Li-ion cells can’t handle them without problems.
Recently most manufacturers have started advertising Li-ion compatibility because they realise it is a plus. If you’re not sure your light can handle Li-ion cells safely, either ask the manufacturer, search on CPF or try it yourself. The last option might kill your light / cell and it won’t be my fault and you won’t be able to sue me over it.
So, you cannot use RCR123’s in every light you can use CR123’s in. Surefire in particular doesn’t like rechargeable Li-ion cells because they make a lot of profit by selling their own top of the line primary CR123 cells, so most Surefire lights do not work very well with Li-ion cells. There are proven-to-work combinations, search here on CPF if you want to know which ones. Especially lights designed for 3 CR123 cells can often be used with 2 17500 cells. 2 17500 cells in series deliver 7.4 volts under load, which is pretty close to what 3 CR123 cells deliver under load. Do your own research about the light you're considering, it will not work in some lights.
As I said before, most cheap chargers do not quit charging by themselves. The following cheap chargers are proven to quit charging by themselves:
Xtar wp6 - The best cheap charger if you're using 18650's IMO. Has a better charging algorithm than the following two chargers, but not yet perfect.
DSD charger – charges pretty slow and often only to 4.10 volts, but won’t overcharge. <=== Watch out with this one, I have received 2 defective chargers in a row, one did not charge at all, one kept charging in constant current mode which overcharged my cells pretty badly. When the charger is plugged in without a cell, the LED should be flashing red-green-red-green, when the cell is in the charger it should be either constant red (when charging) or constant green (when done charging). If the LED is not flashing when the charger is plugged in without a cell inserted you have a defective charger.
WF-139 charger – The old version of this charger had a trickle charge once the green light is on. The new charger apparently doesn’t; but beware, especially when ordering from cheap Chinese sites it is possible that you get old stock and your charger does overcharge. This charger also does not have the best charging algorithm, meaning it will shorten the life of your cells. Not dangerous in itself, but not good either.
The Pila IBC charger is regarded to be the safest and most reliable plug-and-play charger. It’s more expensive, but it’s the best and safest you can get without buying a hobby charger, like the guys flying remote controlled planes use..
Using one of these chargers is no excuse to leave your cells on the charger or leave your cells unattended for prolonged periods. Always take the cells off the charger when the green light comes on.
6 Further reading
Rechargeables & Flashlights
MD's Rechargeable Compatibility chart / guide for popular flashlights: Lists the most common rechargeable chemistries used and has a list of popular flashlights and their known rechargeable configurations.
MDs Lithium-Ion > Incandescent guide + compatibility / comparison chart: Basically the incandescent version of this guide, with a list of possible configurations. This guide was my inspiration for creating my guide. It's a great guide, but aimed at incandescent lights and some things do not count for LED flashlights.
Li-ion cell chemistry and safety measures
Lithium Ion Categories: A list of the most common Lithium-Ion chemistries with their important characteristics explained.
Photos of Protected Lithium Batteries: A thread in which you can see the protection circuit being removed from a Li-ion cell. Good to illustrate the difference between protected and unprotected cells.
Li-ion protection technology and possible dangers: A thread about the different types safety measures in Li-ion cells and how they work. Lots of information, but a lot of work to read it all.
Lithium Battery Safety Guide: A PDF file which contains a lot of safety information about how to treat both primary and rechargeable lithium cells, especially when they're leaking / venting with flame / playing terrorist cell. It's aimed at seafarers, but the information applies to lithium cells anywhere.
A big thank you to everybody who has helped me make this guide. Battery university, various CPF members who did a lot of research and gathered the information I combined and corrected my work, scientists, engineers ... And off course flashlight manufacturers. We all love them, and sometimes hate them. I guess that's it for now. Enyoy your light.