# Fast charging NiMH?



## jawnn (Jan 17, 2015)

Does fast charging NiMH hurt the cells? All my single cell chargers use half an amp for four cells, *why?* It takes about five hours. This device does them in about an hour or less at one amp. And they do not get warm.

As the batteries get a bit old they do not want to take as much of a charge, so i used the discharge function and got better results.

I was able to get these up to 1.4 volts while other cells do not want to take that much. then they drop back to about 1.38v. Is that over charging?


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## Lynx_Arc (Jan 17, 2015)

You can safely charge nimh batteries at C/2 or for a 2000mah battery that equates to 1000ma or 1A charging rate for 2 hours. You can charge faster than that but it can wear the batteries out faster doing so. As far as the batteries and the lower voltage and discharging/recharging it a common practice on smarter chargers to have a refresh function to charge/discharge batteries to help rejuvinate them. I've had batteries only giving me about 600mah (AAs) restored to 1800mah but batteries vary on how successful discharge/recharging them helps.


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## WalkIntoTheLight (Jan 17, 2015)

With quality cells, you can charge them at 2 amps. I bought a Sanyo Eneloop package years ago, and the included Sanyo charger does 2 amps (for AA cells). The cells get warmer than they do at 1 amp, but not hot.

Older cells that have developed high internal resistance won't take a charge that fast. Probably better to charge old cells around 500mA.

You can probably charge Eneloops at higher than 2 amps, but you'd have to reduce the charge rate before they're full. Still, I'd recommend sticking to 2 amps or less. 1 amp seems to be the most recommended rate, to be gentle to the cells.


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## SilverFox (Jan 17, 2015)

Hello Jawnn,

NiMh cells have a limited life span. Storing cells diminishes their life. Using them diminishes their life. Abusing them diminishes their life.

The question is when does normal use cross the line and become abuse?

The charge rate you select depends upon how the charger terminates the charge. A common termination method is -dv/dt and charging in the 0.5C - 1.0C range seems to produce a strong termination signal. If your charger uses peak voltage to terminate, then the charge rate doesn't matter. However if the peak voltage is too high you can abuse the cells through overcharging. If your charger simply charges for a set amount of time and doesn't discharge the cell first, then it should charge at 0.1C to avoid overcharging damage.

The common theme with all of this is that heat is produced when you cross the line between use and abuse. A little warmth is OK but a "hot potato" is not.

To complicate things further, different cells can be different ages, have different design parameters, and have different internal resistance. The higher the internal resistance the more the cell will warm up under higher loads.

Putting some context around all of this, I took some Eneloop cells which are very good quality and charged them in a 15 minute charger. They worked great but did get a little hot. I got about 150 cycles from them.

Evidently this fast rate of charging did hurt them because they should have been good for over 500 cycles. However I didn't notice the damage in actual use. The damage showed up when the charger rejected the cells. The load imposed on the cells by the charger was higher than the load in actual usage so the charger was more sensitive to changes in internal resistance.

What this means to the user depends upon the use. If you do a charge/discharge cycle a day, your cells will wear out in about half a year. If you do a charge/discharge a week you can go for a couple of years. With rapid charging you don't need to invest in spares so your costs are kept down in that regard and the amount of time you need to invest in charging is kept to a minimum.

On the other hand another CPF member took some Eneloop cells and charged them on a charger that had a normal trickle charge rate for standard NiMh cells but it was too high for the low self discharge cells. The light was used on weekends but the cells were left on the charger all week. His cells lasted about 6 months in spite of being "gently" charged.

The bottom line is that since everything "hurts" NiMh cells, a little faster charging rate may hurt a little more but the damage may not be that noticeable in the long run.

Tom


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## WalkIntoTheLight (Jan 17, 2015)

SilverFox said:


> Putting some context around all of this, I took some Eneloop cells which are very good quality and charged them in a 15 minute charger. They worked great but did get a little hot. I got about 150 cycles from them.



Interesting. You charged them at 8 amps? How hot did they get? Too hot to handle for more than a few seconds?


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## SilverFox (Jan 17, 2015)

Hello WalkIntoTheLight,

Actually they didn't get beyond warm. Both the Energizer and Duracell 15 minute chargers have fans to manage the heat and they work very well.

Tom


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## WalkIntoTheLight (Jan 18, 2015)

SilverFox said:


> Hello WalkIntoTheLight,
> 
> Actually they didn't get beyond warm. Both the Energizer and Duracell 15 minute chargers have fans to manage the heat and they work very well.
> 
> Tom



Interesting. I always thought heat was the real threat to the lifetime of NiMH cells when charging. It looks like current is as well, even if heat doesn't build up. Though, 8 amps is pretty extreme, so perhaps current isn't an issue at more reasonable levels?


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## SilverFox (Jan 18, 2015)

Hello WalkIntoTheLight,

The current thinking is that even if you remove the heat from the surface of the cell with a good amount of air flow, the inside if the cell still heats up. This internal heat is believed to cause the loss of cycle life. Although it is possible for high current to produce hot spots that physically damage the internal construction of the cell. 

The key to minimizing the damage caused by fast charging is to use high quality cells. The Energizer 15 minute charger came with Energizer 2300 mAh cells. They survived a little over 25 charge/discharge cycles.

Tom

EDIT to add: 

My current use of the 15 minute chargers is to just add enough charge to get me through the current situation. I put the cells into the 15 minute charge, set a timer for 10 minutes, and when the timer goes off I pull the cells and use them. I fully understand that they won't be balanced but there will be enough charge to finish up my project. After this I charge them normally.

I haven't noticed any premature degradation doing this so the high current theory may not hold up. On the other hand I am not doing this frequently so the degradation may not have shown up yet.


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## thinkFlashlights01 (Jan 18, 2015)

A charge at C.5 should be good for most batteries.


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## SilverFox (Jan 18, 2015)

Hello ThinkFlashlights01,

Once again your suggested charge rate is dependent upon the method the charger uses to terminate the charge. A 0.5C charge rate will work well for new cells, but as cells age it may not give as strong an end of charge signal as charging at a slightly higher charge rate.

We have seen many incidents of the La Cross style chargers where they missed the primary termination and ended up terminating on maximum time. This resulted in overcharging the cell at a rate that was high enough to damage the cell. Missed termination can do more damage than charging at a higher charge rate and generating a strong end of charge termination signal.

In addition, and I always forget to include this in discussions of fast charging, the size of the cell need to be taken into consideration. The discussion so far has focused on AA and AAA sized cells. When you get to C and D cells you need to check the manufacturers specifications to see what charge rate is recommended. For example my 9000 mAh D cells don't do well when charged at 4000 mA. Their internal resistance is too high and the cell gets too hot. They do well when charged in the 1000 - 2000 mA range.

This leaves us with charge rate depending upon the chargers method of termination, the size of the cells, and the age of the cells as measured in internal resistance.

Tom


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## WalkIntoTheLight (Jan 18, 2015)

SilverFox said:


> The current thinking is that even if you remove the heat from the surface of the cell with a good amount of air flow, the inside if the cell still heats up. This internal heat is believed to cause the loss of cycle life.



So do you think it's a waste of time to use a fan to keep cells cool?

Perhaps a fan is still useful to keep the charger circuitry cool, which would add to cell heating (from the outside).




SilverFox said:


> Once again your suggested charge rate is dependent upon the method the charger uses to terminate the charge. A 0.5C charge rate will work well for new cells, but as cells age it may not give as strong an end of charge signal as charging at a slightly higher charge rate.



I've noticed that old cells can't be charged at a higher rate, because doing so will trip the end-of-charge far too early. I think they heat up too much due to high internal resistance. Charging them slowly (4-6 hours) seems to allow a full charge, and I've seldom had a missed termination. I suppose it depends on how sensitive the charger is at detecting -dV or dT.


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## SilverFox (Jan 18, 2015)

Hello WalkIntoTheLight,

Fans help remove heat. Removing heat is good. Fans (when needed) are good. With that said a fan could be used in conjunction with a lower charge rate to achieve a higher (short term) state of charge. The idea is that the fan may slow down the development of the end of charge signal and a "little more" may get stuffed into the cells. In theory this sounds interesting. In actual practice it didn't seem to help all that much.

Let me start by stating that I don't keep trying to use "crap" cells. My definition of crap is when the cell drops below 80% of its initial capacity.

When I observe a cell that terminates early I usually find that issue goes away after a Break In cycle.

Tom


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## jawnn (Jan 20, 2015)

is this formula correct?
2200mAh/1000 = 2.2amp and a 2 amp charge current/2.2=.55 'C'

Are there any 2600 mAh AA NiMH cells that are better than Tenergy? I am amazed at how few mA hours these small batteries hold. One of my cheap cells says that the typical charge is 200mAh over 15 hours....what?

[FONT=Trebuchet MS, sans-serif]So the slower they are charged the longer they last? my i-max terminates at different voltages, but the cells are finished at about 1.37 to 1.43volts. I have it set for 1.4 Volts per cell for termination but I don't think it really pays much attention to what setting I use.[/FONT]


[FONT=Trebuchet MS, sans-serif]How can I determine the internal resistance of a given cell?[/FONT]


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## WalkIntoTheLight (Jan 20, 2015)

jawnn said:


> is this formula correct?
> 2200mAh/1000 = 2.2amp and a 2 amp charge current/2.2=.55 'C'



The formula is correct, but you need a new calculator.


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## SilverFox (Jan 20, 2015)

Hello Juawnn,

I am not sure what you are trying to express with your formula but it is most likely incorrect. It is very difficult to go from amps to temperature assumptions without factoring in mass and internal resistance.

A "standard" charge rate is 0.1C for 16 hours. Assuming that the cell is not completely drained a 2200 mAh labeled cell may end up fully charged by charging at 200 mA for 15 hours. 

The effect of charge rate on the life of the cell depends upon the charge termination method used by the charger. A missed termination at a lower charge rate can often do more damage than a proper termination at a much faster rate.

Internal resistance is determined according to ohms law. To set up the test have a battery connected in a circuit. Measure the difference in voltage between two different current loads and factor in the difference in current and you end up with the internal resistance of the battery.

For example if your NiMh battery has a voltage of 1.25 under a load of 1 amp and a voltage of 1.35 under a load of 0.5 amps. The change in voltage is 0.10 volts and the change in current is 0.5 amps. 0.10/0.5 = 0.2 ohms of internal resistance. 

Tom


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## jawnn (Jan 21, 2015)

so there is no need for a formula? even if my calculator worked it would be wrong?


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## night.hoodie (Jan 21, 2015)

I'm one who has made the mistake of intentionally using too low a current to charge smaller cells (with hopes of increasing cell cycle life), and have a few likely damaged NiMH cells that overcharged missing -dV/dt termination. Now I use 750mAh fast charge for the smaller cells, but avoid going below 300mAh for the slow charge. I hope its a happy medium for keeping cells happy and near new capacities.


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## Grijon (Jan 21, 2015)

night.hoodie said:


> I'm one who has made the mistake of intentionally using too low a current to charge smaller cells (with hopes of increasing cell cycle life), and have a few likely damaged NiMH cells that overcharged missing -dV/dt termination. Now I use 750mAh fast charge for the smaller cells, but avoid going below 300mAh for the slow charge. I hope its a happy medium for keeping cells happy and near new capacities.



For what it's worth, the manual that comes with the Powerex Maha MH-C9000 says that generally the slower the charge, the better it is for the battery - within the following guidelines: don't go below 0.3C or above 1.0C.


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## ChrisGarrett (Jan 21, 2015)

.5C people, .5C.

We've got Global Warming and ISIL to worry about.

Chris


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## jawnn (Jan 23, 2015)

Thanks, this is the best answer, I can actually understand it, sorta. But I will try to calculate the resistance, it sounds important.

The formula above came from the Hyperion owners manual. 



Grijon said:


> don't go below 0.3C or above 1.0C.




I presume the Tenergy cells that claim 2600mAh are false? Or do they do something to the cells that no one else does?


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## Grijon (Jan 24, 2015)

jawnn said:


> I presume the Tenergy cells that claim 2600mAh are false? Or do they do something to the cells that no one else does?



I actually just completed a C9000 break-in on 8 of my 16 Tenergy "2600" cells yesterday. The results are
a low of 1887mAh
a high of 2318mAh
and the average capacity for these eight is 2,167mAh.

I don't know if I got a bad batch, but all 16 of my Tenegy 2600 AAs have broken wrappers that broke within the first few weeks of using them, and you can see how they're testing. I also have a Tenergy TN156 charger that didn't last 2.5 months.

I, for one, will not be buying anything Tenergy in the future.


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## StandardBattery (Jan 24, 2015)

Grijon said:


> I actually just completed a C9000 break-in on 8 of my 16 Tenergy "2600" cells yesterday. The results are
> ....
> I, for one, will not be buying anything Tenergy in the future.


That's not a bad policy.


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## WalkIntoTheLight (Jan 24, 2015)

I've never had much success in the past with high-capacity cells. Better to stick with moderate capacity that will be dependable.


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## jawnn (Feb 6, 2015)

*Capacity determination?*

Can some one tell me how to set up a device to test the cells resistance?
*Where can I get a variable load circuit?*
And then stick my volt meter prongs at the ends of the battery. Unless there is a simple device that does it all?


My i-max charger terminates by voltage first, then capacity or time. And the old Tenergy cells I have will not take as much as the newer cells in the above photo. 


I have a new Tenergy Sub C cell pack of 6 in series, they claim 5000mAh.


*But how do I determine the actual capacity by calculating with the resistance? *


*Doesit take a formula? *


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## markr6 (Feb 6, 2015)

*Re: Capacity determination?*

Recently I've been using 2A on my MH-C801D with 2000mAh Eneloops. I don't bother with all the data anymore; just charge and use. I may put some older ones on another charger to calculate capacity every so often, but not much.


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## MidnightDistortions (Feb 7, 2015)

*Re: Capacity determination?*

IMO getting some good LSD NiMH cells such as Eneloops, charge them at 500-1000mA, keep extras on the side and you won't need charging times at 15 minutes.


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## Raphion (Feb 8, 2015)

*Re: Capacity determination?*

^This. Fast charging is a curious thing, it makes much more sense to keep more cells than you use at any time, so you have a handful charged and ready to go. When something exhausts the cells in it, just take them out, put them on the charger, and grab some from your charged stack of cells. It's like having an instant charger, but without ruining any cells!

Just keep an eye on the charging. I like to watch the accumulated mAh display on the charger, and yank the cell out if I notice it going much over the cell's rated capacity. I wonder if any chargers have an option to set a maximum accumulated charge so as to limit the damage if the -dV/dt termination signal is missed... Would be nice.


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## HKJ (Feb 8, 2015)

*Re: Capacity determination?*



Raphion said:


> Just keep an eye on the charging. I like to watch the accumulated mAh display on the charger, and yank the cell out if I notice it going much over the cell's rated capacity. I wonder if any chargers have an option to set a maximum accumulated charge so as to limit the damage if the -dV/dt termination signal is missed... Would be nice.



You can do that on hobby chargers, but they are way to difficult to use for regular charging of NiMH.
http://lygte-info.dk/info/HobbyChargers UK.html


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## more_vampires (Feb 8, 2015)

*Re: Capacity determination?*



Raphion said:


> ^This. Fast charging is a curious thing, it makes much more sense to keep more cells than you use at any time, so you have a handful charged and ready to go. When something exhausts the cells in it, just take them out, put them on the charger, and grab some from your charged stack of cells. It's like having an instant charger, but without ruining any cells!



+2 This! My majority of battery use is eneloops. I've got the "live box," the "dead box" and get around to swapping cells on the charger about once a month. As a low mode freak, I'm just not burning the runtime on NiMH lights.

In my opinion, having a battery management plan is far more important than fast charging. It's also much easier on your gear.


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## SemiMan (Feb 8, 2015)

*Re: Capacity determination?*

There is another way to look at this:

AA - Eneloops on sale can be had for $2.00 if you are not in a hurry, but pretty much always < $4.00.

If you charge them on a fast charger, 15-30 minutes, let's say you only get 50 or 100 uses per battery. That means the cost is $0.02/usage/battery to a high of $0.08/usage/battery.

I have both a "slow" charger and a super-fast charger. I also have kids. That means that as much as one wants to use the slow one, often there are no charged batteries and the fast charger is put into us.

I know some people put "effort" into ensuring they always have charged batteries, that they are all equally cycled, etc. I have a charged and uncharged container and that is it .... and sometimes it works, sometimes it doesn't. For me personally, the cost/battery does not justify anything more complex.


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## MidnightDistortions (Feb 8, 2015)

*Re: Capacity determination?*



Raphion said:


> ^This. Fast charging is a curious thing, it makes much more sense to keep more cells than you use at any time, so you have a handful charged and ready to go. When something exhausts the cells in it, just take them out, put them on the charger, and grab some from your charged stack of cells. It's like having an instant charger, but without ruining any cells!
> 
> Just keep an eye on the charging. I like to watch the accumulated mAh display on the charger, and yank the cell out if I notice it going much over the cell's rated capacity. I wonder if any chargers have an option to set a maximum accumulated charge so as to limit the damage if the -dV/dt termination signal is missed... Would be nice.





more_vampires said:


> +2 This! My majority of battery use is eneloops. I've got the "live box," the "dead box" and get around to swapping cells on the charger about once a month. As a low mode freak, I'm just not burning the runtime on NiMH lights.
> 
> In my opinion, having a battery management plan is far more important than fast charging. It's also much easier on your gear.





SemiMan said:


> There is another way to look at this:
> 
> AA - Eneloops on sale can be had for $2.00 if you are not in a hurry, but pretty much always < $4.00.
> 
> ...



If you have 100 different devices that need batteries getting 300 cells might be a bit too much but it all depends on how much you need at one time. One solution is to get extra chargers with 8 charging slots, then charge the cells overnight. And there is the factor in how long you want the cells to last. I'd expect to get 10 years out of most of my cells versus having to replenish stock from weeding out crap cells over 2-4 years. In fact i use crap cells in low drain devices, long as they work fine that's all i really care about. Also my collection of devices grow so it'll require getting some new cells for those devices when my emergency stock diminishes. With that being said i have some otherwise decent cells short out on me because they couldn't handle the higher current rate. I prefer to charge them at the best rate which is 500-700mA and 1000mA for partially drained Eneloops. It keeps the cells happy and working well. If you absolutely need charged cells right away, the faster charge rates are there but if you can deal with the longer charge times a lower charge rate should be used. If you can't get enough cells at once and require using higher charge rates, maybe set aside more money towards extra cells and as i said more chargers with more than 4 slots. It isn't an option for everyone especially those new to rechargeables but since i already think i have enough cells, in a pinch i know i can get some Duraloops at my local Target store.

I keep track of what cells i have, what i need.. have some emergency cells off to the side and get more if needed. With that said i got 8 Eneloop Pros (that have fewer cycles but higher capacity) along with a LEDLenser light that takes 4AAA cells. Didn't really need the batteries but they'll come in handy in case of an emergency. It's a great EDC light as well. Also got another smart charger so now i have 3 chargers that'll charge the Eneloops and can charge up 12 cells at once without resorting to a dumb charger or using higher charging rates. It's really up to some folks but my idea of rechargeable cells is to use them, taking care of them in the process and recycle with each recharge until they no longer work in any device, then they are off to battery heaven . If i'm running into lack of charged batteries, buy more or if all the chargers are in use buy more chargers .


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## jawnn (Feb 9, 2015)

*Re: Capacity determination?*

I am starting to think it would be better to some how fit Lithium cells in my devices.


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## jawnn (Feb 14, 2015)

*Re: Capacity determination?*



Raphion said:


> I wonder if any chargers have an option to set a maximum accumulated charge so as to limit the damage if the -dV/dt termination signal is missed... Would be nice.



[FONT=Trebuchet MS, sans-serif]*Myi-max tells what the input capacity is adding up to, but it does notcut off by that unless you set the capacity to cut off. It can tellwhat the capacity is only by what it has put into it, so I think itwould work as a cut off only if you discharge the cells first. Thepeak voltage is always first to cut off. Or the time set, which isalways different. *[/FONT]


[FONT=Trebuchet MS, sans-serif]*FourNiMH cells in series at 1.4 volts cut off sensitivity, is 5.6v, theinput voltage is about 1 volt higher than the actual cell voltage at1 amp input less for .6 amp input. And some times the voltage will goover the set cut off voltage. Unfortunately I can't* *setit to cut off at 1.35volts*.[/FONT]


[FONT=Trebuchet MS, sans-serif]*Ihave never figured out how much capacity there is actually needed tofill the cells, and I always charge four in series. They never getwarm like my Nitecore i4 that does four singles.*[/FONT]


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## happyguy82 (Feb 14, 2015)

*Re: Capacity determination?*

I bought one of these:

https://www.youtube.com/watch?v=hv1tBDghfBc

Uniross 15 Minute Turbo Charger.

The fastest NiMh chargers will only do 15mins so as long as your charge time isn't under this then it's OK.

I see you've used a normal battery holder to connect 4 AA to your iMAX. This isn't a good thing to do. This Uniross Turbo Charger has large contact surfaces that press against the batteries with A LOT force. It's actually quite difficult to insert and remove the cells. This is to ensure good contact when pushing many amps through them.

Hope this helps.


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