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Charging low dischrge NiMh batterries.

overheating

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Robert Putley 102/09/2017 05:52:36
201 forum posts

What is the considered charge rate for eneloop's and other low discharge battaries.

For example, if charging Eneloop 1900's at one amp, there are very warm/ hot when the charging has finished, which cannot be good for the cells.

cymaz02/09/2017 06:41:33
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9283 forum posts
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I have a Turnigy Accucell 6 charger, i leave it on .7amp charging. No problems...

Do all the eneloop batteries get hot or just one in particular? Eneloops don't loose a lot of charge if left for several months. How often do you need to charge it?

Broken Prop02/09/2017 06:45:14
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The recommended rate of charge for all NiMh batteries is quite low, around 0.1 amp, which is why 'wallwart' trickle chargers are often used. Of course this means that they take all night to charge up. Even then, at the end of the charge, the battery can be quite warm.

I use Eneloop batteries and normally charge at a rate of less than half the capacity for a fast charge. So for a 1900 battery I would charge at 0.8 to 0.9amps, not much different to your charge rate. They do get quite warm towards the end of the charge.

The commonly perceived wisdom is that regular fast charging is not a good idea as it reduces battery life. However I have not noticed such.

No doubt others will be along shortly with much better explanations but in the meantime try searching the archives. There is a lot of helpful information there.

Pete

Trevor Crook02/09/2017 07:26:12
990 forum posts
71 photos

I normally charge mine on a peak detect charger at about 700mA. I find that much higher than that can cause the charger to "false peak".

If a charger that doesn't automatically turn off is used, the rate should be lower. When the battery is fully charged, the energy you are still putting into it is dissipated as heat. So, if charging at 100mA, if the pack is at 6V then 600mW will need to be dissipated, so they will get quite warm.

I've always found that NiMh batteries get warm during charging.

Denis Watkins02/09/2017 07:34:44
4539 forum posts
123 photos

The absolutely ideal recommended charge rate is 1\20th C

But 1/10th C is acceptable

Allan Bennett02/09/2017 08:23:28
1703 forum posts
49 photos
Posted by Broken Prop on 02/09/2017 06:45:14:

The recommended rate of charge for all NiMh batteries is quite low, around 0.1 amp, which is why 'wallwart' trickle chargers are often used. Of course this means that they take all night to charge up. Even then, at the end of the charge, the battery can be quite warm.

......

As has been mentioned a couple of posts later, the recommended charge rate for small low-current NiMh packs (such as used in transmitters and receivers) is 0.1C, which means it's not a fixed value, but is 1/10 the pack's mAh capacity. So for a 1900mAh pack I would be looking at 190mA as a sensible charge rate. It will take at least 10 hours to charge, but that rate is low enough that the cells shouldn't heat up significantly if they reach full charge before the charger is stopped.

The "wall wart" chargers that are supplied with transmitter sets usually work on this principle, and they usually recommend 14 hours charging, to ensure a full charge of all cells. A peak-detect charger very often can't reliably detect the peak when used at a low charge rate.

For larger NiMh packs which are designed for higher currents -- e.g. motor batteries -- 1C is often recommended as the optimum charge rate. For this one would usually use a peak-detect charger, though an occasional overnight 0.1C charge is good to ensure that the cells in the pack remain balanced.

Chris Bott - Moderator02/09/2017 08:47:35
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I would not necessarily recommend the low charge rate. It was wise for NiCads as they were perfectly happy to absorb any overcharging. So it didn't matter what the discharge state was when charging commenced. They could just be left on charge say overnight.
Nimhs are more fussy, a delta peak charger is recommended and the lower the charge rate, the less likely that the "battery full" peak will be detected.
Eneloops are fine being charged at 1C. This rate gives the best chance of stopping at the right time.
Robert Putley 102/09/2017 09:31:42
201 forum posts

When I have charged eneloops@ 1c they end up very hot...NOT good.

Denis Watkins02/09/2017 09:40:18
4539 forum posts
123 photos

Robert, with a low battery at the field, charge at 1C occasionally

But at home, as you found, divide battery capacity by 10, to arrive at a charging number of 1/10C

Chris Bott - Moderator02/09/2017 09:41:09
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The most important thing about charging nimhs is that the charge it terminated when the cells are full.

Many dedicated NiMh chargers not only detect a voltage peak, they also sense temperature and stop if the cells start to warm.

If your batteries are getting hot then for whatever reason, they are being overcharged.
Chris Bott - Moderator02/09/2017 09:45:27
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I'd suggest a good read of eneloop101.com

Here's a cut and paste from their "charging" page:-

Is slow charging better than fast charging?

I think this is a very common mistake people make. They think that slow charging will result in longer cycle life. This is not true just by itself! They put their eneloop Pros in a time based charger and think they are treating their batteries nicely. Wrong.

Please look at the Tests Page for the 5000 cycles test. It is not mainly about the charge current but when the charge stops. Charging at a slow current can actually be more problematic when the charger can not detect whether a battery is full. At a higher charge rate it is easier for the charger to detect a Voltage drop and Temperature increase and therefore knows when to stop charging. These 2 checks can be missed when the batteries are charged at low current!
john stones 102/09/2017 09:50:12
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11595 forum posts
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I'm with Chris, use your charger as intended and it'll function correctly, i have nimhs for use in cameras they go on a small plug in charger, forget to unplug when charged, they're HOT. Tx on an intelligent charger, no hot battery.

Robert Putley 102/09/2017 09:50:46
201 forum posts

Dennis, I shall take your advice.

Chris, I would have used a tempeture sensor, had I had one, which I did back in the eighties! Something I might consider in the future.

John Lee02/09/2017 09:58:46
764 forum posts
86 photos

The temperature rise at the end of the charging cycle is characteristic of NiMh battery types and some chargers can use this temp ramp to detect that the battery is fully charged in lieu of, or in addition to, the voltage peak detection.

Enerloops are made by Panasonic and rather naturally they recommend there own chargers. Their 'flagship' charger BQ-CC65 charges in 1.5hours (.6C) and they say that the battery can reach 50degrees C, which is normal & not detrimental to the battery life of 2100 cycles. More at the official site here.

Personally I've a variety of low discharge NiMh's & I charge them all at .5amp which I find gives a reliable peak detect cutoff on my Graupner charger.

Robert Putley 102/09/2017 10:00:44
201 forum posts

John,

When you use the term intelligent chargers, I assume you mean the usual type that we all use for our flight pack and radio batteries, i,e. peak voltage detect.

I also have no problem with NiMh cells. It is only with the low LSD type that this problem occurs.

john stones 102/09/2017 10:08:18
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11595 forum posts
1517 photos

Yes, mine are just warm when charged on them, on the cheap plug in jobbie, with LSD's they get hot if i don't unplug when light go's off, i don't know if them getting hot is bad for them, but it bugs me.

Martin Harris02/09/2017 10:51:01
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9409 forum posts
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Posted by John Lee on 02/09/2017 09:58:46:

Enerloops are made by Panasonic and rather naturally they recommend there own chargers. Their 'flagship' charger BQ-CC65 charges in 1.5hours (.6C) and they say that the battery can reach 50degrees C, which is normal & not detrimental to the battery life of 2100 cycles. More at the official site here.

Interesting - in another thread on this subject, I and/or another Martin were taken to task for suggesting that .5 or .6 C was a reasonable charge rate to prevent false peak detecting!

Tom Satinet02/09/2017 13:37:07
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519 forum posts

If you charge at very low current sometimes the charger doesn't peak at all and just keeps charging.

I've always charged up to 1C and never had any problems. Usually I charge around 800mah. Depends how much of a rush you are in.

The pack last ages.

Martin McIntosh02/09/2017 21:25:10
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3471 forum posts
1216 photos

Did a few experiments on these and came to the conclusion that 600mA is ideal. At 100mA my chargers have trouble detecting a peak. If I am in a rush I charge at 1A but they will always take a bit more if time allows to top up at 600mA. Never got more than 1800mA/hr into 1900`s even after a slow discharge. As mine die off I am changing over to LiFe anyway. Much better/cheaper/lighter.

Richard Wills 202/09/2017 23:18:25
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208 forum posts
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Always charge mine at 1c. Always feel hot, but 50 deg is normal apparently, and that will burn if you hold it, so what you 'feel' as too hot isn't really that hot.

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