Current status of NIMH knowledge re heat and %/C chargin

For general discussion of the UltraSmartCharger
Benediction
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Joined: Thu Nov 03, 2016 12:44 am

Current status of NIMH knowledge re heat and %/C chargin

Postby Benediction » Thu Nov 03, 2016 12:53 am

The last test results I am aware of, regarding battery cycle lifetime and loss of capacity due to stress are from 2010, before current LSD type batteries were at all common. Is anyone aware of more recent testing?

My understanding of the 2010 tests said that if charging was closer to 1.0 C (100% of rated charge, eg: using 2000 mA to charge a 2000 mAh battery) that the lifetime (total # of cycles) would be reduced by 5%. Any of my numbers might be off as this is just my memory.

In any case, I believe those tests were done with a fixed voltage. Many of the current chargers use a pulsed voltage...from 0 to X Amps, and controlling true mA by controlling the duration of the on cycle (eg: 46% cycle of 3 Amps would be ~1400 amps). So does this help or hinder lifetime (max cycles) of either regular or LSD NIMH?

Mark
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Re: Current status of NIMH knowledge re heat and %/C chargin

Postby Mark » Thu Nov 03, 2016 1:02 am

I'm not sure what the effect of using duty cycle on the charging current has on the number of cycles that can be achieved.

In my opinion, using a higher current with a reduced duty cycle will lead to higher cell temperatures which I would expect to be detrimental to the number of cycles that can be achieved. I have seen speculation that higher charging rates lead to smaller crystal formation within the cell which might be beneficial...

Benediction
Posts: 18
Joined: Thu Nov 03, 2016 12:44 am

Re: Current status of NIMH knowledge re heat and %/C chargin

Postby Benediction » Thu Nov 03, 2016 5:01 am

In my opinion, using a higher current with a reduced duty cycle will lead to higher cell temperatures which I would expect to be detrimental to the number of cycles that can be achieved.


I agree, but I believe it was you who stated in another thread that the UltraSmart was charging batteries, with this pulsed current, and even though the effective current was greater than .7 C there was still not much heat in the LSD batteries. Whereas I have observed even with a 700 ma fixed current on a BC-700 the eneloop AAs (4th Gen Japan) get somewhat warm.

I have purchased one of the newer Costo Eneloop charger + battery packages, with a BQ-CC55, which uses a pulsed 1400 ma current (46% x 3A) when charging 2 or fewer batteries, which is a .7C for a (nominally) 2000 mAh Eneloop AA. I suspect it wont even get very warm. What temperatures, assuming a newish Eneloop AA with low resistance, would the batteries typically be in an UltraSmart at 1400 ma?

Mark
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Re: Current status of NIMH knowledge re heat and %/C chargin

Postby Mark » Mon Nov 07, 2016 12:28 am

The way that the UltraSmartCharger works is to use high frequency PWM to step down the voltage (and increase the current) from power supply - the effective output that the cell should be seeing is a constant current with a little bit of ripple.

I don't think that I've done any charging at 1400 mA, so I couldn't really tell you what the temperature increase would be at that charging rate. I could dig out some data for charging at 1000 mA and also at 2000 mA - keep in mind that the temperature increase probably won't be linear with charging current, but it might still give you a bit of an idea...


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