Charge 4S2P NiMH

[High Efficiency]

I've been searching for an answer and haven't been able to find much (which could be an answer in and off itself).

I'm looking at stitching together my own 4S2P pack of NiMH (eneloops) batteries. Is it possible to charge a pack like this with a wall based charger (link below) or an Imax B6?

http://www.powerstream.com/NiMH-3-4.htm

 

[HKJ]

Yes and yes.
You first have to balance the pack, this is done with 16 hours of 0.1C charging. Then you can charge it normally.
If you over discharge the pack it is probably a good idea to rebalance it, it might also be a good idea to rebalance it occasionally, but don't overdo it, it puts extra wear on the cells.
The linked charger will "balance" the cell each time, due to the high trickle current, just leave the batteries on the charger for a few hours when the charger says ready.

 

[High Efficiency]

Thanks for the input.

So, any initial balance charging could be done with the pack in a 4S2P configuration using the linked to charger by simply leaving it plugged in for a few hours after full on it's first charge?

 

[HKJ]

If you start with all the cell charged, that will work, but if you start with unknown charge state the 16 hours at 0.1C current is mandatory.
The idea is that NiMH cells can take 0.1C over charge current for a long time without taking significant damage (This is not completely true for LSD cells like eneloop).

 

[SilverFox]

Hello High Efficiency,

A caution should be stated when using low self discharge cells in your battery. Eneloops will vent if left to trickle charge (even at very low trickle charge rates) for extended periods of time. At the end of the charge you can balance the cells in the battery by letting them trickle charge for 3 or 4 hours, but after that it would be best to disconnect the charger.

If you have a recording voltmeter you can actually track this. At the end of the charge you will see a voltage of around 1.5 volts per cell. As the pack cools down the voltage will drop a little and then under the trickle charge the voltage will start to rise again. At some point it will flatten out. About 30 minutes after the voltage flattens out is full charge.

Tom

 

[HKJ]

Eneloop do not vent easily. I have for some testing been charging 3000mAh into a 2000mAh cell at different rates and it has not vented, but that is not as bad as when people uses 0.1C for 16 hours on a full cell and they do not vent either.
But even if they do not vent, it is best normally to remove them from the charger when it reports full, but to balance a series pack of NiMH you need to overcharge.

 

[SilverFox]

Hello Henrick,

Interesting...

I made a 3S2P pack using the original Eneloop cells and ran into problems with a pulsed balancing charge of 400 mA on a 1/3 pulse rate. This ended up as a 100 mA charge. I was alerted to the venting by a very soft "gurgling" sound. When I went to explore where the sound was coming from I found it coming from a particular cell in the pack. I stopped the charge and the "gurgling" sound stopped.

The pack continued to be used and I did not notice any degradation in performance, but since that experience I have always limited any top off charge with Eneloop cells.

The charger was the Schulze isl 6.

Tom

 

[HKJ]

It it is only a very soft sound I may have missed it, but I do occasionally overcharge eneloops during my charger test and I have never heard (or smelled) anything.
I am going to do a couple more 3000mAh charges on 2000mAh cells and I will "look out" for any gurgling sounds.

My special tests are done with lap equipment and not with chargers.

 

[High Efficiency]

Thanks, this has been very helpful.

Is there any amount of trickle charge that will not hurt LSD type batteries (Eneloops)?

The charger above indicates that it moves to 80mA trickle charge.

Would a 4S2P pack of Eneloops (assuming 2,000 mAh) contain 16,000 mAh or 4,000 mAh for charging purposes? I'm unsure whether to add the capacity of the cells are wires are in series...

Depending on the above an 80mA trickle charge would be a charge rate of either .005C or .02C. Even something this low wouldn't be recommended?

I was hoping to design a light I could leave permanently plugged in (and charged) so that it's always ready to go with a full charge.

 

[HKJ]

No, but with a low level of trickle charge it may take a few years to kill the cells.

Parallel cells add capacity, serial cells add voltage. 4S2P is 4*voltage, 2*capacity.

Permanently plugged in is easier to do with LiIon, a normal LiIon charge chip will turn current off then the cell is full and first resume charging when it has lost some charge.

 

[High Efficiency]

Thanks.

Understood regarding series/parallel voltage/capacity. I just wasn't sure charging in series was any different.

If killing the cells via trickle charge would there be risk of a violent failure or simply a loss in capacity until the cells were no longer functional?

 

[HKJ]

As long as the cells are not sealed air thigh there is no risk of violent failure. NiMH venting is without a big risk of fire, but the pressure must be able to escape.
My guess is that a constant trickle charge would only wear the cells down, not making them went (This will depend on the actual current).