Charging Batteries in Series and Parallel

[Impartial]

I recently bought 80 Panasonic NCR18650A 3.7V 3100mAh 18650 Rechargeable Batteries. My plan is to store them in a handlebar bag to power my 36v electric bike. I've configured the batteries so that there are 8 sets of 10 (in series batteries) and they're all in parallel. That way I achieve the circa 36/37volts for the motor and 24.8Ah.

What I'm unsure about it how to best charge the batteries as one big battery, so what voltage and amperage to use, as opposed to charging them individually. Here are Panasonic's recommended charge conditions for an individual battery:

CVCC 4.2V Max.0.3It (885mA), 60mA cut-off at 25deg.C

Your advice would be greatly appreciated. Thank you.

 

[(*steve*)]

There are a number of issues with doing this, but the best option is probably to place sets of 8 cells in parallel, and then place these in series to make the battery.

Then use a balancing charger to charge it. A balancing charger will ensure that you stop charging as soon as one set of cells is full. Some can also balance between cells (or in this case sets of 8 cells). Whatever you do, ensure that no cell overcharges or over discharges. Both will kill cells rapidly.

 

[Impartial]

Good man. Thanks.

I wasn't aware these things existed. Do you think something like this would work then: http://www.ebay.co.uk/itm/IMax-B6-1...e-Nicd-Battery-balance-Charger-/261239866719? In the specifications it says that it can charge 1-6 li-ion cells. If I set it to at 3.7 volts and 885mA, would it handle a set of 8 in parallel?

 

[(*steve*)]

That looks like the right thing. Before you connect the batteries in parallel, ensure they are in the same state of charge (i.e. they have the same voltage). for 8 cells in parallel, you would charge them at 8 times the current they are rated for. 885mA sounds a bit low, but it should certainly be safe.

A charge at 0.2C would suggest a current of about 4.9A (0.2C would charge them in about 5 hours). The voltage at termination of charge is likely to be around 4V per cell unless they're LiFePO4 or you're going for extended life and can live with the lower Ah capacity.

 

[Impartial]

I see. So if I have 10 lots of 8 sets of batteries in parallel, I could charge 6 sets with the balance charger and then charge the remaining 4? Would the charge time be 3100 / 885 = 3.5 hours? Then multiplied by 2, as I can only charge 6 cells at a time? Forgive me, I'm not fluent in electronics. I couldn't quite get my head around the 'A charge at 0.2C would suggest a current of about 4.9A (0.2C would charge them in about 5 hours)' bit. What do you mean by that?

 

[(*steve*)]

I could charge 6 sets with the balance charger and then charge the remaining 4?

I would recommend you find a balance charger capable of charging them all at once.

Would the charge time be 3100 / 885 = 3.5 hours?

No, You have 8 cells in parallel, so the mAh rating is 8 * 3100 = 24800 (or 24.8 Ah). So at 885mA it would take 24800/885 = 28 hours to charge the battery.

I couldn't quite get my head around the 'A charge at 0.2C would suggest a current of about 4.9A (0.2C would charge them in about 5 hours)' bit. What do you mean by that?

C is the capacity in Ah, it is thus also the current required to charge (or discharge) the batteries in 1 hour (approximately). Charging is often specified in relation to C, so if you want to charge the battery in 5 hours you choose 1/5C = 0.2C. 0.2C is a moderate charge rate. Some batteries can be charged much faster, although they can be damages and won't necessarily be charged completely to capacity.