Proper battery charging measurments

[JThiessen]

Since I started running electrics, I have always (and maybe ignorantly) just used voltage as the indicator of a charged battery. This past weekend, my nephew was telling me that he charges and recharges until the mAh are up to the rated capacity. Me being a dummy, I just wanted to find out if this is infact the correct way to charge.

[BrianG]

I use both ways and usually they agree. If you've put in 6Ah into a 5Ah battery, the voltage would be higher as well. Think of it as another method to ensure proper care.

[JThiessen]

That was my thought also, but on one of his, he went to 8.XXX volts on a 7.2V pack, but it was only showing about 2000mAh, on a 3600mAh pack.

[VintageMA]

Going to assume that you are that you are talking about NiMH and not LiPo here.

LiPo uses a CC/CV method to ensure a safe charge.

NiMH uses a negative-delta-peak-detection CC method of charging. A constant current is applied to the pack and the voltage is monitored by the charger. When the pack reaches it's peak charge the voltage will actually start to decline. Usually the charger will be set at around 3-5 mV / cell, so when the voltage maxes out and then declines .018 - .030 (for a 6-cell pack) V the charger will stop.

It is pretty normal for the voltage read from the charger to be much higher than the rated nominal voltage right before the charge is over. It will settle back down very quickly after removing it from the charger and if you put a volt-meter up to the terminals you will probably read about 1.25 - 1.35 v/cell.

Also - it is not uncommon to put in more than you take out. You have to consider the internal resistance of the cells themselves. With high current voltage will be dropped across the battery pack along with the motor and esc - which is what causes the cells to heat up. That is lost power you will never see on a meter and one reason why you can put in more than you take out.