Voltage dropping while charging?

[RossA]

Okay, so I have a 24v LFP battery pack that I assembled a little while ago and I'm charging it right now. The battery prior to charging was reading roughly 27.2v. I connected it to the inverter charger to charge it up and the voltage on the battery went up pretty quickly as it was getting about 30 amps into it for charging. I've been watching it a little bit. It seemed that the voltage capped out about 27.6v and then the voltage has just been dropping. Which is totally not what I was expecting. I figured they would increase from the 27.2 to 27.5 or so and then slowly climb until the pack was fully charged at 29v since I need to do a proper top balance. But instead, I've seen the voltage drop from 27.65 to 27.3 even though it is receiving 30 amps from the charger. The inverter shows 30 amps in, the battery shunt shows 30 amps in, and the BMS also shows 30 amps in. So I have no explanation for why the voltage is dropping instead of increasing...

Ideas?

 

[time2roll]

Very likely this is normal. If all cells are running cool I would not worry.

 

[RossA]

Very likely this is normal. If all cells are running cool I would not worry.

Okay, that's good. I figured things were probably fine as I am not seeing anything else abnormal. The bus bars are still cool, and I don't feel any heat in the wires either. The BMS temp sensors are showing the cells to be about 24 Celsius.

 

[Warpspeed]

If cells are discharged down very low, and then connected up to a powerful charger, the charger may be powerful enough, and the battery internal resistance high enough for the voltage to be forced right up.

As the battery recovers charge, the internal resistance falls, and that can pull the voltage down.
As it gains even more charge, the voltage will again continue to rise in the expected manner.
Its quite normal...

 

[Bob B]

It also could make a difference where you are measuring the voltage .... are you measuring directly in the battery terminals?