I'm working on fixing a 5Ah battery pack, and while all the individual cells are balanced at 3.8V, my charger indicates that the pack is fully charged when it clearly isn't. I thought it would stop charging once any cell hit around 4.1V, but something seems off here. Any advice?
3 Answers
You might want to check the readings for the ID resistor and the temperature (Th) resistor. The ID should generally be between 600 and 1k ohms, while the Th should be around 10k at room temperature.
If the Th reading is out of range, usually the charger will flash a temperature warning, so that’s worth checking.
That board you linked looks interesting. If it’s not a Battery Management System (BMS), then what is it? There are resistors and even an IC on that board.
After digging a bit more, I found that C3 appears disconnected somewhere. I’ve poked at an old board and discovered it connects via a 4.7k resistor to that IC. So, if I connect the battery terminal directly to the C3 pin, it likely won't work since the IC wouldn’t have connection. I’m going to try reflowing that terminal first.
That’s actually an over voltage protection board, not a BMS. There’s no balancing or management going on there.
Hey! If all your cells are at 3.8V and the charger isn’t working, try using a different charger first. If that fails, you might need to replace the BMS. Good luck!
Just a heads up, 5Ah batteries typically don’t come with a BMS.
The ID is at 800 for me, and I’ve managed similar issues with a 1k resistor before.