Figure C.1: LiFePO4 Cell Spikes During Charging
Q: OK, so how would one go about top-balancing their cells?
A: There are several ways to manually balance cells, depending on what equipment you have access to:
The best way in my opinion, is to use a regulated power supply to charge the cells to 3.65 volts each. The cells
would be connected in parallel as a single cell and charged together (without the BMS), then re-assembled
into the series-connected pack with the BMS. Will Prowse demonstrates in this video:
https://youtu.be/x5ABvbbics8
Cheapest way: Connect a load to the high cell in your pack to quickly bleed off the excess energy. I tried this
method using a random car light bulb with some alligator clips on the leads. (see Figure C.3 below) You need
to watch the cell voltages closely because it’s easy to go too far.
Figure C.3: Using a car light bulb to bleed off excess cell voltage
What does NOT work is the old recommendation of connecting your new cells in parallel and letting them
passively equalize for hours or days. This does not work because of the flat charge curve. They are at almost
the same voltage even if they are far apart in state-of-charge. Basically the cells don't know that they aren't
balanced unless you can push them into the very top end of the charge cycle.
Q: What about cell matching?
A: Cells have a certain internal resistance. Grade-A cells are tested at the factory to confirm that their internal
resistance is acceptable, usually <1 milliohm. If your battery pack is made of grade-B cells or cells of different
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