Master EcoWalker
Join Date: Dec 2012
Location: Nieuwegein, the Netherlands
Posts: 3,998
- '11 Honda Insight Elegance90 day: 53.26 mpg (US)
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I have built 3 12 cell battery packs using 12 Ah LiFePO4 cells and a BMS for each.
All of my 36 cells came out of the same battery factory box and are closely, but not exactly, matched in capacity.
What you need to understand is that you can destroy a LiFePO4 cell by overcharging, and that the voltage over a cell rises very rapidly indeed when it is full.
I charge my cells at 3A.
During the 4 hours that it takes to charge the packs the voltage per cell rises from 3.20 Volt when nearly empty to 3.35 Volt when nearly full, then within seconds it jumps to 3.85 Volt where the BMS cuts off the pack.
It will then top balance the cell down to 3.65 Volt, which takes a minute or so. The other cells will still be around 3.35 Volt.
If the BMS had not cut off the charge the full cell would have gotten over 4 Volt, which would start to damage it; the higher, the worse. Still the other cells would not be full so the total voltage over the pack would not show you it is about to get damaged...
The chargers cut off when the BMS cut off so I need to reset each charger to get it charging again. The full cells will quickly hit 3.85 Volt again and kill the charge, but the other cells will one by one also reach 3.85 Volt. With every cycle the total voltage rises a bit. Then finally all cells are balanced and full.
You really need a BMS, especially if you do not constantly measure voltage at the cell level yourself.
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