Overkill Solar 120A 4s 12V BMS, Instruction Manual

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Figure C.6:​ The effects of a badly-balanced cell
Steve’s advice to the customer, thinking that these might be crappy cells, was to top-balance the batteries.
The customer did so, and reported that three of the cells took approximately 7.5Ah, but cell #2 charged for
over a day, and when it was finished took a total of around 140 Ah. With that, we can work out a timeline of
what happened:
1. The supplier from China shipped three of the batteries at 90% charge, and one battery at 50% charge.
This should never happen. But it happened.
2. The supplier’s reassuring video of the cell voltages didn’t ​mean​ anything. Recall that LiFePO4 battery
discharge curves are ​ extremely​ flat. This means that the capacity can vary wildly, but the voltage
won’t change much. We can actually see in the measurements from the vendor that the delta was 30
millivolts. Consulting the discharge curve for the battery above, a 30 millivolt delta (which we saw in
cell #2) can mean as much as a 40% state of charge difference.
3. During the battery pack bring-up, the cell overvoltage protection kicked in on cell #1, and cut off the
charging current. This happened because the cells were not balanced.
4. Steve recommended a top-balance, which was done, taking over a day at a charge rate of 10 amps.
After the pack was reassembled, the cell delta was around two millivolts (much better). The battery
successfully discharged down to its cutoff limit, and charged back up to 100% with no issue.
This, dear reader, is why you top balance. Don’t trust voltages when working with LiFePO4 batteries. When
it comes to vendors, the rule is: ​
Trust, but verify​ . Verify by top-balancing.