Concerned about a couple cells in one of my 24v LiFePo4 batteries. Cells 4 and 5 once above a certain voltage just race into overvoltage. This battery will be relatively well balance until it hits 28V, then a minute later it's seriously out of balance and gone into overvoltage protection at 29V.
Any thoughts?
@exador23
There is a strange contradiction about the best way to use Li cells.
Rule 1 keep cells balanced
Rule 2 prefer to keep voltage range of cells within 20 to 80% to extend lifespan.
Unfortunately most balancers work at max voltage, which you try to avoid. Therefore I advise and started using active balancing circuits that pump power from the higher cell to their lower neighbour. They work at every voltage.
The (cheap) circuits you buy, for example on ebay, are 1 Amp max. Although that is not much it gets the work done over time with cells that are not too different.
Cells that got bad will lose capacity, they are full earlier on in the charging process and race up when current isn't taken back because total voltage/avg cell voltage hasn't reached max. Maybe that's the problem you face...
The 20/80 rule holds true for the older lithium chemistries like Lithium Nickel Manganese Cobalt Oxide (NMC) used in power tools, phones, and most US EVs. The LiFePo4 chemistry has so little degradation at the SOC extremes that calendar aging is going to reduce your capacity much faster than using the full 100% will.
But improper cell balancing will rapidly degrade your capacity as well as aging the cells at different rates. let's say you have a 12.8V LFP. The 100% charge voltage is 3.65/cell and 3.4/cell 100% at rest...
If your cells are 3.45/3.55/3.55/3.65, the BMS shuts down charging at 14.2V to protect the high cell from overvoltage. At rest the cells drop to 3.2/3.3/3.3/3.4 which is 13.2V or only 70% SOC. So you've already lost 30% of capacity before considering that a similar early shut down of discharging will happen to protect the lowest cell. The small degradation from the cells spending enough time near 3.65V to do proper balancing is nothing compared to a 30-50% capacity loss and uneven cell aging.
FWIW, I fixed my issue by opening up my battery and top balancing the cells by hand to 100% and within 0.001V of each other. It was a painstaking process, but now the cells have been cycling daily from 100%-20% for the last few weeks without issue, and the balance is never more than 0.010V out throughout the full range.
The voltage/SOC curve is so flat through most of the cycle with lifepo4 cells that there's not much point in trying to balance unless the cells are above 3.4-3.45V.
Hippy Steve
Merlin Makes