The following is my attempt to reason about cell balancing in a battery. I do not know whether that reasoning is correct. It seemingly violates common recommendations and conventional wisdom.
Consider a lithium iron phosphate battery (LiFePO4, or LFP), which is a collection of cells wired in series. Suppose that when the battery is fully charged, each cell is also fully charged. That is, each cell is at 100 percent state-of-charge (SOC), and has the exact same voltage as the other cells in the battery.
Now place that fully charged battery in service to power loads. As the battery is discharged, the current out of each cell is exactly the same as the current out of the entire battery. Because the cells are wired in series, it cannot be otherwise. Thus the Ah out of each cell is also the same.
Due to constraints in manufacturing, it's possible and maybe likely that the individual cells will have slightly different capacities, despite the manufacturer's best efforts to ensure they're identical. For a particular discharge current, a lower capacity cell will experience a greater voltage decrease than a higher capacity cell. Thus under discharge, the voltages and states of charge of the battery's cells can diverge somewhat. The cells become unbalanced.
But isn't that situation entirely reversed during recharge? During charging, the current into each cell is exactly the same as the current into the entire battery. As with discharging, it cannot be otherwise.
It's often said that at the beginning of a recharge, cells with higher starting voltages will experience a voltage spike compared to cells with a lower starting voltage. But is that so? Cells with a higher starting voltage have that higher voltage precisely because they have a larger capacity than cells with a lower starting voltage.
But as the charge proceeds, all cells receive current that exactly replaces the current they generated under discharge. Just as current out of each cell was identical during discharge, current into each cell is identical during recharge. Their higher starting voltage, where "starting" means at the beginning of the recharge, is because they retained higher SOC during discharge. They did so because their overall capacity is greater than cells with a lower starting voltage.
Suppose for the sake of this thought experiment that the charge efficiency of a LiFePO4 battery is 100 percent. (It's actually very close to that in the real world.) With 100 percent charge efficiency, if a cell was discharged n Ah, then an n Ah recharge would restore it to 100 percent SOC. Because n is the same for every cell in the battery, both during discharge and recharge, then restoring n Ah to any one cell restores n Ah to every cell, and bringing any one cell back to 100 percent SOC means that all the cells are brought to 100 percent SOC. If that is so, then at the end of the recharge, every cell is at the same SOC and hence the same voltage.
And yet, battery management systems (BMSs), supposedly seek to "balance" the cells. Typically this is done at the higher voltages that occur toward the end of the charge process. Balancing might involve bleeding off charge in higher voltage cells to allow lower voltage cells to catch up, or some such. Such balancing would result in each cell having the exact same final voltage when fully charged.
But is this even necessary? Assuming the cells were balanced to begin with, doesn't "n Ah out, n Ah back in" keep the cells balanced during normal discharge and recharge?
And if the cells become slightly unbalanced while in use, does that even matter either? Suppose in normal use one keeps his batteries between 50 percent and 70 percent SOC over long intervals, as a strategy to maximize the battery's lifetime. If there's a small amount of SOC variation between cells with a battery in this interval, does it matter at all?
Thus I question the value of cell balancing in many or most circumstances. I suppose my logic depends on the all cells being at the same SOC and voltage when the battery is fully charged, at least once. But after that, if they diverge, so what?
P.S. Balancing during discharge seems particularly perverse. That's because it will cause smaller capacity cells to become over-charged during recharge.
