On 2/14/2013 10:25 PM, David Nelson wrote:
I beg to differ with you Cor. Trying to "float" a LiFePO4 cell at 3.45V will definitely overcharge it! The absolute highest voltage a LiFePO4 cell should be held at is 3.40V and 3.38V would be best.
More to the point; the self-discharge rate of lithium cells is so low that float charging them is not recommended. It isn't going to do any good, and may easily do a lot of harm!
Back to the original question: How to balance four lithiums in series being used as a 12v battery. If you do nothing else, at least put a zener diode and light bulb in series across each cell. This will serve as a crude BMS. It's a poor regulator; but dirt cheap and better than nothing. It will a) bypass at least some of your excess charging current, b) tend to equalize the voltage across all four cells, and c) give you a visual indication if your voltage is just plain too high.
A 3.9v 5w zener (1N5335) and #PR2 flashlight bulb should be about right. The 1N5335 shunts about 25ma at 3.3v, 100ma at 3.6v, 300ma at 3.9v (bulb dimly glowing), and 500ma at 4.1v (bulb lit).
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