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I've read that the most typical approach to charging, is to provide as much current as you'd like (lower current causing less loss of capacity over time), and to stop when the cell reaches 3.6V. The cell should then come to a resting voltage around 3.3V.

My problem is that my car alternator may only be able to provide 13.6V, or even less.

If I charge a cell at 3.4V, or 3.3V for that matter, obviously the charge current won't be as high, which is acceptable. But how do I know when to stop charging?

One guy did some tests and charged to different voltages, and stopped only after leaving the current was down to 30mA (C/100), and found that charging at 3.4V was still able to charge until 99% of capacity.

But is C/100 the correct current level to stop at?

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  • \$\begingroup\$ Are you trying to replace your lead-acid car battery with LiFePO4 in a real car? \$\endgroup\$ – Ale..chenski Jan 7 '17 at 22:32
  • \$\begingroup\$ No. Although I've considered it. For that purpose, it would be necessary to turn off the alternator, rather than disconnecting the battery, yes? \$\endgroup\$ – user95482301 Jan 8 '17 at 7:23
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    \$\begingroup\$ turn off the Alt's Field current, never cut off its output while running \$\endgroup\$ – HansBKK Jan 22 '17 at 4:38
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Actually you will get MUCH better longevity charging at 13.6v (3.40vpc), at that level you can get to higher than ~98.9% full anyway. 13.8-13.9V is maximum AFAIC.

Stopping at C/50 would be better than C/100, and in fact many suggest a half hour absorption once the battery hits your 13.6 would be fine - less is more, no need to push to "full-full", actually harmfull for LFP.

If you "have to" float, 13.2 or 13.3 probably doesn't reduce lifespan much, but better to "just stop" from the LFP bank's POV.

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