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I have a 3.7V, ~1500mAh Li-Ion battery that I am planning to charge with a small photovoltaic cell that will output roughly 100mA when under full sunlight. As I understand it, a Li-ion battery should be charged at a constant current of about 0.5C until its voltage has reached 4.2V, at which point it should be charged at a constant voltage. (I realize fully charging this battery will take 15+ hours)

My question is what will be the effects on battery life if this battery is being charged at 100mA/1500mA = 0.067C during the constant current phase? Keep in mind that the PV cell will not always be under full sunlight, meaning the output current will fall below 100mA and vary. Follow up question: will variations at low (<.067C) charging rates during the constant current phase cause substantial harm?

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You can charge Li-Ion cells at very low currents, as long as you respect the maximum voltage, and the dwell time at maximum voltage. The cell will take longer to charge than at the recommended rate, but it will still charge as efficiently.

If you expect your setup to remain on charge for a long time, then you will need to reduce your maximum voltage, 4.2v is not a safe level for a Li-Ion cell to be left at indefinitely, regardless of current.

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  • \$\begingroup\$ Yes. I built a charger using intermittent power source, and used 4.05V as cutoff voltage. It preserves most of the capacity. 4.2V would not have been safe. Also, don't forget the temperature sensor! \$\endgroup\$ – bobflux Mar 12 '17 at 10:08
  • \$\begingroup\$ 4.2V seems to be a safe voltage to let the battery float indefinitely, see this question. Using a 4.15V for floating the battery is absolutely safe according to my experiment. \$\endgroup\$ – ceremcem Mar 14 at 22:54
  • \$\begingroup\$ One experiment on a second-hand cell does not contradict the advice from every manufacturer which is not to float at 4.2 V. Was this a LiPo or a LiLo, which uses 4.1 V as its terminal voltage, or a LiPoHV, which is 4.3/4.35? \$\endgroup\$ – Neil_UK Mar 15 at 0:26

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