I have ~100 mAh 6 V solar cell that I want to use to charge 18650 lithium-ion battery. Purpose is to use a PIR sensor to turn on light when someone passes by,

I am not very confident about the efficiency of circuits I found online or using something like a TP4056 as I only have limited energy to work with that I don't want to waste as heat while charging so I was looking for a simple solution that cuts off or dumps excessive solar energy to some load when battery is at 4.2 V.

I tried reverse-engineering professional products but they seem to use unlabeled ICs and might have some peak power harnessing magic going on as they work with puny solar cells (50 mAh) yet work without a fail all night, I can reuse such product but

  • My solar cell is twice as powerful
  • Light is twice as big
  • PIR sensor looks different, mine is bigger
  • There is no fun in slapping everything on existing circuit without learning anything

What kind of simple circuit can be used here? I don't care about battery health as much as LEDs I have turn off at around 3.3 V anyway.

  • \$\begingroup\$ Simplest 4.2v cut-off is to buy a battery with protection circuit build-in. There is no problem to connect solar panel directly to it. Just add a series diode to batt. to not discharge overnight. \$\endgroup\$ Oct 1 at 20:55
  • 2
    \$\begingroup\$ Built-in protection features are mostly absolute maximum and will destroy battery if frequently reached, using diodes is way to crude of a solution@MichalPodmanický \$\endgroup\$
    – asim
    Oct 1 at 22:03
  • \$\begingroup\$ A LDO of 4.2V and a diode would work, but would not be optimal. When the sun is not so strong could be that the panel produces less than 4V, then the battery will not complete the charge. You should do some calculations about capacity and time. Maybe it will be ok anyway. \$\endgroup\$
    – Gos
    Oct 2 at 20:50

1 Answer 1


Simple ideas:

  1. Zener diode. Problem is low voltage Zener diodes are terrible with wide voltage vs current

  2. Add an ldo regulator set to 4.2V. When the cell is less than 4.2V it will be in pass-through mode. NCP718 may work. Solar->regulator->diode->cell. That way quiescent current from the regulator doesn’t discharge the cell.

Of course best is to get a real chip or board.


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