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I am planning on recycling some cheap low voltage solar panels that came out of a gardening light. I have two of these modules, that soldered in series provide a voltage of around 3.5v (on direct light).

I want to be able to charge two (or at least one) 18650 batteries, the problem is that I've seen some comercial solutions but they all require 5v input. I wanted to know if there is a way of achieving what I intend to do.

Thanks

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  • \$\begingroup\$ Li-Ion Cells require a charging voltage of about 4.2V \$\endgroup\$ – Alexander von Wernherr Aug 29 '18 at 12:30
  • \$\begingroup\$ Isn't there a way of boosting up the voltage of the solar cells for instance by using a capacitor? \$\endgroup\$ – Fjallbacka Aug 29 '18 at 12:37
  • \$\begingroup\$ My advice would be to buy a TP4056 module and a third cheap 5v panel. Put the three panels in series on input of TP 4056 module for a 4-6V input (will happily drop to around this level under load). You can charge for maybe 3 hours at a time. I wouldn't recommend charging a Li-Ion battery too much longer than that. \$\endgroup\$ – TestDeviant Aug 29 '18 at 13:07
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The solar garden lights I have charge one AAA 300mAh Ni-MH cell in 10 hours from a 2V Solar panel in series with a 0.3V Schottky diode to prevent the battery from discharging into the solar panel in darkness. Your 18650 Lithium battery needs much more voltage and current to charge. If your lithium cell has a capacity of 3000mAh then a few solar panels like I have in series will charge it in 10 days of sunlight. Sixty of these little solar panels in series/parallel will charge the Lithium battery cell in a couple of hours before it explodes and catches on fire from overcharging. A Lithium cell needs a battery charger IC for safety.

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