I have a 18V solar panel which is connected to a step-down buck converter, which drops the voltage from 18V to 3.5V. When clouds block the sun, the voltage drops to less than 3.1V. I have added a diode on the positive terminal to make sure that the battery doesn't give back any current, but I was wondering if there is any risk in having such a fluctuation. Li-ion is sensible to trickle-voltage, and I am worried that this kind of of fluctuations will destroy the battery. Is this fear motivated by reality?

Right now, I'm only connecting the wires to the battery when it is full sun, because I have a constant voltage more or less. Can I keep this connection all the time or do I need other components to keep the battery safe?

  • \$\begingroup\$ Li-ion requires more than just a constant electric potential (constant voltage). At lower charge levels and other charging states, a constant current is commonly used. \$\endgroup\$ Commented Mar 5, 2018 at 15:15
  • \$\begingroup\$ Yes, but the current is related to the voltage and resistance. The resistance is the same, but the voltage will be different. Therefore different current, right? And that worries me, because different voltage mean different current, ergo no CC charging regime. Or did I understand this one wrong? \$\endgroup\$
    – Physther
    Commented Mar 5, 2018 at 15:19
  • \$\begingroup\$ A battery is not a resistor. Until it's got to the state of charge where you can use CV charging you need to use constant current charging. Applying too large a voltage in that time could result in using far too much current. \$\endgroup\$
    – Finbarr
    Commented Mar 5, 2018 at 15:27
  • 2
    \$\begingroup\$ If you don't REALLY understand how to charge lithium batteries, either learn (good resources on ti.com) or don't mess with them. Alternately, place your whole assembly in a concrete fireproof bunker and stay out when it's operational. \$\endgroup\$
    – John D
    Commented Mar 5, 2018 at 15:47
  • 1
    \$\begingroup\$ @mkeith good advice, but the OP states there is a diode in series with the supply, so presumably he/she is trying to charge at around 2.8 (Si diode) or 3.2 (Schottky) volts, which is unlikely to be satisfactory. Far better to buck or buck-boost to a higher voltage and use a dedicated Li battery charger or Li MPPT charger to manage the charge profiles. \$\endgroup\$
    – John D
    Commented Mar 5, 2018 at 20:10

1 Answer 1


Instead of what you are using just use a propper board. It will charge better and the risk is going to be smaller.

Search on eBay or Aliexpress for CN3791. The board is less than 4$ and does eveything properly.

If you are curious to learn about it, read it's datasheet.


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