Circuit: solar panel + battery to power LED light

Thread Starter

Rasťo Masaryk

Joined Jan 27, 2016
Hi, I have built this simple circuit The LED light (tiny 5-7V light) in this circuit should always be on. It is powered by solar panel and battery. During day the solar panel alone can light the led and during night it is powered from battery. Is my solar panel able to charge the battery and not overcharge it? Do I need to use protection circuit? Battery -> two big 3.7V 8000mAh Li-Ion batteries connected in series. I do not need to charge the battery to top capacity from the panel. I am assuming that the panel won't be able to charge the battery above 6V (3V each battery) (this is ok for me). Do you think that it is ok to leave it as it is or is there any danger that batteries will be damaged or will explode?



Joined Apr 24, 2011
LI-ion batteries should not ever never be overcharged, so they are the wrong choice for this application. Ni cads work very well this way, you will find those in every garden light you open.

7.4 volts is way too much voltage for the led, it will kill it in an instant.

A 6 volt panel will not charge a 7.4 volt battery.

Let's start at the led: what is it, what voltage and current does I need?


Joined Aug 7, 2008
The open circuit V of the panel should be 9 to 12 V, & short circuit current should be 3 to 4 times LED current.
LED should have a current limiting resistor in series.
There are IC's available to monitor Li ion batteries.

Thread Starter

Rasťo Masaryk

Joined Jan 27, 2016
Hi thx for help :) The circuit is already built and seems to be working. The LED light is shining for about 2 weeks now, but I am not sure if the batteries are charging. As I said, batteries are quite big 3.7V cells and there is practically no load on them, just the light. They alone should be able to power the light for a few months. I assumed that keeping the batteries charged to 3V should do the trick like they would still be able to power the led for a few days and I wouldn't have to worry about overcharging.

Btw led light I used


Joined Sep 9, 2010
Especially true when you consider the drop across the blocking diode. The SOC of the battery might not ever get above 25%.