hello guys,

I'm trying charge a lithium battery with a solarpanel. This is the solarpanel that i'm currently using(12V;50mA:

The is the mppt solar charger that I use:

But the problem is that when I connect the solarpanel to the mppt, the voltage of the solarpanel decreses to 4 V. When the solarpanel isn't connected is produces 9,6V. Because of the 4 V input the mppt won't work. It only works with a minimal voltage of 9 V. My question is why does the voltage decrease so much?

The mppt is not broken I first tried it with a power supply unit and it works with an input of 9V and 0,005 mA. This gives an ouput of 4,2V and 0,04A.

I hope you guys can help me

with kind regards,


by Bram12
November 20, 2018

1 Answer

Answer by mikerogerswsm

This is the datasheet:

It looks like the series combination of R3 and R4 in Figure 1 is too small in your application. It is permissible to increase them in the same proportion, say to two or three times the existing values. If this doesn't work, increase or remove R1. The CN3791 takes 1mA into the Vcc pin.

+1 vote
by mikerogerswsm
November 20, 2018

PS - First check the output current of the solar panel. Are you getting a suitable current out of it? Is it in direct sunlight as requited in the spec?

If you don't know how to measure the current ask your teacher.

by mikerogerswsm
November 21, 2018

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