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# Charging 2s Li Ion with solar panel

#### I14R10

##### Full Member level 3
I have a small self made wireless weather station that I want to be powered by two 18650 Li Ion batteries. It uses around 10-20mA.
EDIT: I need 3.3V for my circuit. I could use single 18650, but then I would need to use very low dropout linear regulator for when battery is at let's say 3.7. So I'm using 2 in series.

I have a CN3722 based solar charger (MPPT) for 2s batteries. Right now I have the batteries connected in series. I read a lot about how it's not a good solution and that I need balanced charger which CN3722 is not. I'm checking every day the voltages of each battery and so far they are staying close together. My solar panel is 10W 12V panel (about 18-20V open circuit voltage).

So, batteries are charged in series right now. But I'm concerned they might suddenly start charging differently and have different voltages.

Then I started reading about connecting them in parallel which has its own set of problems. They need to be at the exactly same voltage and if not I should use schottky diode. Then they say they shouldn't be charged in parallel together, instead each should have its own charger.

I'd rather not have them in parallel. So I read about BMS for 2 in series. There are boards like this one on Aliexpress https://www.aliexpress.com/item/1005003648953550.html but they are not made for solar panel. Now I don't know what to do.

Maybe I'm blind but I haven't seen solar charge controller (with balancing) for 2s Li Ion batteries. Any advice?

Last edited:

##### Super Moderator
Staff member
uses around 10-20mA.
EDIT: I need 3.3V for my circuit. I could use single 18650, but then I would need to use very low dropout linear regulator for when battery is at let's say 3.7.

You may find you can get by simply installing a low-ohm resistor inline. Say 10 to 15 ohms. At 15mA you drop 0.2 volt.

Thus when the battery is 3.7 V it becomes 3.5 V seen by your device.