Hi, I use esp-wroom-32 to remotely controll via mqtt some sensors and relay. I use 4 parallel plugged 18650 (12000mha, 2,7 discharge voltage, 4,2 charging voltage) they last 1 day even if I put the controller in deep sleep state and wake it every 30 minutes remaining active roughly 10 minutes. I measure the voltage and send via mqtt but there should be some problem since the controller switch off when measured voltage is around 3,9v.
Assuming that the battery are in good shape I would like to add solar charger: I have at hand 12v/5v, 200W panel.
What is the most secure/efficient configuration to charge the battery and provide power to battery with the solar panel?

Thanks for attention and best regards

 

4 parallel plugged 18650

bad idea - connect 'em in series , use step-down

200W solar panel ?