How to charge two 18650 cells in series with a 12 V solar panel for an ESP32 and NPK sensor project?

Question

I'm a beginner and I'm trying to figure out how to charge two 18650 cells in series using a solar panel with a 12 V output. My goal is to power a circuit for an NPK sensor that operates at a voltage range of 5-30 V, along with an ESP32.

Currently, my circuit is already functioning with the two 18650 cells in series. To power the ESP32, I use a DC-DC step-down converter to reduce the battery voltage to 5 V.

Which components do I need for this setup? Specifically, I'm looking for information on:

1. What type of charge controller is suitable for charging two 18650 cells in series with a 12 V solar panel?
2. Any additional protection circuits or components I might need to ensure safe and efficient charging.
3. How to properly connect the solar panel, charge controller, and batteries together.

Any detailed explanation or pointers to relevant resources would be greatly appreciated.

Answer 1

I use a step-down circuit direct connected to solar panel to stablize voltage at 5v, and use a 2s charge board to charge for 2 18650 batteries. The charging board also has overcharged and overdischarged protection, and you can pay more for balancing mode.

Answer 2

You must use two devices:

1. A protector BMS
2. A charger

The protector BMS protects each cell by cutting off the battery current before any cell is operated unsafely.

The charger limits the charging current and the charging voltage.

Right now you have neither. That is a fire danger.

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But, really, my best advice for the sake of your safety is to abandon this approach and instead buy a ready-made solar-charged 5 V power bank. Safe, properly enclosed, meets all regulations, and has a warranty.