Hello,
I have a solar panel of 5v and 500mA and a lipo battery of 3.7v and 800mAh.
What do I need to charge the lipo battery in the day and in the same time power the esp32 using the solar panel,
and then at night power the esp32 using the battery?

 

Presumably the solar panel specification is 5V at 500mA for it's maximum power. I believe you can charge a lipo battery at 1C which means up to 800mA so the solar panel voltage will drop if the battery is connected across it. You could use the esp32 to switch the charging on and off through a transistor. Periodically, turn off the charging and check the battery voltage - if it's fully charged, don't turn the charging on again.

 

Welcome to AAC.

Ideally, you would use an MPPT charge controller module, like this one. MPPT is Maximum Power Point Tracking and is needed for PV (PhotoVoltaic) cells (solar cells) because the voltage and current from a PV cell are highly interdependent and the amount of power (Watts) produced will have a specific combination of the two.

So you can’t just connect to the cell if you want to get the maximum power from it and the MPPT controller adjusts the parameters for maximum power instead of voltage or amperage.

You also need to be sure you have a protection board on the LiPo to prevent overcurrent, overdischarge, or overcharge. This is a critical safety device. Many LiPos will come to you with the board already in place. It will be visible if it is there, often under seem Kapton (polyimide) tape, often an amber color.

 

Ideally, you would use an MPPT charge controller module, like this one. MPPT is Maximum Power Point Tracking and is needed for PV (PhotoVoltaic) cells (solar cells) because the voltage and current from a PV cell are highly interdependent and the amount of power (Watts) produced will have a specific combination of the two.

I do think this a bit over complicated for the solar panel and battery mAhr described. Fair enough if squeezing the maximum power out of the solar panel is a priority, or the solar panel can supply more current than the battery can stand, but if the solar panel is rated to give 500mA at 5V in ideal conditions then I believe it would be safe to connect it directly across the battery with a simple transistor switch. If the maximum power is 2,500mW and the battery voltage is 3.0 volts, the current will be way less than 1C = 800mA. It depends on the power required by the overall system.

A part of the question we didn't answer is how to use the solar panel to power the processor during daylight. The ESP32 has an on-board DC/DC converter which generates its 3.3V from 5V into Vin. If the solar panel is connected to Vin which can accept up to 7.0V there's maybe a danger that the open circuit voltage of the solar panel is too high. Again, it depends on the current drawn by the application. Typically, the maximum power point is around 2/3 of the open circuit voltage which the ESP32 DC/DC converter should cope with 7.5V - I read somewhere it can cope with 15V but I wouldn't recommend it.

The fully charged battery should have a fully charged output voltage of 4.2V which is higher than the 3.3V input of the processor which can tolerate up to 3.6V so a diode should be connected between the battery and the processor for night time operation.

 

That rating would be at the MPP, not into an arbitrary load. The module costs 75 cents US. The ESP32 costs more and seems a lot more complicated. I don't see the need to guess at MPP when the module will do it automatically.

 

A LiPO battery is not fully charged when its charging voltage reaches 4.20V. It will continue charging at 4.20V until its chrging current drops to a low amount. Then the charger must be disconnected from the charger.