How hard would it be to charge 2 batteries successively ?

[X-Ryl669]

Let's say you have a primary battery to charge first and when it's charged, switch to charging a second battery. Typical motorhome or RV scenario.

Is that possible ?

I guess 2 mosfet will be required to select which battery to charge, but that would consume 2 more GPIO (or maybe, use a I2C I/O expander on the UEXT interface, that board containing the 2 mosfet and the large trace width).

[martinjaeger]

Yes, with extra hardware as you described it should be possible to switch between two batteries. However, you have to make sure that you maintain power supply to the MCU during switching, so you can probably only do it while there is solar power available.

There are 8 GPIOs in the UEXT connector, so if you don't need I2C, SPI and UART at the same time, you should be able to use two of them instead of an I2C I/O expander.

I know there are some charge controllers out there which can charge a second battery with lower priority and less current. But I don't know exactly how they realize the current limitation to the second battery. It could be possible to charge the second battery via the load output, limiting the current by a series resistor. However, this would probably be quite wasteful.

[X-Ryl669]

I guess maintaining the power supply is not really an issue since the goal is to top the 2 batteries' charge. So if there's no solar power, the main idea of switching is kind of useless. If the PS fails, then upon solar resumption it'll restart on the first battery. Maybe it'll require more IO to measure the second battery voltage (so it does not switch if it's not present).

I don't think it'll work if we try to PWM the load output to limit the current on it (or it'll require another H-Bridge and an inductor + sense resistor and so on).