Other power sources than PV

For commercial PWM and MPPT charge regulators (and inverters) we find most time a maximum PV short circuit current in datasheet. I do not know, if this value is only given for the case of MPPT reverse polarity protection.

I understand, all PWM designs relies on high intrinsic resistance of its source (like PV panels have). This is the only thing what limits the current to a battery with smaller voltage during Mosfet on state.

In comparison, all MPPT circuits use inductors to delay the current until Mosfet swiitch off. I have a 48V PV battery system and like to support this in case of foggy days. Instead of PV I wish to use my 80V forklift PzS battery to charge the 48V system. Assume this is ideal battery with unlimited current, there is no sense to find a maximum power matching for mppt. Is there any chance this sources works on the MPPT designs ? Simply using this as non isolated buck regulator.

If there is any accidental delay in software by switching the mosfet gates, this could destroy the hardware what does not occur if higher impedance PV power sources are in use. Any experience with this or other commercial MPPT?

Charging from battery to battery will most probably not work properly with most MPPT charge controller hardware. In order to do that you need a very fine-grained PWM duty cycle control. As you said already, batteries behave almost like voltage sources (unlimited current), so a very small decrease in the terminal voltage of the battery will lead to a large increase in its output current.

Within the Libre Solar MPPT controllers, only this prototype hardware would be able to properly control the charging between two batteries if the STM32G474 is used. Its current-sensor is fast enough to provide cycle-by-cycle current limitation and the HRTIM provides very high-resolution PWM control.