Another approach to the selecting a source is to avoid microcontrollers altogether and just rely on non-programmable logic. This is inspired a bit by the Saturday Clock, or rather, my thinking of how it could be done without an MCU.
In selecting a source, we really only care about one thing: is the battery voltage high enough? If no, we need to hunt for one that is.
This question can be answered by a simple analogue comparator such as the LM311, a shift register, and a few other logic gates.
Here, we have such a circuit. Up the top, is our shift register, set up as a ring counter. The buffers there are stand-ins for diodes, if any of them is a 1, the output is a 1 and the NOT gate on the input outputs a 0.
The outputs of the shift register are used to select a battery, which has its own comparator and select logic. The comparator is represented here by the D flip-flop at the extreme left: in essence I’m using this as a switch, Logisim doesn’t provide one, only a momentary button. We need a signal that is high when the battery is above acceptable voltage. We also need its inverse.
The select line from the shift register controls the gate on two tri-state buffers, allowing us to inhibit the comparator’s output. The buffered “good” signal is used to SET the “enable” D-flip-flop that drives the switch turning the battery on. This same (buffered “good”) signal also passes thorough a diode-OR arrangement that indicates whether a source is “available”.
To emulate make-before-break, inverted “select” signal and the “source available” signal pass through an AND gate and into the RESET of the “enable” flip flop, so it gets turned off when another source is turned on.
Finally, the buffered “bad” signal from all modules is fed back on one shared line, inhibiting the clock until a battery drops below the minimum level.
A glitch here is if multiple batteries are initially turned on with none above the minimum voltage, this will cause multiple sources to be selected. This is not too hard to manage in software, and the solution might in fact be to implement this on an ATTiny24A as mentioned in the previous post; this logic circuit can be implemented quite easily in C, with comparators in hardware or using the ADC as a software comparator as I’m doing in the charge controller.