Solar Panel as power supply for multiple borehole pumps

I think that a very good option is to take advantage of the PIC uC that you already have in your design and its analog (ADC) input ports for monitoring the batteries voltage and to drive the switching between those batteries using MOSFETs instead of BJT transistors. Using the uC you should not need the two comparators that you show in the picture and you will have a more intelligent charging/monitoring battery circuit that could lead to improve the batteries duration. This uC could stay in sleep mode (or low power mode as second option) all the time using interrupts to read the ADC channels. You should run the uC from its internal RC oscillator operating at the lowest frequency as possible to decrease the power consumption.

I also recommend you using Schottky diodes instead of 1N4148 and to put a small bulk capacitor at the output of the power selector circuit (or switching circuit). Those Schottky diodes will drop less voltage than the 4148.

If you decide to use the uC for the battery voltage level monitoring, try to use the internal fixed voltage thresholds that the uC provides for the internal comparators, because if you don't do that you could have some problems reading wrong analog values because those batteries are used to power the uC too (you will have ratiometric voltages).

I hope this help.

These things are the dumbest solar charge circuits possible. See a tear down here https://youtu.be/OSR2ofEAJKg

Solar cell to reverse protection diode to 6x series AA NiCD battery pack. No charge controller. It depends on the battery pack getting trickled charged, and being stronger than the solar panel.

Connecting the same panel to two lights will cut the current to both, and likely tripling the charge time (it won't be exactly half). The built in diodes on the input board will prevent one battery from charging the other, just as they prevent the battery from back feeding the solar panel in normal use. No modification or precautions needed.

These lights will already deep discharge the batteries over night as is, which is why NiCD are chosen. Their runtime will be reduced due to less power charging them.

FYI you can buy similar panels from overseas online for a few bucks. Probably from the same place HF gets theirs.