Better yet is to use diodes so that both batteries will always be "on"
in parallel and you're always pulling from the best battery with no
fiddling required from the pilot. Relatively low voltage drop diodes
are available with 18 Amp forward capacity. For redundancy, I used two
in parallel on each battery.

If it is always drawing from the best battery, what is the time
interval between switching.
What controls the switching. Please advise.
Udo

There is no switching. As the higher voltage battery drops in value,
gradually a greater % will start drawing from the other. From that point,
they will discharge equally. Assuming there is slightly different capacity,
current will gradually increase from the battery with the greater capacity.

Most diodes have about a .6 volt drop, which may be significant. Therefore,
you will need diodes with a low threshold. Schottky diodes should be about
..3 volts drop. You can measure the actual drop using a digital
volt-ohmmeter. There is a diode test range and it will show the voltage
drop.

Colin Lamb