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