On Wednesday, April 8, 2020 at 12:55:49 AM UTC-4, kinsell wrote:

The really funny thing is, most people reading this have no need at all
to ever switch batteries. Instead of two small batteries, one big one
is so much easier to manage. If they have to be broken into multiple
units, then just wire them in parallel and let them all provide power
until depleted. If you switch them, then you risk switching too soon
and wasting capacity in the first one, or switching too late and ruining
a flight log or messing up a flight computer when you key the mike and
don't realize how weak the battery was.

-Dave

Funny that nobody responded to this comment from Dave. I think he's right on. Other than motorgliders that need an engine-starting battery separate from the avionics, why do we need a 2-battery setup? I've flown with a single battery for 25 years now, and have NEVER had a problem with that. Had plenty of other glitches in flight recorders etc, but not the battery's fault. Having two batteries (perhaps one in the tail) with separate wires to the panel can add capacity and redundancy even if they are simply paralleled within the panel.

Can one battery go bad (shorted cell) and load down the other one? Theoretically yes. Not likely, if you test your batteries once a season and stop using any that show a decline in capacity. But you can have a separate on/off switch for each battery, and normally have both turned on for the whole flight. If you have a voltmeter in the panel (some radios have it built-in) you can turn off one battery at a time just to check the condition of the other one.