A true emergency battery should be as far above the waterline as
possible. While an isolated battery such as you describe would be
perfectly acceptable, I think there is a lot to be said in simplicity
for a dry cell set. Anything you charge may vent, overcharge, self
discharge, sulfate, etc. Dry cells replaced at proper intervals would
be very reliable and give you more flexibility to locate them above
the radio so you can be transmitting up to the last minute.

I don't know myself which way I will go but it's food for thought.

--

Roger Long