Sjouke Burry writes:

Comments? Any one tried it?
I have used a small (~12AH) battery, connected to the main battery
with a Scottky diode(80 amp), or a break relay.
The break relay was connected to the radio power, which is
switched off while starting, and so can feed the relay coil.
Your starting switch might have such a contact.
That way, starting the engine, or using an electric winch, does
not mess up your power.

What is the source of the battery? What is it's size.

I assume it's lead acid so it can tolerate the charging voltage range
of the main system.