Hi Ian,
it has backing, it was designed that way
and it's as simple as 1-2-3:
1. buy 2nd battery
2. make sure both batteries have the same state of charge—e.g. charge one after the other inside the radio
3. install and connect both batteries
The charger is actually just an external power supply, the charging circuit proper is inside the radio.
So yes, you use the same adapter, it just takes about twice as long to charge, because of the double capacity.
Best,
Eddi
Born to fly ...
forced to work.