Kapag battery,- theoretically,- unlimited power....

Ang rating ng battery refers to capacity - measured in Ampere-Hours.... So, a 40 A-Hr battery can theoretically deliver, limited by its material/physical construction: 1A for 40 hours or 20A in 2 hours, etc

And during its normal operation,- it is the alternator which is delivering the power, regulated/supported by the battery

Bro- I do not see a need for a backup... It's like having a backup gasoline tank,- when the OEM is more than enough...

Afraid of not being able to start a vehicle? Jumper cables or replace the battery when its warranty period expires for peace of mind...