But what do you suggest for the extra 12v battery?
I would be tempted to use a 12V 18Ah Deep Cycle Sealed Lead Acid battery, as fitted to various types of mobility scooters and powered wheelchairs etc.
These batteries can be used in any position and usually recover well from a deep discharge.
Nominal Capacity
20-hr. (900mA to 10.50 volts) 18.0 AH
10-hr. (1.7A to 10.50 volts) 17.0 AH
5-hr. (3.2A to 10.20 volts) 16.0 AH
1-hr. (11.1A to 9.00 volts) 11.1 AH
15-min. (34.3A to 9.00 volts) 8.58 AHInternal Resistance (approx.) 14 milliohms
Max Discharge Current (7 Min.) 54.0 amperes
Max Short-Duration Discharge Current (10 Sec.) 180.0 amperes
(Length = 181mm, Width = 76mm, Height = 167mm and Weight = 12.6lbs/5.72kg)
I bought one of these jump starter/tyre inflators recently from a local discount store for £23 (approx 35usd).
Cheaper than buying the 12v 20Ah battery on its own and it came with an air compressor, tripple LED light, a slow 500mA charger, voltmeter, switches, charging socket and leads etc.
Now that's what I call good value!
If it can supply enough Amps to start a car, I'm sure it would work fine as an extra boost battery.
Alan