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.
![](http://www.arhservices.pwp.blueyonder.co.uk/12v18a.gif)
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!
![](http://www.arhservices.pwp.blueyonder.co.uk/JS1.gif)
If it can supply enough Amps to start a car, I'm sure it would work fine as an extra boost battery.
Alan