You're better off with a separate battery in the boot to run your sounds. It needs to be a leisure battery though as designed for caravans/camper etc as these can handle a deep discharge unlike a normal car battery. You can get a 110AH leisure battery for about £40 from most car spares shops. This battery could supply 110amps for one hour. An ICE setup is never going to draw it's maximum power for long (only during certain peaks in the music etc) so as a really rough guess I reckon one of those batteries would run your system for a good four hours. It should really be hooked up to a split charger so it recharges when the engine is running.
I don't think a 240VAC - 12VDC supply is really gonna be suitable for running your sounds unless it is REALLY hefty. It would probably need to be capable of supplying over 100A in short bursts and this isn't the sort of thing you're gonna find cheaply in maplins.
Go with a leisure battery to get yourself out of this hole. Simply wire all your amps etc to it in the boot.
Spot on bud!
Get yourself a split charging relay box from ebay (Mine was £18 I think) which only charges the spare battery when your starter battery is fully charged. It also isolates the leisure battery so if it goes flat your ICE won't draw anything from your starter battery.
Nick
