6KW will be PMPO that'll roughly equate to 3KW rms or 1500 W ACTUAL POWER CONSUMPTION.
so your ICE will be wanting 105 AMPS @ 14.4 volts the car will need 30-50 AMPS
my sugestion would be to go and buy a battery from a diesel version of your car as it'll proably fit in the std battery location but it extra capcity and power delivering ablity will help keep voltages more stable, and then get a bigger alternator idealy somethign in the 150A+ range
multipul batterys won't help if your alternator isn't big enough because all the power has to come from the alternator at the end of the day. also that split charging system thats linked to is NASTY it looks like it just isolates 1 battery from the other when the car isn't runniing so you can flatten the battery your useing to run stuff and still start the car but when the car is started and that battery is completly flat its gping to try and pull onw hell of a current trying to charge itself back up, at best thats really bad for the batterys service life and it's not exactly safe, uncontrolled charging can result in batterys getting very hot and going bang
