I have an 09 silverado crewcab v8 thats about to go form 3k to 6k so i think its time to upgrade the charging system. DC has so many options im a little lost. i currently have my factory battery and a deka 9a31, going to add another group 31 in the back with the amp. how much alternator do i need with 2 group 31s and a large OEM starting batt?