Here's one for ya. I put a second, 210RC, 125A hour, deep cycle battery in my van to power the 800W amp. My original battery is a Die Hard Range Handler 700CCA, 875CA. I haven't upgraded the alternator yet. The system and the whole car seems to be running and starting a lot better than it was, so far. I think the alternator is 130A. Do you have any tips or suggestions? I heard something needing a battery isolator device, but I don't know much about them.
Unless you have 2 identical sized battery, it's always a good idea to isolate them. That way you won't left stranded with dead batteries. You may think the problem's gone till the batteries need charging. Since you have 2, it'll take that much longer to recharge. Do a google search on isolators. Some are designed better than others. Don't go cheap.
As long as you don't have heater/air conditioner/spot lights or anything else that eats up lot of current on at the same time. You'll know when it can't, the lights will dim badly or the engine rev will sound odd.