Join Date: Apr 2006
Thanked 181 Times in 178 Posts
Rep Power: 13
"WHat is the limiting factor in what current a given alternator can put out?"
The carrying capacity of the output wire or fuse.
But in conventional installations you don't have to worry about that, you no doubt have an automobile type alternator with integral regulator and they are designed to work as a unit. You can't burn it out UNLESS you bypass or replace the output lead, output fuse, or fusible link, whatever protects it.
And in common use, if it has to put out too much it will overheat and throttle back on its own, unless you've got a real cheap old regulator design in there.
Group31 batteries fall within the realm of "automotive" and a single one won't be a problem. Two of them probably won't be a problem (in parallel) unless you've run them down too far. If you want some fun, run both down about 80%. Start the engine on one, then throw the switch to "both" (assuming you've got alternator protection so you can do that) and watch the engine shudder and the fan belt jump like a snake from the extra load on the alternator.
Even with group 24's.
Mixing one wet battery and one AGM is generally a bad idea, using two of the same kind is all you really need to worry about in your situation. The rest matters--but isn't critical. Plenty of web sites about batteries, charging, mixing chemistries, when your head stops hurting.