Bigger banks are generally better because:
1. you'll be using less of their capacity
than you would with a smaller bank, therefore increasing the available amp-hours and extending their life;
2. you'll be able to replenish them faster
IF you have the charging capacity onboard, thereby reducing costs associated with charging (engine time and maintenance
costs, etc.); and
3. you'll have considerable reserve capacity
in case you want to or need to go longer between charges.
Note that AGMs can absorb a very large charging current
, often exceeding their amp-hour rating. A 400AH AGM
bank could easily absorb a 400-amp charging current
or more. If you're not careful, this could place an unsustainable load on your alternator and burn it out. To avoid this, you need to somehow: (1) limit the output current; and/or (2) reduce alternator heating
. There are several strategies to do this, both mechanical (e.g., regulating pulley size) and electrical
(e.g., using smart regulators like the Balmar
MC-612 to de-rate the alternator, or using external rectifiers).
With an AGM
bank the size you contemplate and a 70-amp alternator, I think I'd want to find a way to de-rate it by about 20% in order to avoid overheating
and burnout. Temperature sensors on the alternator and batteries would be highly desirable as well.
If you were designing the system from new, you'd want a much larger alternator...perhaps 4 to 5 times the capacity.
Bottom line: the 70A alternator will do, but charging times will be longer than would be ideal, especially since you'll want to derate the alternator somewhat to avoid burnout.