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1) How many batteries
2) Battery Types (FLA, AGM, Gel?)
3) Battery capacity (Amp hours)
Ideally, you would want a charging circuit per battery. A charger that can sense and charge each battery individually. Be appropriate for the types of batteries being charged. (i.e if you have AGM batteries, the charger will need to support AGM). The larger the charger, the faster the batteries will recharge. You will also be able to support more DC load and charge even when connected to shore power.