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Chargers whether it's the alternator or inverter/charger look at the voltage at the battery and put out its charge based on that. IOW if the battery is at 12.5 volts it will assume it needs a full charge. But if battery is at 13.8 because of one charger working first, the other charger will assume it is almost fully charged and should cut back the amps and voltage. So I guess it depends on what source starts charging first. At least that's what my feeble mind thinks.