Alternator on a boat generating 90Amps for three batteries ACR system

You could do this, but you should leave one battery connected directly to the alternator so it functions correctly, then use a split-charge system for the other two batteries.

Companies who supply winches do split-charge systems and so do caravan builders.

The risk of overcharging a weak 2V cell increases with age and total % of imbalanced ESR and Capacitance times current output. Therefore 50mV / 100A wire current sense using “Kelvin method” can be used to determine the imbalance charge.

If the batteries are connected prior to alternator for say 10 minutes then risk of imbalanced over charge is significantly reduced if batteries are OK. If a small lead acid “expired” battery is attached to an oversized Alternator , chances are that only 1 cell will outgas from excessive voltage (>2.5V) and it won’t accept a charge for long nor supply discharge very long. The ideal situation is that all cells are perfectly balanced much less than 1% difference in V and specific gravity. But once the weakest cell starts to differ, the aging from overvoltage accelerated faster. Thus it is best to have separate current limiters and cell balances to slow down the aging imbalance exponential effect but can be alternatively reduced currents as per above ( connect batteries until current sense drops then charge and monitor results)

Or get a smart ESR/Ah Battery charger that displays results with charger and load dump test.