Marine House Battery Load and Alternator Recharge

The consumption side is straightforward bookkeeping: sum each load's amps times its hours, divide by the usable depth of discharge, and the bank size follows. The usable fraction is the part that is easy to get wrong, because it differs by chemistry -- about half for flooded lead acid, considerably more for lithium iron phosphate -- and a bank sized on nameplate amp-hours rather than usable ones is half the bank it looks like. The charging side is where the surprises are. A rated alternator does not put its rated output into a lead acid bank for the whole recharge: it does so during bulk, and then the battery's own acceptance limits the current as it approaches full. Returning a flooded bank from half to about eighty-five percent might take an hour, and getting it from there to a genuine hundred can take several more -- so the nameplate hours figure is the optimistic end of a range, and the realistic figure at an entered average acceptance is reported beside it. The consequence is a habit rather than an event. Cruising boats run the engine for the bulk portion, see the ammeter fall off, shut down, and leave the bank in a partial state of charge day after day -- and chronic partial state of charge is what kills flooded banks. It is a consequence of the acceptance curve rather than of anything the owner is doing wrong, which is why the arithmetic is worth seeing rather than the advice. Lithium changes it entirely: it accepts near full current to a high state of charge, so the recharge is short and the bulk figure is nearly right -- and it requires alternator temperature protection precisely because it will accept everything the alternator can make, for as long as the alternator can make it. This does not model the acceptance curve itself, which depends on chemistry, age, temperature and the charge profile; it does not size the alternator, its belt or its regulator, evaluate solar or wind contribution, or address the wire sizing that the charging current needs. ABYC E-11, the battery manufacturer's charge acceptance data, and the alternator manufacturer govern.

usable capacity = bank amp-hours x the usable depth of discharge; autonomy = usable / daily consumption; the bulk recharge time = consumption / the alternator's rating, and the realistic time = consumption / (rating x the average acceptance).

The house bank balance as marine electrical practice writes it. The acceptance fraction is ENTERED because it depends on chemistry, age, temperature and the charge profile. It does not model the acceptance curve, size the alternator, its belt or its regulator, evaluate solar or wind contribution, or size the charging wire.

A capacity, a division, and one entered acceptance fraction.

Estimate. AHJ and licensed professional govern.

Field names used by the API: daily_consumption_ah, bank_ah, usable_dod, alternator_a, acceptance_fraction, bulk_target_soc_pct, usable_ah, autonomy_days, bulk_hours, realistic_hours, bank_for_one_day_ah

• Acceptance is entered it depends on chemistry, age, temperature and the charge profilethe battery manufacturer's charge acceptance data
• Loads are entered the consumption side is bookkeeping this does not do for youthe boat's own load list
• Lithium changes the arithmetic it accepts near full current to a high state of charge and needs alternator temperature protectionABYC E-11