Plan the night from the loads
Fridge, lights, electronics, and maybe an inverter are a known number of amp-hours if you are honest. BU-105’s RC and Ah ratings size the lead bank. Wikipedia’s depth of discharge is how far you empty it. Dawn should still leave a starting battery that was not invited to the party.
Refill in the morning
Engine charging, solar, or a run to a pedestal has to put those amp-hours back. A charger on the wrong chemistry never finishes (BU-403). Lithium house with a lead float never gets a proper rest (BU-409b). Overnight design without a refill is a one-night trick.
Voltage sag is not always empty
An inverter or a windlass can pull a bank down in voltage while amp-hours remain. Cables and the inverter cutoff matter. We look at the run when the complaint is “it died at 2 a.m.” and the charger filled it at the dock the next day.
Anchor is not a slip
Slip boats in Cape Coral can hide a dead charger behind “we were on shore power.” At anchor there is no pedestal. The bank and the engine or solar are the whole story. Those are different quotes.
What we ask
What you run after dark, how you refill, and whether start and house are split. Pursuit and Sea Ray overnight boats are this layout. We size from that, not from a romance about anchoring.
Start battery still has to crank at dawn
The house bank can be empty-ish at first light if you planned that depth of discharge. The starting battery should not be. A switch left on BOTH overnight makes that split a story instead of a design. We check the switch position in the quote the same way we check amp-hours. If you invert a coffee maker at 5 a.m., say so — that load belongs in the house math, not on the starter.
Sources
- Battery University BU-105 — Battery Definitions (RC at 25 A)
- Wikipedia — Depth of discharge
- Battery University BU-403 — Charging Lead Acid
- Battery University BU-409b — Charging Lithium Iron Phosphate
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.