Marine Battery Bank Calculator

Sizing a Marine Battery Bank

A marine battery bank must supply enough energy to run all onboard electrical loads for the desired number of days without recharging — your autonomy period.

Formula: Required Ah = (Daily Ah × Reserve Days) / Depth of Discharge

Where:

• Daily Ah = sum of all electrical loads × hours used per day
• Reserve Days = how many days without shore power or charging
• Depth of Discharge (DoD) = maximum safe discharge % (typically 50% for lead-acid, 80% for lithium)

Estimating daily amp-hours: Add up each device: (Watts / Voltage) × Hours per day = Ah per day. Common loads: VHF radio = 2-3A, chartplotter = 2-5A, LED cabin lights = 1-3A, refrigerator = 4-8A, bilge pump (auto) = 0.5A average.

Depth of discharge guidelines:

• Flooded lead-acid: Do not discharge below 50% (use DoD = 50%)
• AGM / sealed lead-acid: DoD = 50–60%
• Lithium (LiFePO4): DoD = 80–90% safely
• Discharging deeper shortens battery lifespan significantly

Battery types:

• Flooded lead-acid: Cheapest, requires maintenance and ventilation
• AGM: Maintenance-free, spill-proof, good for most boats
• Gel: Similar to AGM, sensitive to overcharging
• Lithium (LiFePO4): Lightest, longest life (2000+ cycles), highest upfront cost

Rule of thumb: For a typical coastal cruiser, plan for 100–150 Ah of usable capacity per day. Offshore or liveaboard sailors often need 200–400 Ah per day.

Wiring configuration: To increase capacity: connect batteries in parallel (same voltage, capacity adds). To increase voltage: connect in series (capacity stays the same, voltage adds). Most 12V systems use parallel connections to add capacity.