A properly sized marine battery bank is the foundation of a reliable electrical system on any boat. Unlike a house that can draw from the grid, a boat at anchor has no external power source. If the battery bank cannot support the electrical loads between charging cycles, the crew loses lighting, navigation, autopilot, refrigeration, and communication. An undersized bank suffers deeper discharge cycles, which dramatically shortens battery life.
Marine battery sizing follows a systematic process: inventory all DC loads and their daily run times to calculate total daily amp-hour demand, apply a depth-of-discharge limit appropriate for the battery chemistry, add a safety margin for aging and temperature effects, and then select a battery type and configuration that meets the calculated capacity while fitting the available space and weight budget.
Electrical Load Analysis
Create a table listing each DC load, its current draw in amps, and expected daily run time in hours. Include navigation lights, cabin lights, instruments, autopilot, refrigeration, water pumps, bilge pumps, entertainment, and accessories. Record amp draw from equipment nameplates and estimate daily run time based on your cruising pattern.
Some loads run continuously, some intermittently. A refrigeration compressor drawing 4 amps with a 40% duty cycle averages 1.6 amps over 24 hours, consuming 38.4 Ah/day. An anchor light drawing 0.5 amps for 12 hours consumes 6 Ah/day. Sum all daily amp-hour figures to get total daily consumption.
A typical 35-foot cruising sailboat with LED lighting, basic instruments, autopilot, and small refrigerator might consume 80-120 Ah/day. A similar boat with radar, watermaker, and entertainment system might consume 200-400 Ah/day. The daily consumption figure is the basis for all subsequent sizing calculations.
Marine Battery Bank Sizing Calculator
Size house and starting battery banks for boats. Calculates CCA, Ah capacity with DoD limits, temperature derating, and chemistry comparison.
Battery Chemistry Comparison
Flooded lead-acid (FLA) batteries are inexpensive and widely available but require upright installation, periodic watering, and ventilation for hydrogen gas. They should not regularly discharge below 50% SOC. Quality FLA batteries deliver 800-1,500 cycles to 50% DOD.
AGM (Absorbed Glass Mat) batteries are sealed, maintenance-free, and can be installed in any orientation. They accept charge faster than flooded batteries and are more vibration resistant. They should not regularly discharge below 50% SOC and cost 1.5-2 times more than equivalent FLA batteries.
Lithium iron phosphate (LiFePO4) batteries deliver 80-90% usable capacity (versus 50% for lead-acid), charge 3-5 times faster, weigh one-third as much, and last 3,000-5,000 cycles. A 200 Ah lithium bank provides the same usable energy as a 400 Ah lead-acid bank at half the weight. They cost 3-5 times more and require a battery management system (BMS).
Marine Battery Bank Sizing Calculator
Size house and starting battery banks for boats. Calculates CCA, Ah capacity with DoD limits, temperature derating, and chemistry comparison.
Battery Bank Sizing Calculation
The sizing formula: Required Capacity = (Daily Ah × Days of Autonomy) / (Max DOD × Temperature Factor × Aging Factor). For 120 Ah/day, 2 days autonomy, AGM batteries (50% DOD), moderate temperature (1.0), aging factor 0.85: Required = (120 × 2) / (0.50 × 1.0 × 0.85) = 564 Ah. Round up to three 200 Ah batteries for 600 Ah total.
Days of autonomy depends on cruising style: 1 day for coastal cruisers who run the engine daily, 2-3 days for blue-water sailors. The temperature factor accounts for reduced capacity in cold weather: at 32°F, lead-acid batteries deliver 70-80% of rated capacity. Apply 0.75-0.85 for cold-climate cruising.
The aging factor (0.80-0.85 for lead-acid, 0.90-0.95 for lithium) ensures the bank still meets requirements near end of service life. Do not add more than 10-20% safety factor beyond these calculations, as excessive oversizing wastes money and weight without proportional benefit.
Marine Battery Bank Sizing Calculator
Size house and starting battery banks for boats. Calculates CCA, Ah capacity with DoD limits, temperature derating, and chemistry comparison.
Charging System Requirements
The primary charging source on most boats is the engine alternator. Stock alternators (50-70A) with internal regulators typically deliver only 25-35 amps to the house bank after reaching absorption voltage. At 30 amps, recharging a 600 Ah bank from 50% SOC takes approximately 10 hours. Upgrading to a high-output alternator (100-175A) with an external smart regulator reduces charge time dramatically.
Lead-acid bulk charge rate should not exceed C/5 (120A for 600 Ah). Lithium batteries accept C/2 or higher, limited by alternator capacity. A smart regulator implements proper multi-stage charging (bulk, absorption, float) and is essential for any upgraded alternator.
Solar panels provide silent, free energy. A 400-watt array produces roughly 100-160 Ah/day in tropical latitudes. Wind generators produce power day and night: a 500-watt unit produces 50-120 Ah/day in 15-25 knot sustained winds. Both reduce engine charging time significantly and complement each other well.