Auxiliary Battery Sizing Calculator

An auxiliary battery is a secondary power source used in campervans, boats, and off-grid setups to run devices independently from your vehicle's starter battery. Enter up to 3 devices with their power ratings and daily hours of use, then configure your battery capacity, number of batteries, battery type, and depth of discharge to get the Recommended Battery Capacity. Secondary outputs include daily power consumption, daily amp hours required, current usable capacity, maximum runtime, and overall system status.

W

Check device data plate or multiply Amps × 12V

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hrs
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hrs
Ah

Per battery capacity in Amp Hours

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Max 50% for lead acid, up to 100% for lithium

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Additional capacity buffer for unexpected usage

Results

Recommended Battery Capacity

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Daily Power Consumption

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Daily Amp Hours Required

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Current Usable Capacity

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Maximum Runtime

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System Status

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Results Table

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Frequently Asked Questions

What is an auxiliary battery system?

An auxiliary battery system is a secondary electrical system separate from your vehicle's starting battery. It powers camping equipment, lights, fridges, and other accessories without draining your main battery, ensuring you can still start your vehicle.

How do I find the power consumption of my devices?

Check the device's data plate or manual for wattage. If only amperage is listed, multiply the amps by 12V to get watts (e.g., 4A × 12V = 48W). Many modern devices also list power consumption in their specifications.

What's the difference between lead acid and lithium batteries for auxiliary systems?

Lead acid batteries are cheaper but heavier, with a maximum 50% depth of discharge. Lithium batteries cost more upfront but are lighter, last longer, and can be discharged up to 100%, providing more usable capacity.

Why is depth of discharge important?

Depth of discharge (DoD) determines how much of your battery's capacity you can safely use. Exceeding recommended DoD levels significantly reduces battery lifespan. Lead acid batteries should only be discharged to 50%, while lithium can go to 100%.

Should I add a safety margin to my battery capacity?

Yes, adding a 20-30% safety margin is recommended to account for battery aging, temperature effects, unexpected usage, and system inefficiencies. This ensures your system remains reliable over time.

Can I mix different battery types or sizes?

No, never mix different battery types (lead acid with lithium) or different capacities in the same system. This can cause uneven charging and discharging, potentially damaging your batteries and creating safety hazards.

How long will my auxiliary battery system last?

Runtime depends on your daily power consumption and usable battery capacity. The calculator shows your estimated runtime in days based on your inputs. Factors like temperature, battery age, and inverter efficiency can affect actual runtime.

What size auxiliary battery do I need for weekend camping?

For weekend camping, calculate your daily consumption and multiply by the number of days, then add a safety margin. A typical setup with LED lights, water pump, and phone charging might need 50-100Ah for a weekend trip.