Dual Battery System Calculator

A dual battery system keeps auxiliary power (for fridges, lights, winches, and other accessories) separate from your vehicle's starter battery so you're never stranded with a dead engine. Select your battery type, battery capacity, number of auxiliary batteries, and system voltage, then enter your total load power and desired runtime to get your total usable capacity. Secondary outputs include estimated runtime, current draw, recommended isolator rating, and minimum wire gauge for safe wiring.

Ah
W

Total power consumption of all devices

hours

Below 0°F or above 85°F

Results

Total Usable Capacity

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Estimated Runtime

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Current Draw

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Recommended Isolator Rating

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Minimum Wire Gauge

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Frequently Asked Questions

What's the difference between AGM and lithium batteries for dual battery systems?

AGM batteries are more affordable and reliable but can only be discharged to 50% capacity. Lithium batteries cost more but can be discharged to 100%, offering twice the usable capacity and longer lifespan.

How do I calculate the power consumption of my devices?

Find the power rating on your device's data plate in watts. If only amps are listed, multiply amps by 12V to get watts (e.g., 4A × 12V = 48W). Add up all devices for total consumption.

What size isolator do I need for my dual battery system?

The isolator should be rated at least 25% higher than your alternator's maximum output current. Most vehicles need a 100-140A isolator, but check your alternator specifications.

Can I mix different battery types in my dual battery setup?

No, never mix different battery types (flooded, AGM, gel, lithium) in the same system. They have different charging requirements and will cause premature failure.

How does temperature affect my dual battery system performance?

Extreme temperatures reduce battery capacity and efficiency. Cold weather below 0°F can reduce capacity by 20-30%, while heat above 85°F shortens battery life significantly.

What wire gauge should I use for my dual battery installation?

Wire gauge depends on current draw and distance. Higher current and longer distances require thicker wire. Use 4 AWG for most installations, 2 AWG for high-current applications over 100A.

How long will my dual battery system run my devices?

Runtime depends on battery capacity, number of batteries, and load power. The calculator accounts for safe discharge limits: 50% for lead-acid batteries, 100% for lithium.

Do I need shore power charging for my dual battery system?

Shore power charging is recommended for extended camping or when the vehicle alternator can't fully charge the batteries. It ensures batteries are maintained at optimal charge levels.