Battery Replacement Calculator (12V)

Sizing a replacement battery correctly means accounting for your load, runtime, and the real-world factors that reduce usable capacity. Enter your device power, desired runtime, system voltage, and battery type into the Battery Replacement Calculator, then set your max depth of discharge and inverter efficiency to get the required battery capacity in Ah. Secondary outputs include total energy needed, energy after efficiency loss, and a recommended battery rating adjusted for temperature and age.

W

Power consumption of your device in watts

hours

How long you want to run your load

%

Safe discharge level (50% for lead-acid, 80% for lithium)

%

Use 100% for DC loads, 85-95% for AC through inverter

Check if battery operates below 32°F or above 85°F

Older batteries have reduced capacity

Results

Required Battery Capacity

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Total Energy Needed

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Energy After Efficiency Loss

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Recommended Battery Rating

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Frequently Asked Questions

What's the difference between AGM, Gel, and Flooded batteries?

AGM batteries are maintenance-free and handle vibration well, making them ideal for automotive use. Gel batteries work better in extreme temperatures but are more expensive. Flooded batteries are the most affordable but require regular maintenance and proper ventilation.

How deep should I discharge my 12V battery?

For lead-acid batteries (flooded, AGM, gel), limit discharge to 50% to maximize lifespan. Lithium batteries can safely discharge to 80-90% without significant degradation. Deeper discharges reduce battery life significantly.

Why do I need to consider inverter efficiency?

If you're powering AC devices through an inverter, energy is lost in the DC to AC conversion process. Most inverters are 85-95% efficient, so you need a larger battery to compensate for this loss. DC loads don't require this adjustment.

How does temperature affect battery capacity?

Cold temperatures reduce available capacity by 10-20%, while excessive heat accelerates aging and can reduce lifespan. If your battery operates in extreme conditions, you'll need additional capacity to maintain performance.

When should I replace my car battery?

Most car batteries last 3-5 years. Replace when it struggles to start the engine, shows corrosion, has a swollen case, or fails a load test. Regular testing after 3 years helps prevent unexpected failures.

How do I calculate amp-hours needed for my device?

Divide your device's wattage by the system voltage to get current draw in amps, then multiply by runtime hours. For example: 60W ÷ 12V = 5A, so 5A × 8 hours = 40Ah needed (before safety margins).

Can I use a larger capacity battery than calculated?

Yes, using a larger battery is always safe and beneficial. It provides longer runtime, reduces discharge depth (extending battery life), and offers reserve capacity for unexpected needs. Never use a smaller battery than calculated.