Automotive
Battery reserve capacity & runtime calculator
A battery's reserve-capacity rating comes from a standardized 25-amp test — not from your specific accessories. Enter the battery's RC minutes and the load in amps or watts to see a simple runtime estimate, with an adjustable planning fraction. The calculation is a rough comparison, not a safe-discharge target or a guarantee your vehicle will start afterward; temperature, battery health, charge level, and electrical conditions all matter.
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Your estimate
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Watts are divided by nominal system voltage when you enter power.
RC minutes × 25 A ÷ 60; not interchangeable with a manufacturer’s 20-hour amp-hour rating.
A rough constant-load estimate based on the standard RC test; real runtime may differ substantially.
The math, plainly
How it works
- 01
Reserve capacity is the minutes a fully charged battery sustains a standardized 25-amp discharge at 80°F (26.7°C) until 10.5 volts. The calculator turns that test result into a simple amp-hour equivalent: RC minutes × 25 ÷ 60.
- 02
For a device specified in watts, current is estimated as watts ÷ nominal system voltage. Runtime then scales the RC equivalent to your load and the capacity fraction you select.
- 03
This is only a constant-load planning estimate — not a battery discharge test, a promise of real-world runtime, or a guarantee the vehicle will restart. Real battery behavior varies with chemistry, age, temperature, charge, and wiring.
Try a real scenario
120-minute RC battery, 5-amp accessory
Inputs120 min RC, 5 A load, 50% planning fraction
ResultSimple RC equivalent: 50 Ah. Estimated runtime at the selected fraction: about 5 hours. This is not a safe-discharge recommendation or a starting guarantee.
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