Some battery labels show reserve capacity (RC) in minutes instead of amp-hours. RC measures how many minutes a fully charged 12V battery can deliver 25 amps before dropping below 10.5V at 80F (27C). This calculator converts RC to approximate amp-hours so you can compare batteries that use different rating systems.
What Reserve Capacity Actually Measures
Reserve capacity was designed for a specific automotive scenario: your alternator fails while driving, and the battery must power the headlights, ignition system, and fuel pump (roughly 25A combined) long enough for you to pull over safely.
The RC test is standardised by BCI (Battery Council International). A fully charged 12V battery at 80F is discharged at a constant 25 amps until the voltage drops to 10.5V. The elapsed time in minutes is the RC rating.
The conversion to amp-hours is approximate because RC tests at a specific rate (25A) while Ah ratings use a 20-hour rate (a much slower discharge). Due to the Peukert effect in lead-acid batteries, the RC-derived Ah is typically 10-15% lower than the rated Ah. The formula RC x 25 / 60 gives a close estimate: 120 RC minutes = roughly 50Ah.
Reserve Capacity to Amp-Hours Conversion Chart
| RC (minutes) | Approx. Ah | Wh at 12V | Typical Battery Group |
|---|---|---|---|
| 60 min | 25 Ah | 300 Wh | Small car (Group 26R) |
| 90 min | 37.5 Ah | 450 Wh | Compact car (Group 35) |
| 120 min | 50 Ah | 600 Wh | Midsize sedan (Group 24) |
| 150 min | 62.5 Ah | 750 Wh | SUV/Light truck (Group 27) |
| 180 min | 75 Ah | 900 Wh | Large truck (Group 31) |
| 210 min | 87.5 Ah | 1,050 Wh | Heavy-duty (Group 4D) |
| 240 min | 100 Ah | 1,200 Wh | Marine deep cycle |
These figures use the formula Ah = RC x 25 / 60. Actual Ah at the 20-hour rate may be 10-15% higher because the slower test rate is more favourable to lead-acid chemistry.
Frequently Asked Questions
Need to size a starting battery by CCA instead? The <a href="/battery/cold-cranking-amps-calculator">cold cranking amps calculator</a> handles that by engine size and climate.
Reserve capacity is a useful spec for automotive batteries but a poor metric for deep cycle applications. Convert to amp-hours using the formula above, then use the Ah figure for all your sizing and runtime calculations.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.