Convert battery reserve capacity to amp-hours or amp-hours to reserve minutes using the 25-amp test basis, with an optional C20 lead-acid estimate.
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Battery Reserve Capacity to Amp Hours Formula
The base conversion from reserve capacity to amp hours at the 25-amp discharge rate is:
This factor (0.4167) comes from RC x 25 amps / 60 minutes per hour. Since BCI defines reserve capacity as the number of minutes a battery sustains a 25-amp load before dropping below 10.5 volts, multiplying RC by 25 gives amp-minutes, and dividing by 60 converts to amp-hours.
The direct conversion measures charge delivered during the RC test; no efficiency multiplier is needed. The optional C20 mode uses a rough lead-acid comparison described by Discover Battery:
For example, 120 RC minutes equals 50 Ah delivered at 25 A, while the rough C20 estimate is 64.8–69.6 Ah. Manufacturer ratings at matching current, temperature and cutoff take precedence; the empirical range is not a guaranteed error bound.
| RC (min) | RC (hours) | Amp-Hours (Ah) |
|---|---|---|
| 30 | 0.50 | 12.50 |
| 45 | 0.75 | 18.75 |
| 60 | 1.00 | 25.00 |
| 75 | 1.25 | 31.25 |
| 80 | 1.33 | 33.33 |
| 90 | 1.50 | 37.50 |
| 100 | 1.67 | 41.67 |
| 110 | 1.83 | 45.83 |
| 120 | 2.00 | 50.00 |
| 150 | 2.50 | 62.50 |
| 180 | 3.00 | 75.00 |
| 200 | 3.33 | 83.33 |
| 240 | 4.00 | 100.00 |
| 300 | 5.00 | 125.00 |
| 360 | 6.00 | 150.00 |
| Formula: Ah = RC (min) × 25 / 60. Values rounded to two decimals; this is not the C20 rating. | ||
What Is Battery Reserve Capacity?
Reserve capacity is the number of minutes a new, fully charged battery at 80°F (about 27°C) can supply 25 A before reaching 1.75 V per cell, or 10.5 V for a nominal 12 V lead-acid battery. See the Interstate Batteries glossary. Compare manufacturer test conditions before using ratings from different products.
Why Reserve Capacity and Amp Hours Are Not Interchangeable
RC gives duration at a specified 25 A load. An amp-hour rating gives charge at its stated discharge rate; a C20 rating uses a 20-hour test. For example, 100 Ah at C20 corresponds to 5 A for 20 hours under its specified conditions. Lead-acid capacity generally decreases at higher currents, so the 25 A test charge and the C20 label can differ.
The arithmetic RC × 25 / 60 is exact for charge delivered during the stated constant-current test, apart from input and rounding precision. Inferring a C20 rating from RC is approximate because the test rate changes. Use the rating matching the intended load and manufacturer discharge curves for system sizing.
The Peukert Effect on Battery Capacity
Peukert’s law approximates the effect of discharge rate: t = H × (C / (I × H))^k, with time t and rating period H in hours, capacity C in Ah and current I in A. The exponent k should come from battery-specific data, as explained in the Victron manual. The following is a mathematical illustration, not chemistry-specific test data.
| Assumed exponent k | Modeled duration (h) | Modeled charge (Ah) |
|---|---|---|
| 1 | 4.0000 | 100.0000 |
| 1.1 | 3.4054 | 85.1340 |
| 1.2 | 2.8991 | 72.4780 |
| 1.3 | 2.4681 | 61.7034 |
| Hypothetical inputs: H = 20 h, C = 100 Ah, I = 25 A. Charge = 25 × modeled hours; rounded values. | ||
At k = 1 this model has no rate-dependent charge loss. Higher k shortens the modeled duration at currents above the rated current. Chemistry alone does not specify a reliable exponent or percentage loss; temperature, cutoff, condition and model also matter. This calculator’s C20 comparison uses the stated empirical factors rather than fitting a Peukert model.
Battery Group Size and Conversion Examples
Group size describes a battery’s physical fit and terminal arrangement, not a unique RC, C20 or CCA rating. Check the exact manufacturer model and vehicle requirements. The table below gives calculated examples of the two conversion bases; it is not a group-size specification chart.
| RC (min) | 25 A test charge (Ah) | Rough C20 lower (Ah) | Rough C20 upper (Ah) |
|---|---|---|---|
| 60 | 25.00 | 32.40 | 34.80 |
| 90 | 37.50 | 48.60 | 52.20 |
| 120 | 50.00 | 64.80 | 69.60 |
| 150 | 62.50 | 81.00 | 87.00 |
| 180 | 75.00 | 97.20 | 104.40 |
| 200 | 83.33 | 108.00 | 116.00 |
| 240 | 100.00 | 129.60 | 139.20 |
| 300 | 125.00 | 162.00 | 174.00 |
| C20 comparison uses RC × 0.54–0.58. Prefer the exact model’s manufacturer ratings; values are not BCI specifications. | |||
Lithium vs Lead-Acid: Reserve Capacity Comparison
Lead-acid ratings are generally more sensitive to discharge rate than lithium ratings, but two products with the same nominal Ah need not have the same RC. Compare published discharge curves, cutoff, temperature and battery-management limits. A lithium battery’s protection may disconnect the load before a lead-acid RC-test voltage endpoint is reached.
Recommended depth of discharge for cycle life is a separate design choice from the RC test endpoint. Do not turn an RC test rating into a safe cycling limit, or assume a universal weight, capacity advantage or replacement equivalence. Follow the manufacturer’s operating limits and charging requirements.
