Planning estimate only—not a safe remaining-starting-time prediction.
Test-point Ah is not necessarily C20 Ah. Nominal Wh is an energy proxy. No result guarantees remaining starting ability.
Calculations run locally. Battery and load inputs are not uploaded, stored, or included in analytics events.
These are charges delivered at 25 A, not necessarily C20 Ah ratings. Nominal Wh = test-point Ah × 12 V, not measured energy over a discharge.
| RC (minutes) | Ah at 25 A | Nominal 12 V Wh |
|---|---|---|
| 60 | 25 | 300 |
| 75 | 31.25 | 375 |
| 90 | 37.5 | 450 |
| 100 | 41⅔ | 500 |
| 120 | 50 | 600 |
| 150 | 62.5 | 750 |
| 180 | 75 | 900 |
| 200 | 83⅔ | 1000 |
| 240 | 100 | 1200 |
The BCI reserve-capacity definition uses a full battery, 25 A and 80°F (27°C), with a 1.75 V per-cell endpoint (10.5 V for six cells). See Interstate’s BCI glossary. The automotive battery test-method standard is SAE J537_202309: Storage Batteries; its full procedure requires access to the standard.
The first conversion is charge = current × time, not a prediction of a different discharge test. ODYSSEY’s rating explanations distinguish reserve capacity from the 20-hour Ah test.
For runtime, let R be RC minutes; s the current charge fraction; a the fraction of present charge permitted for accessories; and c the condition/temperature capacity fraction. The accessory budget is f = s × a × c. For example, 80% charge × 20% accessory use × 90% retained capacity = 14.4% of the rated basis. Holding back 80% of present charge is not the same as an 80% state-of-charge floor.
I is battery current while on; d is the on-time fraction; k is the Peukert exponent. Simple mode uses k = 1, d = 1 and 12 V with no conversion losses. The advanced model is anchored to the same 25 A RC point. Victron’s technical explanation describes rate-dependent capacity and cautions that Peukert scaling is approximate.
η is inverter efficiency and ℓ is wiring power-loss fraction. Battery-side amp inputs already include these losses, so they are not applied again. Duty-cycle modeling assumes no draw while off; include idle loads separately. Constant-watt devices draw more current as voltage falls, which this nominal-voltage model does not simulate.
The advanced range evaluates both entered exponent bounds and sorts the outcomes; below 25 A, the larger exponent can produce the longer runtime. It is a sensitivity range, not a confidence interval or guaranteed minimum. Charge and condition scaling are planning approximations, not an electrochemical state-of-charge model. Use manufacturer discharge curves at the required cutoff for important sizing decisions.
Input: 90 RC minutes, one 12 V battery.
Formula: 90 × 25 ÷ 60 = 37.5 Ah; 37.5 × 12 = 450 nominal Wh.
Meaning: 37.5 Ah delivered in the 25 A RC test, not a C20 rating.
Input: 120 RC minutes, one 12 V battery.
Formula: 120 × 25 ÷ 60 = 50 Ah; 50 × 12 = 600 nominal Wh.
Meaning: two hours at 25 A under the RC test conditions, down to the test cutoff.
Input: 50 Ah measured at 25 A.
Formula: 50 × 60 ÷ 25 = 120 RC-equivalent minutes.
Meaning: the inverse test-point conversion. If 50 Ah is a C20 label, 120 minutes is only a linear equivalent, not verified RC.
Inputs: 120 RC, 10 A continuous, 100% charge, 20% accessory use, 100% condition; Simple model.
Formula: 120 × 25 ÷ 10 = 300 minutes at the theoretical endpoint; 300 × 0.20 = 60 minutes accessory budget.
Meaning: this budget holds back 80% of present charge in the model. The 10 A load is far from 25 A, and a restart is not guaranteed.
Inputs: 120 RC, 12 V, 120 W, 85% inverter efficiency, 5% wiring loss, 100% duty, k = 1.10–1.30, full charge/condition and 20% accessory use.
Formula: I = 120 ÷ (12 × 0.85 × 0.95) ≈ 12.38 A. t = 120 × (25 ÷ 12.38)k × 0.20 ≈ 52–60 minutes.
Meaning: the full-charge endpoint is about 260–299 minutes. This is a sensitivity interval for the assumed exponents; idle consumption and a higher inverter cutoff can shorten it.
| Rating | Measurement and conditions | Use |
|---|---|---|
| RC | Minutes at 25 A and 80°F (27°C), to 10.5 V for a 12 V battery. | Sustained reserve under one test. |
| Ah / C20 | Charge at a specified rate; C20 uses a 20-hour discharge. Check the maker’s temperature and cutoff. | Capacity comparisons at the same test rate. |
| CCA | Current for 30 seconds at 0°F (−18°C), maintaining at least 7.2 V for a 12 V lead-acid battery. | Cold engine starting. |
| CA | Current for 30 seconds at 32°F (0°C), with the same 7.2 V threshold. | Starting at a warmer test temperature; not interchangeable with CCA. |
| Wh | Energy delivered over time; measured Wh depends on changing voltage. Nominal V × Ah is only a proxy. | Energy comparison with matched conditions. |
RC, CCA and CA conditions: Interstate / BCI definitions. C20 explanation: ODYSSEY Battery FAQs. There is no reliable universal CCA-to-RC or CCA-to-Ah conversion.
Simple conversions are for one 12 V automotive battery. Matched batteries in series add voltage; their Ah at the same string current does not add. Parallel strings retain voltage and add Ah at matched per-string conditions. A parallel bank’s RC at a total 25 A cannot be inferred by simply multiplying one battery’s RC because each battery sees a different current. Changing the voltage field alone does not establish a bank RC rating. Advanced bank estimates require a documented 25 A bank rating with the correct per-cell cutoff.
RC is the minutes a fully charged battery supplies 25 A at 80°F (27°C), down to 10.5 V for a 12 V battery. It is a controlled test rating, not a safe starting-reserve endpoint.
100 RC delivers 41⅔ Ah and 120 RC delivers 50 Ah at the 25 A test rate: RC × 25 ÷ 60. These are not necessarily the manufacturer’s C20 Ah ratings.
No. It means 120 minutes at the specified 25 A test point. Linear scaling gives 300 minutes at 10 A before planning adjustments; discharge-rate effects, charge level, temperature and equipment cutoff can change the actual runtime.
No universal formula reliably converts CCA to RC or Ah. CCA measures short-duration cold starting current; RC and Ah describe sustained discharge under specified conditions. Use the battery’s data sheet.
For many automotive lead-acid batteries, the 20-hour test uses much less than 25 A. Lower discharge current often yields more measured Ah. The difference depends on the battery and test conditions; there is no fixed correction factor.
Use the vehicle manufacturer’s specified battery type, fit, CCA and minimum RC. Among otherwise compatible batteries, higher RC means longer discharge at the same 25 A test point. There is no universal good RC value for every vehicle.
Ah × 60 ÷ 25 gives a linear equivalent only. The reverse conversion here assumes Ah measured at 25 A. A C20 rating requires manufacturer discharge data or a validated rate model to estimate actual RC.
No. Even the Preserve starting reserve preset is only a planning budget. Battery condition, temperature, engine requirements and voltage under load determine starting ability. Follow the battery and vehicle guidance and monitor the battery.
Methodology revised: .