⏱️ Reserve Capacity (RC) Calculator¶
Estimate how many minutes a battery can support a load, and get an SAE-style reference reserve capacity at a 25A load.
Conservative estimate: Minutes = (Ah × usable depth) ÷ load (A) × 60
:material-help-circle: Frequently Asked Questions¶
What is battery Reserve Capacity?¶
Reserve Capacity is the number of minutes a fully charged 12V battery can deliver 25 amps at about 27°C before terminal voltage drops below 10.5V. It is commonly measured to SAE J537.
How do I estimate RC minutes from Ah?¶
A common rule of thumb for flooded SLI batteries is RC minutes ≈ Ah × 1.5. Actual reserve capacity varies with plate design, temperature, and discharge rate, so this calculator gives a planning estimate rather than a guaranteed test result.
Does battery type affect usable runtime?¶
Yes. AGM tolerates deeper discharge (about 80% usable depth in repeated cycling), while a standard SLI battery should be kept near 50% to protect its service life. Deep-cycle batteries are built for deeper cycling.
What happens if my load is higher than 25A?¶
Runtime falls faster than a simple linear estimate because high discharge rates reduce effective capacity (the Peukert effect). The calculator still returns a conservative planning estimate, but the datasheet should be checked for heavy loads.
Are these results exact?¶
No. Results are honest planning estimates based on rated capacity and typical usable depth. Actual runtime depends on temperature, battery age, state of health, and load profile.
Related resources¶
- Reserve capacity testing — the RC test explained
- C20 capacity testing — how stored energy is measured
- Battery model database — reference RC values