Battery Reserve Capacity Calculator
Convert RC to amp-hours, find runtime at specific loads, or calculate battery size needed. Essential for RVs, boats, solar, and backup power.
What do you want to calculate?
RC rating from battery label (minutes at 25A)
Battery capacity in amp-hours
Current draw in amps
System voltage
How long you need battery to last
Cold cranking amps
How deeply you can discharge (50% for most batteries)
What is reserve capacity on a battery?
Reserve capacity (RC) is the number of minutes a fully charged battery can deliver 25 amps at 80°F before dropping to 10.5 volts. A 120 RC battery can run a 25-amp load for 2 hours. This is more realistic than cold cranking amps (CCA) for deep cycle and accessory use.
How do I convert reserve capacity to amp-hours?
The relationship between RC and Ah is approximate. For smaller batteries (RC < 80): Ah ≈ RC / 2. For larger batteries: Ah ≈ RC × 0.025 + 2. The formula accounts for the non-linear discharge curve. A 100 RC battery is roughly 50-55 Ah, while a 150 RC battery is about 75-80 Ah.
How long will a battery last at a given load?
Runtime ≈ (RC / 25A) × (rated RC / actual load). At 25A, runtime equals RC. At 50A, runtime is roughly half. At 12.5A, runtime is roughly double. Always apply a depth-of-discharge factor (50% for lead acid, 80% for lithium) for battery longevity.
What size battery do I need for my application?
Calculate watt-hours needed: Watts × hours. Divide by voltage for amp-hours. Apply depth-of-discharge: lead acid batteries should not be discharged below 50%. A 100W device running 4 hours needs 400Wh = 33Ah at 12V. With 50% DoD, you need a 66Ah battery minimum.