Battery Reserve Capacity to Amp Hours Calculator

The Battery Reserve Capacity to Amp Hours Calculator is a handy tool that helps you estimate how many ampere-hours (Ah) a battery can provide over an extended period, based on its reserve capacity rating.

Battery Reserve Capacity to Amp Hours Calculator Convert reserve capacity in minutes to amp-hours delivered during the 25-amp test. Use the custom option only for a discharge duration measured at another constant current.

Calculation type Standard reserve capacity (25 A) Custom constant-current test Use the RC minutes printed on the battery or its datasheet. The test current is fixed at 25 A (amps), not your device's current draw.

Reserve capacity (minutes) Enter the rated test duration. Decimals and zero are accepted.

Duration unit Minutes Hours Seconds Changing units converts the entered duration. It does not run the calculation.

Example Presets Fill an example, then select Calculate.

100 min RC at 25 A 60 min RC at 25 A 120 min RC at 25 A 180 min RC at 25 A 240 min RC at 25 A Custom: 100 min at 5 A

Calculate Reset

Formula, worked examples and limits

The short formula Ah = minutes × amps ÷ 60 Here, 60 means minutes per hour: dividing the time in minutes by 60 converts it to hours. A means amperes (amps), the unit of current. Ah means amp-hours, a quantity of electric charge, not power. 100 minutes at 25 amps 100 × 25 ÷ 60 = 41.666... Ah ≈ 41.67 Ah That is about 41.7 Ah to one decimal place. A separate test lasting 100 minutes at 5 amps would deliver 8.33 Ah. The current must match the duration's test conditions. Why the longer formula also works Ah = (minutes × 60 × amps) ÷ 3,600 In this longer version, the first 60 means seconds per minute, and 3,600 means seconds per hour. Multiplying by 60 and then dividing by 3,600 is the same as dividing by 60 in the short formula. What reserve capacity means The usual automotive RC rating is a test at 25 A and about 27°C (80°F), ending at 1.75 V per cell, or 10.5 V for a 12 V lead-acid battery. Interstate Batteries: RC and amp-hour definitions. Not the same as a 20-hour capacity rating This calculation finds the charge delivered during the stated test. It does not determine a battery's rated capacity at another discharge rate. Lead-acid capacity depends on current, and a 20-hour (C20) rating uses different test conditions. Victron Energy: capacity and discharge rate. Use the manufacturer's discharge data for equipment runtime. This tool does not model temperature, ageing, voltage changes, an inverter or other losses, and the RC test endpoint is not a recommended everyday discharge target. Follow the battery and equipment manufacturers' limits.

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What Is the Battery Reserve Capacity to Amp Hours Calculator?

This calculator converts a battery’s reserve-capacity test duration into the electric charge delivered during that test, in amp-hours (Ah). For a standard 25-amp reserve-capacity rating, enter the RC time from the battery label or datasheet. You do not need your appliance’s current draw.

The usual automotive reserve-capacity test measures how long a fully charged battery can supply 25 amps at about 27°C (80°F), down to 10.5 volts for a 12-volt lead-acid battery. These are test conditions, not a recommendation to discharge a battery that deeply in everyday use. See Midtronics’ explanation of reserve capacity.

How to Calculate Amp Hours from Reserve Capacity

or the standard automotive reserve-capacity test, use a current of 25 amps. Use the duration measured at that current, not the current drawn by a different device.

Amp-hours (Ah) = reserve capacity in minutes × 25 ÷ 60

Dividing the time in minutes by 60 minutes per hour converts it to hours. Multiplying those hours by 25 amps gives the amp-hours delivered during the test.

The same formula using seconds

The longer version of the formula gives the same result:

Ah = (BR × 60 × 25) ÷ 3,600

Here, BR is the battery reserve capacity in minutes, 60 is the number of seconds in a minute, 25 is the test current in amps, and 3,600 is the number of seconds in an hour.

This version first converts minutes to seconds, then divides the amp-seconds by 3,600 to obtain amp-hours. Both formulas are equivalent; the role of 60 depends on which version is being used.

Worked example: 100 minutes at 25 amps

A battery with a standard reserve-capacity rating of 100 minutes delivers 25 amps for that duration under the specified test conditions.

Ah = 100 × 25 ÷ 60 = 41.67 Ah

Using the longer formula:

Ah = (100 × 60 × 25) ÷ 3,600 = 41.67 Ah

The charge delivered during this test is therefore approximately 41.7 Ah.

Separate example: 100 minutes at 5 amps

For a separate constant-current test that actually runs for 100 minutes at 5 amps, the calculation is:

Ah = 100 × 5 ÷ 60 = 8.33 Ah

This is a custom test, not the standard 25-amp reserve-capacity test. Do not take a duration measured at 25 amps and reuse it at 5 amps to calculate the same battery’s capacity at that different discharge rate.

What the result means: this calculation gives the electrical charge delivered during the stated test. It does not automatically give the battery’s 20-hour Ah rating or its runtime with another load. Delivered capacity depends on discharge rate; use the manufacturer’s ratings or discharge data for the intended conditions.

References: Interstate Batteries: reserve capacity and amp-hours; NIST: minute and hour definitions; Victron Energy: discharge rate and battery capacity.

Why Is Reserve Capacity Important for Batteries?

RC gives a comparison under a defined load. It is not a promise of how long a particular appliance, vehicle accessory or inverter will run. Use the battery manufacturer’s discharge curves and the equipment’s requirements for runtime planning. This calculator does not account for temperature, ageing, voltage changes, usable discharge limits or conversion losses.

What Are Amp Hours and How Are They Used to Measure Battery Capacity?

A means amperes (amps), a unit of current. Ah means amp-hours, a quantity of electric charge: constant current in amps multiplied by time in hours. For example, a measured 5-amp discharge lasting two hours delivers 10 Ah. Ah is not power in watts or energy in watt-hours. See Interstate Batteries’ amp-hour definition.

A battery’s delivered capacity can depend on discharge rate. A manufacturer’s 20-hour (C20) Ah rating is therefore not automatically equal to RC × 25 ÷ 60. That calculation gives Ah at the RC test rate, not an independently determined C20 rating. Victron Energy explains how discharge rate affects battery capacity.

To use the calculator, you’ll need to know both your battery’s reserve capacity rating and current draw from whatever system or device it powers. The calculator then takes these values and estimates how many amp-hours your battery can provide before needing recharge.

Here’s an example of how to use the formula:

Suppose you have a battery with a reserve capacity of 100 minutes and a current draw of 5 amps.

To calculate the amp hours of this battery, you would use the formula:

AH = (BR x 60 x 25) / 3,600 AH = (100 x 60 x 25) / 3,600 AH = 4.17 Ah

Therefore, this battery can provide 4.17 amp hours of power before needing to be recharged, assuming a constant current draw of 5 amps.