Battery Reserve Capacity to Amp Hours Calculator

| Added in Physics

Understanding Battery Reserve Capacity and Amp-Hours

Battery reserve capacity and amp-hours are two different ways to measure battery performance and capacity. Understanding the relationship between them helps you compare batteries and determine which specification is most relevant for your application.

The Conversion Formula

The relationship between reserve capacity and amp-hours is based on the standardized testing conditions:

$$\text{Amp-Hours} = \frac{\text{Reserve Capacity (min)} \times 60 \times 25}{3600}$$

This simplifies to:

$$\text{Amp-Hours} = \text{Reserve Capacity} \times 0.41666$$

For the reverse conversion:

$$\text{Reserve Capacity (min)} = \frac{\text{Amp-Hours} \times 3600}{60 \times 25}$$

Which simplifies to:

$$\text{Reserve Capacity} = \text{Amp-Hours} \times 2.4$$

Example Calculation

If a battery has a reserve capacity of 90 minutes:

  1. Multiply by the conversion factor: 90 × 0.41666
  2. Result: 37.5 Ah

Conversely, if a battery is rated at 37.5 amp-hours:

  1. Multiply by 2.4: 37.5 × 2.4
  2. Result: 90 minutes reserve capacity

Why Both Ratings Matter

Reserve Capacity is particularly useful for:

  • Automotive applications
  • Emergency backup scenarios
  • Understanding how long a battery can sustain critical systems

Amp-Hours is better for:

  • Comparing total energy storage
  • Calculating runtime for specific loads
  • Deep-cycle battery applications like RVs and boats

Both measurements provide valuable information, and this calculator helps you translate between them for easier comparison and planning.

Frequently Asked Questions

Reserve capacity (RC) is the number of minutes a fully charged battery at 80°F can deliver 25 amps before falling below 10.5 volts. It measures how long a battery can power essential systems if the alternator fails.

Amp-Hours = Reserve Capacity (min) × 60 × 25 / 3600, which simplifies to RC × 0.41666. The formula assumes a constant 25-amp discharge rate per the reserve capacity test standard.

Reserve capacity focuses on emergency backup time at a specific discharge rate, while amp-hours measure total energy storage capacity. Both ratings help consumers understand battery performance in different scenarios.

The conversion is a standardized approximation based on the 25-amp discharge rate used in RC testing. Actual performance may vary slightly depending on battery chemistry, temperature, and discharge rate.