Battery Reserve Capacity To Amp Hours Calculator

What Is Battery Reserve Capacity (RC)?

Reserve Capacity tells you how long a battery can deliver 25 amps before its voltage drops to 10.5 volts.

• It is measured in minutes
• It assumes a steady 25-amp load
• It is mainly used for automotive and marine batteries

Example:
If a battery has an RC of 120 minutes, it can supply 25 amps for about 2 hours before it is considered discharged.

RC is useful when you want to know how long a battery can keep essential systems running if the charging system fails.

What Are Amp Hours (Ah)?

Amp Hours measure total energy storage in a battery.

• It shows how many amps a battery can supply for one hour
• It is commonly used for deep-cycle, RV, solar, and backup batteries

Example:
A 50 Ah battery can deliver:

• 50 amps for 1 hour
• 25 amps for 2 hours
• 5 amps for 10 hours

Amp hours are easier to work with when planning device runtimes.

Why Convert Reserve Capacity to Amp Hours?

Manufacturers do not always list both values. Many car batteries list RC but not Ah. Many deep-cycle batteries list Ah but not RC.

Converting between the two helps when:

• Comparing different battery brands
• Sizing batteries for RVs or boats
• Estimating how long devices will run
• Planning backup or off-grid power

A calculator saves time and avoids math errors.

The Basic Conversion Formula

The calculator uses standard industry formulas.

RC to Ah

Amp Hours = (Reserve Capacity × 25) ÷ 60

Example:
RC = 90 minutes
Ah = (90 × 25) ÷ 60 = 37.5 Ah

Ah to RC

Reserve Capacity = (Amp Hours × 60) ÷ 25

Example:
Ah = 40
RC = (40 × 60) ÷ 25 = 96 minutes

These are baseline values before adjustments.

Why Adjustments Matter

Real batteries do not behave the same in all conditions. This calculator improves accuracy by applying three key adjustment factors.

Battery Type Adjustment

Different battery chemistries deliver power differently.

• Flooded lead-acid: baseline performance
• AGM: usually more efficient
• Gel: slightly lower output
• EFB: improved flooded design
• Lithium-ion: highest usable capacity

The calculator applies a multiplier based on the selected battery type.

Temperature Adjustment

Cold batteries lose capacity. Hot batteries behave differently but may suffer long-term damage.

For example:

• 80°F is considered standard
• 32°F can reduce usable capacity by about 25%
• 0°F can cut capacity in half

Selecting temperature helps the calculator reflect real-world performance.

Discharge Rate Adjustment

Batteries deliver less usable energy when discharged quickly.

• 20-hour rate is standard
• 10-hour or 5-hour rates reduce capacity
• 1-hour discharge drastically lowers usable Ah

This follows Peukert’s law, even though the calculator keeps it simple and practical.

What the Calculator Actually Shows You

This calculator does more than convert numbers.

1. Standard and Adjusted Values

• Standard RC or Ah from the formula
• Adjusted RC or Ah after all factors

2. Runtime at Common Loads

• 10 amp draw
• 5 amp draw
• 1 amp draw

3. Device Runtime Estimates

• LED lights (0.5A)
• Car stereo (5A)
• Laptop (3A)
• Portable fridge (4A)

4. Battery Size Interpretation

It explains whether your battery is:

• Small
• Medium
• Large
• Very large

This helps non-technical users understand what the numbers actually mean.

Example Use Case

You have a battery rated at:

• 100 minutes RC
• AGM type
• 60°F temperature
• 10-hour discharge rate

The calculator:

1. Converts RC to Ah
2. Adjusts for AGM efficiency
3. Reduces capacity for colder temperature
4. Accounts for faster discharge

The result gives a realistic estimate of how long your devices will actually run.

When This Calculator Is Most Useful

This tool is ideal for:

• Car and truck owners
• RV and camper setups
• Marine battery planning
• Solar and backup systems
• DIY power projects

It turns confusing specs into usable information.

Key Takeaways

• Reserve Capacity measures time at 25 amps
• Amp Hours measure total energy
• Simple formulas give estimates
• Battery type, temperature, and discharge rate matter
• A calculator provides faster and more realistic results

If you rely on batteries for anything important, converting RC to Ah correctly can save you money, prevent power loss, and help you choose the right battery the first time.