Is a battery with higher reserve capacity always better?

For starting batteries, higher RC provides a larger safety margin if your alternator fails. For deep cycle use, RC is less relevant because it only measures capacity at one specific discharge rate (25A). A deep cycle battery with 120 RC and 100Ah may outperform one with 150 RC and 80Ah in a slow-discharge application because the Ah rating at the 20-hour rate better represents sustained-use capacity. To estimate the cost per kWh of energy stored in your battery over its lifetime, convert RC to Ah first, then factor in cycle life.

Why do some batteries list both RC and Ah while others only list one?

Automotive (SLI) batteries traditionally use RC because it reflects real-world driving emergencies. Marine and deep cycle batteries more often list Ah because their users care about sustained runtime. Dual-purpose batteries usually list both to appeal to all buyers. If a battery only lists RC, it is probably designed primarily for starting — check whether it is rated for deep cycling before using it for sustained loads.

How accurate is the RC to Ah conversion formula?

The formula Ah = RC x 25 / 60 gives an estimate within 10-15% of the rated Ah for most lead-acid batteries. The discrepancy exists because RC tests at 25A (a relatively high rate) while the Ah rating uses the 20-hour rate (typically 2.5-5A for automotive batteries). The Peukert effect means the battery delivers less total energy at higher discharge rates, so the RC-derived Ah is usually lower than the official 20-hour Ah rating.

Reserve Capacity to Amp Hours

4 min read

Some battery labels show reserve capacity (RC) in minutes instead of amp-hours. RC measures how many minutes a fully charged 12V battery can deliver 25 amps before dropping below 10.5V at 80F (27C). This calculator converts RC to approximate amp-hours so you can compare batteries that use different rating systems.

Reserve capacity to amp hours conversion showing minutes input and battery output.

What Reserve Capacity Actually Measures

Reserve capacity was designed for a specific automotive scenario: your alternator fails while driving, and the battery must power the headlights, ignition system, and fuel pump (roughly 25A combined) long enough for you to pull over safely.

The RC test is standardised by BCI (Battery Council International). A fully charged 12V battery at 80F is discharged at a constant 25 amps until the voltage drops to 10.5V. The elapsed time in minutes is the RC rating.

The conversion to amp-hours is approximate because RC tests at a specific rate (25A) while Ah ratings use a 20-hour rate (a much slower discharge). Due to the Peukert effect in lead-acid batteries, the RC-derived Ah is typically 10-15% lower than the rated Ah. The formula RC x 25 / 60 gives a close estimate: 120 RC minutes = roughly 50Ah.

Conversion reference for reserve capacity values from 60 to 240 minutes to amp hours. — Reserve capacity converts to amp-hours at roughly 0.42× the RC value — a 120-minute battery holds approximately 50Ah.

Reserve Capacity to Amp-Hours Conversion Chart

RC (minutes)	Approx. Ah	Wh at 12V	Typical Battery Group
60 min	25 Ah	300 Wh	Small car (Group 26R)
90 min	37.5 Ah	450 Wh	Compact car (Group 35)
120 min	50 Ah	600 Wh	Midsize sedan (Group 24)
150 min	62.5 Ah	750 Wh	SUV/Light truck (Group 27)
180 min	75 Ah	900 Wh	Large truck (Group 31)
210 min	87.5 Ah	1,050 Wh	Heavy-duty (Group 4D)
240 min	100 Ah	1,200 Wh	Marine deep cycle

These figures use the formula Ah = RC x 25 / 60. Actual Ah at the 20-hour rate may be 10-15% higher because the slower test rate is more favourable to lead-acid chemistry.

Worked Examples

Converting RC to Ah for Battery Comparison

Context

Battery A is rated 180 RC. Battery B is rated 100Ah. You want to compare them on equal terms. RC is measured at 25A draw until voltage drops below 10.5V.

Calculation

RC to Ah (approximate): Ah ≈ RC x 25A / 60 x correction factor

180 minutes at 25A = 180/60 x 25 = 75 Ah at 25A discharge rate

At the standard 20-hour rate, actual capacity is higher: ~100 Ah (Peukert correction adds ~25-35%)

Interpretation

Battery A is roughly equivalent to Battery B. But RC and Ah are measured at different discharge rates, so the conversion is always approximate. RC uses a higher rate (25A) which makes the Ah number appear lower.

Takeaway

For a precise Wh comparison, convert both to watt-hours. Use our battery capacity calculator to convert between Ah and Wh at any voltage.

Does a 90 RC Battery Have Enough Capacity for an RV?

Context

You found a cheap Group 35 battery rated at 90 RC. You need at least 80Ah for your RV house loads overnight. Is this battery sufficient?

Calculation

Estimated Ah: 90 x 25 / 60 = 37.5 Ah at 25A rate

At 20-hour rate: roughly 50-55 Ah

Interpretation

At ~50-55 Ah, this battery falls well short of your 80Ah requirement. A Group 27 at 180 RC (~100Ah) or Group 31 at 215 RC (~120Ah) is what you need.

Takeaway

Once you have the right capacity, check how long it powers your specific RV loads with our RV battery runtime calculator.

Frequently Asked Questions

Glossary

Reserve Capacity

The minutes a fully charged 12V battery can sustain a 25-amp draw at 80F before voltage drops below 10.5V. RC is a BCI standard designed to show how long a car's electrical system runs if the alternator fails.

Peukert Correction

An adjustment needed when converting between ratings measured at different discharge rates. RC is measured at 25A (a high rate), while Ah is typically rated at the 20-hour rate (a much lower draw). The correction accounts for the fact that lead-acid batteries deliver more total energy at lower discharge rates.

Battery Group Size

A BCI standard specifying the physical dimensions, terminal type, and position of an automotive or marine battery. Group 24, 27, and 31 are common deep-cycle sizes. The group number does not indicate capacity — it is purely dimensional.

Need to size a starting battery by CCA instead? The <a href="/battery/cold-cranking-amps-calculator">cold cranking amps calculator</a> handles that by engine size and climate.

Reserve capacity is a useful spec for automotive batteries but a poor metric for deep cycle applications. Convert to amp-hours using the formula above, then use the Ah figure for all your sizing and runtime calculations. For a step-by-step walkthrough of turning those amp-hours into actual hours of runtime, see our battery runtime guide.

Last updated: March 19, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.