What RC means on a battery
The short answer
On a 12-volt automotive battery label, RC stands for reserve capacity. It is the number of minutes a fully charged battery can deliver a constant 25-amp current at about 80°F (roughly 27°C) before its voltage drops to 10.5 volts, the point at which it is treated as effectively discharged. A label reading "RC 120," for example, means the battery met that 25-amp load for 120 minutes under those test conditions.
The figure applies to conventional 12-volt lead-acid starting batteries, including flooded, EFB and AGM types, and is a rating convention used mainly in the North American market. Batteries sold elsewhere may list amp-hours (Ah) instead of, or alongside, reserve capacity. RC also does not describe the high-voltage traction battery in a hybrid or electric vehicle, which is rated in kilowatt-hours.
How reserve capacity works and what it tells you
Reserve capacity is a measure of endurance rather than power. The test simulates a situation in which the battery must run the vehicle's electrical load on its own, such as when the alternator has failed or the engine is off with accessories running. The 25-amp draw stands in for a basic set of loads like ignition, lights and other essential electrics. The longer the battery sustains that draw, the higher the RC number.
In practical terms, a higher reserve capacity gives more margin before the battery is too depleted to keep the vehicle running or to restart it. That margin is useful if the charging system stops working while driving, if the vehicle often sits with accessories in use, or if it carries extra electrical equipment that draws power when the engine is not charging.
How RC differs from other battery ratings
Rating | What it measures | What it tells you; RC (reserve capacity) | Minutes at a steady 25-amp load until voltage falls to 10.5 V | How long the battery can support electrical loads without charging; CCA (cold cranking amps) | Current the battery can deliver for 30 seconds at 0°F (-18°C) while staying above 7.2 V | How well it can start an engine in cold weather; Ah (amp-hours) | Total charge delivered over a set period, commonly 20 hours | Overall storage capacity at a low, steady discharge rate
These ratings are complementary, not interchangeable. A battery can have strong cranking performance and modest reserve capacity, or the reverse, depending on how it is built. CCA addresses a short, heavy burst of current for starting; RC addresses sustained supply over a longer period.
Some readers try to convert RC into amp-hours. Multiplying the RC minutes by 25 amps and dividing by 60 gives the charge delivered during the 25-amp test, but that result is not the same as a battery's published 20-hour Ah rating. Lead-acid batteries deliver less total capacity at higher discharge rates, so a figure derived from the 25-amp test will generally differ from the manufacturer's Ah figure. Treat any such conversion as a rough comparison only.
Limits on what the number means in real use
RC is a laboratory rating for a new, fully charged battery at a specified temperature; an older or partly charged battery will deliver less. Cold temperatures reduce how much usable capacity a battery can deliver, so real-world endurance in winter can be shorter than the label suggests. Actual vehicle loads vary widely and are rarely a steady 25 amps, so the rating does not predict how many minutes a specific vehicle will run on battery power alone. The figure is most useful for comparing batteries of the same group size and type against one another.
Using the RC figure when choosing a battery
When replacing a battery, start with the specification the vehicle manufacturer lists for the applicable model year, engine and trim, usually found in the owner's manual or from a parts supplier's fitment data. Matching or exceeding the recommended reserve capacity, while also meeting the correct group size, terminal layout, battery type and CCA requirement, is the generally safe approach. Vehicles with start-stop systems or heavy electrical demand may specify a particular battery technology, such as AGM or EFB, and that requirement takes priority over choosing on RC alone.
No vehicle-specific reserve capacity values were supplied for this article, so the required RC for any particular car, truck or SUV should be confirmed against the manufacturer's documentation or the battery maker's fitment information for that exact vehicle.