Last Sunday our 2002 Mercury Sable would not start. There was not even a click when I turned the key. I called AAA, and they came out and used a jumper to start the car. I immediately drove to an auto-repair center to buy a new battery.

The salesman looked for the correct battery replacement on his computer. It showed group 36R as the correct one. He did not have this battery in stock, so he substituted a group 24F. He said the main difference was that the “cold-cranking power” was higher in the 24F.

Is this a proper replacement for this car? I don’t want to have problems caused by an improper battery.

-R. Greco, Felton, Calif.

BCI (Battery Council International) group numbers indicate the physical size of an automotive battery, its voltage, terminal type and terminal placement.

The 24F battery installed in your Sable was a hair longer, an inch less in width and an inch taller than the specified 36R battery. As long as the battery case and terminals don’t come into contact with an unintended surface, it is retained properly and the cables reach the terminals with ease, the exact physical size shouldn’t be of concern.

What’s of greater importance is the battery’s CCA and RC ratings, indicating the cranking ability and reserve capacity of the battery.

Carmakers specify a CCA (cold-cranking amps) rating for a vehicle depending on engine size and the quantity of electrical accessories. CCA is the number of amps (quantity of electrical current) the battery can deliver for 30 seconds at zero degrees Fahrenheit, while maintaining at least 7.2 volts.

Depending on a battery’s physical size, plate composition and design characteristics, CCA can range from about 400 to 1,000.

Reserve capacity also might be specified. This is a measure of the battery’s ability to deliver current without being recharged, perhaps while the vehicle is parked, or driven with a charging system failure. RC is the number of minutes the battery can deliver 25 amps of current without individual cell voltage falling below 1.75 volts. (That’s 10.5 volts at the terminals of a typical six-cell battery.)

As long as the 24F battery installed has an equal or greater CCA and RC rating than the specified 36R battery, you should be in good shape. The increased capacity might come in handy during a blustery day’s cold start during ski season and increase the chance of a successful start-up, should you accidentally leave the headlights on while parked.

Be sure to check the clearance between the terminals and any nearby metal surfaces (I’d prefer at least one inch). Accidental contact between the positive terminal and vehicle body would be catastrophic, resulting in a possible battery explosion and electrical system damage. Proper mounting is also essential, to ensure terminal clearance and to protect the battery from excessive vibration.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.