How are car batteries rated?

A typical car battery

LSI (Light, Starting, Ignition) car batteries must supply specific amounts of electrical energy in different vehicles. Therefore, battery manufacturers and regulatory bodies the world over have developed a standard system to rate batteries for specific applications.

Although there are currently more than a dozen rating parameters in use around the world, you only need to consider the four most important ones. Below are some details of these ratings-

CCA (Cold Cranking Amps)
Cranking a cold engine requires more energy than cranking a hot engine. Therefore, battery manufacturers use low temperatures when they test a battery’s ability to crank a cold engine.

Therefore, a CCA rating refers to the number of amperes a battery can deliver for 30 seconds before the battery’s voltage falls below 7.2 volts. This test is carried out when the battery is at a temperature of -18 degrees celcius.

CCA ratings for batteries in passenger vehicles typically range from 300A to 500A, but do not install the battery with the highest possible CCA rating.

The best thing to do is to follow the vehicle maker’s recommended CCA rating for that vehicle. This information is usually contained in the vehicle’s user manual, or on a sticker under the bonnet.

CA (Cranking Amps)
Also sometimes known as MCA (Marine Cranking Amps), a CA rating is similar to the CCA rating. However, the CA rating is based on a battery’s power output at 0 degrees celsius, as opposed to at a temperature of -18 degrees celcius for the CCA rating.

Because of the higher temperature limit, CA ratings are about 25% higher than the CCA ratings on most batteries. On some batteries, CA ratings may be stated as MCA (Marine Cranking Amps), or sometimes as HCA (Hot Cranking Amps). "HCA", is, however, no longer in common use.

RC (Reserve Capacity)
This rating refers to the number of minutes that a car battery can maintain a current draw of 25 amperes before the battery’s voltage falls below 10.5 volts. This test is performed when the battery is at a temperature of 25 degrees celcius.

A current draw of 25 amperes is considered a typical electrical load on most vehicles. Thus, this rating is an indication of how long (measured in minutes) a vehicle can be driven after the alternator stops working.

Note that a 25-ampere load only applies to essential functions that keep the car running. This does not include loads by the internal fan(s), audio system, or repeated opening and closing of windows, etc. Put simply, the fewer electrical consumers are used, the longer the battery’s reserve capacity will last.

AH (Ampere Hour)
An Ampere Hour rating is a combined measure of the amount of electrical energy stored in the battery, and the number of hours a battery can deliver a current of 1 ampere.

For example, if a 60 Ah battery is discharged at 1 ampere per hour, that battery will deliver power at 1 ampere per hour for 60 hours. If, however, the current draw is increased to 3 amperes, the battery will deliver a 3-ampere current for only 20 hours.