automobile batteries

The physical dimensions and some other characteristics of an automobile starter battery are specified by its group number. Stores that sell batteries have tables showing what group number a given make of automobile, model and year requires. Battery groups are established by the Battery Council International, which publishes the annual BCI Replacement Battery Data Book (see  A table showing the dimensions of many group numbers is at  Note that an “R” following a group number indicates a battery with the same dimensions as the same group number without the R, but with reversed polarity.

For each battery group the SAE specifies two minimum CCA (cold-cranking amps) ratings:

After the battery is brought to the test temperature it is discharged at the rated CCA current for 30 seconds. To pass the test, each cell must then still have a voltage of at least 2.1 volts. Because starting an engine on a cold morning is one of the most severe demands placed on a battery, these ratings are a good indication of the battery's capacity.

A special class of automobile batteries are deep-discharge batteries, which are designed to be repeatedly drained completely, on electric trolling motors for fishing, for example. Such use would quickly destroy an ordinary car battery. Instead of a CCA rating, these batteries have an MCA (for marine cranking amps) rating.

Terminal posts may be threaded or unthreaded. If they are unthreaded, they have a 1:9 taper, are at least 0.625 inch high, and the positive terminal is slightly larger (0.688 inch) than the negative terminal (0.625 inch). If they are threaded, the thread is 3/8-16 UNC 2A.

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