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automobile batteries

Physical dimensions: BCI group numbers

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, and a used battery should have a label saying what group number it is.

Battery groups are established by the Battery Council International, which publishes the annual BCI Replacement Battery Data Book . A table showing the dimensions of many BCI group numbers can be found at www.jegs.com/Sizecharts/bcigroup.html or www.batterysales.com/bci.cfm 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.

Amperage

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

one at a temperature of 0°F
one at a temperature of −20°F.

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.

Terminals

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.