Battery basics

Motor,  Apr 2002  by Layne, Ken

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Temperature has an important effect on both battery performance and the specific gravity of the electrolyte. High temperature increases electrolyte loss and shortens battery life; low temperature can damage a battery.

If electrolyte freezes, it often expands and cracks the battery case. The freezing point of the electrolyte depends on its specific gravity and the battery's state of charge. A fully charged battery with a specific gravity of 1.265 won't freeze until the temperature drops to -60 deg F. A discharged battery with a specific gravity of 1.120, on the other hand, can freeze at temperatures as high as 18 deg F.

Low temperature also increases the resistance of the electrolyte, which reduces battery cranking power. A double whammy is that an engine needs increased cranking power to start at lower temperatures. Specific gravity specifications are based on a standard temperature of 80 deg F At this temperature, a fully charged battery has 100% cranking power. The same battery at 32 deg F has only about 83% cranking power, although the engine needs 165%. Electrolyte resistance also means that a cold battery is harder to recharge than a warm one.

Battery Ratings

Current-capacity ratings indicate a batterys ability to provide cranking power for an engine or reserve power for the electrical system, or both. Three basic rating methods exist:

Ampere-Hour Rating. Also called the 20-hour discharge rating, the amp-- hour rating is the oldest current-capacity rating method. It indicates the steady current flow that a battery will deliver for 20 hours at 80 deg F before cell voltage drops below 1.75 volts. For example, a battery that continuously delivers 4 amperes for 20 hours is an 80-- amp-hour battery.

The amp-hour rating has been largely replaced by other rating methods for automobile batteries, but it's still the best way to rate a cycling battery such as those used in electric vehicles.

Cold-Cranking Rating. This rating indicates the power a battery can supply to start an engine in cold temperatures. Cold-cranking ratings are given in total amperes (375, 425, 500, etc.). They're a function of a battery's ability to deliver a set current for 30 seconds at OOF before cell voltage drops below 1.2 volts.

Reserve-Capacity Rating. This rating method indicates the number of minutes that a fully charged battery can deliver 25 amperes before cell voltage drops below 1.75 volts. These ratings are given in minutes (60, 110, 120, etc.). The reserve capacity also indicates how long a car can be driven if the charging system fails.

Battery Size & Group Number

Batteries of various current-capacity ratings are built in different sizes and shapes, which are indicated by battery group numbers. Group numbers are developed by the Battery Council International (BCI). A group number indicates length, width, height, terminal design and location, holddown location and other physical features. A battery maker may build several batteries of the same group number but with different numbers of plates and plate surface areas. Thus, any single group number may have several different current-capacity ratings.