The Car Battery

The primary function of a car battery is to provide power to start the engine. The secondary functions in order of importance are:-

To help flatten out the fluctuating DC power generated by the car's charging system.

To provide additional power for lighting and electrical accessories when the power being used exceeds the power being produced by the charging system. This may be when the engine is idling or during short journeys on a dark winter night.

To provide power when the engine is not running.

Car manufacturers normally design cars' charging systems with about 25% more capacity than is required for factory fitted equipment. The additional load which a carputer represents should not present a problem. However, if you have plans to add a high power sound system, or have already done so, you may need to look at upgrading your charging system

Introduction to Batteries and Charging Systems courtesy of Sure Power Industries Inc.

SLI v Deep Cycle

If you intend to use your carputer extensively while the engine is not running, you need to fit a deep cycle battery, (also known as a deep discharge or leisure battery), in addition to your normal battery. If consistently undercharged or overcharged, a normal car lead-acid battery (sometimes called SLI, for starting, lighting, ignition) will lose its ability to hold a charge and ultimately fail.

SLI batteries need to supply a high starting current for a short time. They have a large number of thin "spongy lead plates giving a high surface area. If this type of battery is deep cycled, this sponge is quickly eroded and drops to the bottom of the cells. SLI batteries normally fail after 30-150 deep cycles, but will last for thousands of cycles in normal use.

Deep cycle batteries have much thicker plates of solid lead. this means there is a lower surface area and less instant power of the type required for starting engines. The design does however lend itself to deep discharging repeatedly without the battery failing.

Click on the illustration above for a comprehensive introduction to batteries and charging systems