Car encyclopedia

Battery Replacement

The battery, in conjunction with the alternator, supplies the electrical power needed for the lights, stereo, and all other electrical components.

Periodically check battery cables to ensure they are clean and tight; also, check the fluid level in the battery. If you have a maintenance-free battery, there is a window on top that shows the state of charge.

No battery should be replaced without proper testing (e.g. individual cell voltage, load, and conductance tests). A properly functioning charging system (including cables and wiring) is a prerequisite to battery testing. Electrical faults that cause electrical current flow—sometimes in very low amounts after the vehicle is shut off—can cause chronic dead battery problems.

When replacing the battery, battery cable, or the battery cable end, apply the emergency brake, remove the keys from the ignition, and turn off all electrical components. Before disconnecting the battery, make sure the radio anti-theft code is available; some vehicles require certain systems to be reset or "learned" after the battery is installed. These can include engine and/or transmission adaptation, power window pinch protection, power seat/mirror settings, and radio station presets.

There are may types of memory devices available to help retain this often critical information when the battery must be disconnected. Please see Preventing Memory Loss for more information on these memory saving devices and how different control modules can be affected by loosing their memory.

To replace the battery, remove the hardware securing the battery, the battery cables, and the old battery. Install the new battery, reinstall the battery cables and hardware, and secure and tighten the new battery connections.

Mechanics' Corner: More Technical Detail

An automotive battery is an electro-chemical device that uses a mixture of sulfuric acid and water that surrounds lead plates in six separate areas called cells. Ideally, each cell develops 2.1 volts for a total of 12.6 volts.

When removing an automotive battery, the key should be removed from the ignition, the electrical loads turned off, and all vehicle doors closed. The negative cable is always removed first, followed by the positive cable. After removing both cables, the hardware holding brackets are removed. If you carefully follow these steps, any reserve voltage in the electrical system will equalize.

When installing the new battery, always connect the positive cable first, followed by the negative cable because the computer memory and adaptive functions will start to energize as soon as the new battery is connected. To avoid damaging the computers or software in the vehicle, it is important to have the correct polarity.

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