How a car’s battery works

A car’s battery, like all batteries, stores electrical energy in the form of chemicals. We aren't going to get into detail, but the energy is stored and discharged via bonds of hydrogen and oxygen that make water, and the reaction of sulfuric acid and lead plates to make lead sulfate and lead oxide.

Withteh engine running , the alternator puts electricity back into the battery, causing the water to become more acidic and stripping sulfur off of the lead plates.

Every car is different, and our manuals have full instructions. Find yours…

When to change your battery

Even with nothing to cause them to run down, car batteries only have a finite number of years to live. Lead acid car batteries are designed to last for about five years, but unlike cell phone batteries they don’t like to be repeatedly drained and charged. So if you repeatedly run it down by leaving the lights on or have a ‘mystery drain’ somewhere in the car, it’s likely that your battery won’t be in the best of health for very long.

It’ll be obvious when your battery is dead - the engine won’t turn over. But that doesn't always mean the battery needs to be replaced. Once the car has been started (via jumper cables) take the car for at least a 20-minute run to charge the battery. Turn off the engine and try to restart it. If it starts easily, shut it down and wait at least an hour, then try again.

It can be hard to tell the difference between a battery that won't hold a charge, a battery with a bad cell, and a car that has a short or accessory that is draining the battery. Modern cars all have some parasitic loss due to computers, clocks, and infotainment memory. With a multi meter you can find out many things about the health of your car's battery and charging system, here's how.

All cars are slightly different, so if it is time to change your battery, use our before you begin checklist, and find your car for specific instructions.

Make sure you follow the correct battery disconnection procedure to avoid causing problems with the car's electrical components

Why you should change your battery

Aside from the obvious inconvenience of being stranded somewhere with a car that won't start, a dead battery may require you to reset the car’s electronic systems. Modern engine and transmission control systems learn your driving habits and adapt to them, so the car may drive oddly if the battery goes dead for any length of time. Often the car's memory will be retained for the few moments it takes to swap out a battery.

Batteries are susceptible to damage due to extreme heat or cold. If you live in the snowbelt it is possible for a battery to freeze and crack, though if it is fully charged and healthy the temperature will need to be below -70 degrees. The liquid acid in a battery is 75% water, and if the battery is not fully charged it can start to freeze at 32 degrees, just like regular H2O. An over charging condition, or just an engine that is running hot, can cause some of the liquid in the battery to boil away. A battery low on water may put out more than 12 volts, but it may not have the amps needed to crank the motor.

Before you begin

Replacing the battery is usually an easy task which requires only basic tools. The hardest part will be literally removing the battery, as they can weigh more than 40 lbs. and often lack a handle to lift. In most cases this procedure will take only half an hour or so.

Tools you will need

Only basic tools are required for this job

• Socket and ratchet set
• Wrenches
• Wire brush

Parts you may need

• Battery
• Dielectric grease
• Felt terminal washers
• Hold down clamp

Every car is different, but our manuals have full details...