A car's battery is the heart of an automotive electrical system. Just like a battery in any electronic device, a car battery must be replaced periodically as its performance will degrade over time. Some batteries require periodic maintenance and must be serviced regularly to obtain optimum performance, while others are 'maintenance-free,' meaning that they are not user-serviceable. There are a number of automotive problems that may occur with symptoms that mimic those of a dead battery but, with the proper tools and knowledge, these can easily be diagnosed before a new battery is purchased unnecessarily.
Test the battery's voltage with a digital multimeter. A battery in perfect working condition should ideally show 12.66 volts DC between the positive and negative terminals. This number may be slightly higher or lower depending on its size and manufacturer, but should be no less than 12 volts, and no more than 13 volts. If a battery has been thoroughly charged and shows a voltage less than 12V at rest, check the battery's water level and add as necessary. If the water level is OK, it may require replacement.
Check the battery terminals for corrosion and proper connection. It is very common for corrosion to build up on the battery. In many cases it is easily visible, but it is possible for corrosion to be hidden between the terminal and the connector. This may lead to insufficient charging, as well as a loss of power to the car. Disconnect both terminals and clean them with a battery terminal cleaning solution and a wire brush. Reconnect the battery and tighten the terminals.
Check the ground. Follow the negative battery wire from the terminal to the chassis ground. Use the multi meter to test the resistance between both ends of the wire. A good ground will show 0 ohms, or very close to it. Any reading greater than 1 ohm indicated a poor connection. Repeat this procedure between the negative terminal and the vehicle's body ground.
Perform a load test. Place the battery load tester between the positive and negative terminals. When the load test is performed, the meter will show the battery's performance under a load. If the battery has been sufficiently charged and fails the load test, replacement is necessary.
Test the alternator. Use the multimeter to test the battery's voltage with the vehicle turned off. Start the car and test the voltage again. When the car is running, the voltage should read approximately 14.4 VDC. If the voltage is the same when the car is running as it is when the car is not running, or if the voltage drops when the car is running, the alternator may not be working properly and may need replacement. If the alternator fails to properly charge the battery, the symptoms produced will be very similar to those that occur with a failing battery.
Dirty/loose battery terminals are the most common cause of battery related car problems. Always clean and tighten the battery terminals before replacing the battery. Always dispose of dead batteries properly.
Wash your hands after handling a battery. Car batteries may contain sulphuric acid, and the terminals are commonly made of lead. Do not smoke when servicing a battery. Hydrogen gas may be released from certain types of car batteries, and an explosion may occur.