The average car battery has a life expectancy of three to five years. Extreme temperature changes or frequent, short trips can adversely affect a car battery's lifespan. There is no way to determine exactly how much longer a battery will perform in regards to time. You can determine if the battery is still at optimal performance or if it is time to purchase a new battery.
Protect yourself by donning protective clothing (i.e. long sleeves, trousers and safety glasses). Battery acid can cause severe damage.
Grab your voltmeter and set the dials on the 0-50 DCV range.
Disconnect the car battery according to the directions in your car's manual. Disconnect the negative terminal first, because if you disconnect the positive terminal first, you risk shorting out the battery. Keep the positive terminal covered with a rag to avoid shorting out the battery.
Connect positive (red) lead of voltmeter to the positive terminal on the battery (marked with a "+" sign) and the negative (black) lead of the voltmeter to the negative terminal on the battery (marked with a "-" sign).
Check the voltmeter. If your battery is charging properly, it should read between 12.6 and 12.8 volts. Anything under 10 indicates that your battery needs to be charged.
Check the fluid levels in the battery while it is still disconnected if the voltmeter reading is not adequate. Accomplish this by lifting up the battery cover and peeking into the compartments. If the electrode plates are not submerged, add distilled water until the fluid is approximately a quarter-inch above the electrodes.
Reconnect the battery, positive terminal first, then start the car. With the vehicle idling, check the voltage again. If it is above 14 volts, the system is charging properly. If it is below 12 volts, your charging system may be at fault. This can mean either that the battery needs replaced or you have a problem with your alternator.
On some models equipped with theft-lock audio systems, be sure the lockout feature is turned off before unhooking the battery. This information can be found in the user's manual. Some automotive parts stores will load test your battery free of charge. A load test will determine whether the battery is supplying sufficient amperage for its rating. This is a more reliable method of determining battery condition.