Sometimes nothing seems more frustrating than a car that won't start. To diagnose the problem, run through your normal start-up routine, and determine what seems different. Write down anything that seems abnormal in case you need to tell a mechanic about it later. If you think the problem might be the battery, look for the telltale signs of a bad car battery.
No Noise from the Starter
When you turn the key in the ignition, you should hear the starter crank (it has a loud, whirling sound). If you hear nothing, the battery might be dead. Jiggle the key a little, and try the ignition again, just in case the ignition somehow is jammed. Also, check any antitheft device to see if it is disabling the start-up process. If everything checks out, chances are you have a bad car battery.
If you hear a clicking noise when you turn the key, but not the loud noise of the starter, the car battery might be dead. The clicking noise most likely comes from the solenoid or relay, which are parts of the starter. To find the source of the clicking, ask someone to turn the key while you listen under the bonnet. If the click comes from the starter, then you may have a faulty solenoid. If you don't know where the starter is in your car, ask a mechanic or consult your car's manual. But no matter where the click is coming from, Popular Mechanics advises that you "begin your real diagnosis at the battery," which might be the ultimate cause of the problem.
Lights Left On
If you notice that you left the headlights or interior lights in the "on" position, and the car will not start, your car battery probably is dead.
Sometimes a car will start, but it takes a while. To see if slow starts are because of a bad battery, turn off the engine, then observe the behaviour of the electronic system. Dim dashboard lights, slow wipers and any other sluggish behaviour of electronic devices are signs of a dying battery. Ask an auto-parts shop or a mechanic to test your battery to see if it still is capable of holding a full charge. If not, replace it.
Check the Charge Indicator
Some batteries have charge-indicator lights built into their casings, allowing you to check the power status. Follow the battery manufacturer's instructions to identify whether your battery is bad. For example, some car batteries present a green light when there is a normal charge and a black light for low or no charge.