Test the voltage of a car battery when the vehicle is off and when attempting to start it after you disconnect the ignition or fuel injector pump. Batteries should test in the normal range without a load, and the reading should remain above 9.6 volts when cranking for a maximum of 15 seconds. If you do not have a voltmeter and the car starts, take it to an auto parts store and have the representative check the battery.
When working with batteries, wear eye protection and gloves. The liquid inside the unit is sulfuric acid and it can cause burns.
If the battery passes the test, use a charger to bring it back to full charge. Attempting to charge an almost dead battery using the alternator of the vehicle can lead to permanent damage. Manufacturers design alternators for maintaining the battery's charge.
If the battery requires replacement, choose a battery with the same type of posts as the original and one that fits the existing battery tray and hold-down. The replacement should provide at least the same number of cold cranking amps to ensure the vehicle starts when temperatures are very low. A battery's warranty length typically relates to the number of cold cranking amps it provides.