A typical 12-volt car battery is composed of six cells that chemically produce voltage and amperage to start your car. When one or more of the cells fails, the battery is incapable of producing the needed amperage to start the vehicle to get you to work in the morning. A battery is charged continually by the alternator while the automobile is running. Cell failure is one of the most common types of battery failure. A telltale sign that one or more cells is not producing sufficient amperage comes when the vehicle is reluctant to start, or "turn over," to get the motor running. If the car starts and runs fine when the alternator is running but after being shut off for a time (like overnight), has a hard time starting, the likely cause is that the battery is dying; that is, the battery is not holding the charge it gets from the alternator.
Test the battery with a voltmeter. A voltmeter is the most reliable way to determine if a battery is holding its charge. After the battery has been idle overnight or for a few days, it should retain its charge. If new, it should hold a charge a lot longer. A voltmeter is like the filament in a light bulb, with a meter in the middle of the filament. The current runs through the filament, and the meter registers the voltage. For a battery, all you need is a simple voltmeter with an analogue gauge. Attach the positive end of the voltmeter to the positive lead from the battery and the negative lead from the voltmeter to the negative lead from the battery (red to red, black to black). If the voltmeter shows no reading or a reading below 12.4 volts, the battery is dying. If the voltmeter reads 12.4 volts or higher, the battery is fine and the problem lies elsewhere.
Start your car. If the vehicle is reluctant to "turn over," it could mean that the battery lacks the cold-cranking amperage (CCA) to start the car. The amount of CCA required for each vehicle is different, and the CCA is specified on a sticker on top of the battery. It would read something like CCA 650 (Cold cranking amperage 650). If one or more cells is no longer holding a charge, the CCA is reduced and eventually will drain all the cells, killing the battery. Again, the voltmeter is the most sure way to tell.
If the car stops running completely after you've started it and driven it for a while, it's likely a sign that the battery is dead and not holding any of the charge it receives from the alternator. Even though the vehicle operates on the electricity provided by the alternator once it starts, that electricity runs first through the battery, which acts as a form of capacitor to regulate the amperage that goes to lights, cigarette lighters, inside dome lights, windshield wipers and electronics. Too much amperage will burn out these electrical components and stop the car from running if it can't channel the necessary amperage into the vehicle. If it can't allow the charge to run through it, that's usually a sign that the battery is dead or very close to it.