When Should You Replace the Battery in Your Car?

The battery in a vehicle simply provides the power to start the engine, after which it becomes redundant. Once the engine is running, the alternator supplies all the operating voltage and recharges the battery. A battery discharges much faster than it charges because of the heat created in the charging cycle. This remains the biggest problem with totally electric vehicles: The vehicles usually will travel 50 to 80 miles before they need a recharge (taking 7 to 10 hours on average). The batteries in an engine and electric hybrid last longer since they are not required to be the sole source of power. The engine does much of the work while the batteries assist. In a hybrid vehicle the braking action is used to generate electricity used to help charge the batteries.Operation

In a normal vehicle, starting the engine requires the greatest amount of power from the battery. Once running, the alternator takes over and recharges the battery. An alternator that has one failed diode will have a seriously lowered output. When an alternator fails, a bad diode is usually the cause as opposed to a sudden complete failure. It will take a lot longer to charge the battery with an alternator with a bad diode.Intermittent Low Charge Causes

A battery that seems low, causing a hard start or slow turning engine when a start is attempted could have a bad cell in the battery. A bad cell the battery, in many cases, can be recharged with the engine running. It will not hold the charge for very long though. When the vehicle sits for several hours the charge will bleed off the battery and it will once again be low.Checking the age and service life:

Batteries all have a certain life expectancy. The cells' anodes and cathodes will wear thin over a period of time and no longer maintain a charge. The lifetime date is stamped on the side of the battery starting with a letter code for the month if was produced. The letter A is for January, B for February and so on. The next number is the day and the last number is the year. The sticker on the top of the battery should say the month and year put into service. The top sticker will also have the cold cranking amps (CCA) and the reserve capacity (RC) in terms of time. This is the most important.Identifying the right battery for the vehicle:

The battery will have a group number on the top sticker of the battery. This group number, for example, group 54, is the size and shape of the battery made for a particular car. It also identifies whether or not the battery is a top or side post connection.Diagnosing a battery for a bad cell:

The best way to check the battery and alternator is with a voltmeter. Place the leads of the voltmeter across the terminals of the battery. Have a helper start the vehicle while you watch the voltmeter. If the voltage drops below 10 volts when the starter is engaged, the battery has a bad cell and needs to be replaced. If the voltage stays within limits above 10 volts, check the voltage with the engine running. It should be about 14.5 (plus or minus a few tenths) if the alternator is good and charging the battery adequately. Turn on the air conditioning and headlights and see if the voltage drops below 13.5 volts. If so, the alternator needs to be replaced.