The two most common causes of dead car batteries are driving habits that result in a low charge and acid stratification. Factory defects accounts for less than 7 percent of dead batteries.
Low charge in a car battery is the result of continually using vehicles, particularly small cars, for short trips. Often, compact cars have batteries that only store enough immediate charge to start the vehicle and run a few basic functions. The batteries rely on longer trips to recharge themselves after starting. During short trips in which drivers use extra functions such as radios, air conditioning or heating, and defrost, the batteries do not have a chance to fully charge and become depleted to the point of becoming "dead."
Acid stratification is when the electrolytes in a battery settle to the bottom of the battery. The upper half of the battery becomes too low in acid, and as a result, cannot produce enough charge to start the vehicle. This condition can occur when the battery doesn't get a lot of use and doesn't get a full charge. Safely disconnecting the battery and turning it upside down to allow the acids to properly distribute again should help. The battery may need to rests a few days or receive a trickle charge.