AutoTalk: Bad battery? Maybe not

You've just returned home from a well-deserved vacation. For the first time in weeks, it's time to start up the car and get back to work. But when you turn the key, nothing happens—even though you installed a new battery only a short time ago. Did you get sold a defective one?

Probably not. After a lengthy period of disuse, even a new battery will lose its charge. Like people, batteries need regular exercise to remain healthy. Wet-charged batteries such as those in automobiles can self-discharge approximately 4% of their stored electrical capacity per week in summer temperatures. Compounding this loss of capacity is the fact that modern cars have electrical demands that need to be met at all times, whether the car is being driven or not. The current used for these needs is called "parasitic load." Some examples of components that draw current even when the car is turned off:

the powertrain control module
the anti-lock brake system
the presets for the various radio stations you have chosen
the memory power seats
the inflatable restraint system
the car alarm and even the generator.

While the electrical current used by these components individually is low, when they are added together the parasitic draw of the typical modern car is around 50 milliamps. That may not seem like much, but over an extended period it can drop the total voltage in a battery below the threshold necessary to start the vehicle.

Like people, batteries need regular exercise to remain healthy.

If a vehicle sits for a couple of weeks or longer, a new battery can be completely dead. In winter this can cause further problems, as a dead battery can freeze, damaging its case or plates.

So what can be done to avoid this inconvenience? Regular vehicle use will keep the battery fully charged and capable of starting your car, provided of course that there are no other mechanical issues. Just starting the car and letting it idle for a while isn't usually enough to keep a battery fully charged.

There are also various battery chargers available that can help to keep your battery at the correct operating voltage. If you often go long periods without using your car, it would be best to consult with a professional auto technician to identify an appropriate charger for your needs.