Every gasoline-powered or hybrid car today contains a rechargeable 12-volt battery. This battery powers everything electrical: the engine's control computer, the ignition system, the radio, the headlights, etc. So, the battery is vital to the car's operation.
Like all batteries, a car's 12-volt battery would eventually go dead if it were not recharged, so your car has a built-in recharging system. The alternator and voltage regulator charge the battery and provide electricity to the vehicle when the engine is running. A typical alternator can put out between 500 and 1,000 watts when necessary.
Because your car is so dependent on the battery, any car with a combustion engine and a 12-volt battery has a light on the dashboard that warns you if the recharging system fails. It looks like a little battery, or sometimes it says "Battery." A simple circuit looks at the voltage that the alternator is producing and turns the battery light on if that voltage is low. So, the light is telling you "your alternator is not properly charging." It's not telling you "your battery is bad or needs to be replaced immediately." You don't have enough information yet.
The battery light comes on when you start your car, and it might linger for a second or two, but it usually turns off pretty quickly. If it stays on while the car is running, it probably means there's something wrong with the alternator. It could also mean there is a minor problem with the battery like corrosion. Either way, it's something you want to check out.
If there was enough juice for you to start the car, then you can safely drive the car on however much charge is remaining, but it won't last long — maybe 30 to 60 minutes max. If the battery isn't being charged, the electrical systems will start to fail.
Use whatever charge is remaining in the battery to get somewhere safe, either an auto repair shop or your home. Once you turn the engine off, it might not come back on again. To eke out every minute of charge, turn off as many electronics as you can: radio, air conditioning, heated seats, even the headlights if it's daytime. Unplug your phone too, since every little volt counts.
Originally Published: Apr 1, 2000