If the battery warning light (a light in the shape of a battery) comes on while you're driving, that means the charging system isn't working, but the fault may lie in something other than the battery.
The cause could be a loose or corroded battery cable or other wire connecting components of the charging system, or it might be a problem with the alternator or voltage regulator. The alternator generates the power that is stored in the battery. If the alternator fails, or the accessory belt that drives the alternator is loose or broken, then your battery isn't being recharged.
The battery itself may be the cause if it has corroded cable terminals, damaged cells or plates inside or if it is leaking electrolyte.
The charging system warning light should come on for a few seconds when you start the car, but if it turns on while you're driving, that signals a problem. Among other signs that the charging system isn't working are dim headlights or if the clock loses time.
If the light comes on, you might be able to make it home or to a service facility. The car will continue running as long as some juice is left in the battery, but if the charging system isn't working, your car will stop running once the battery is drained. If you turn off the engine, you won't be able to restart it if the battery doesn't have enough power left to run the starter motor.