What does the battery warning light mean?

The battery warning light (a battery symbol) comes on when the car's charging system isn't charging the battery properly. Despite the symbol, the fault is more often the alternator than the battery itself.

What it means in practice: the car is now running on whatever charge is left in the battery, which will run down. If this comes on while you're driving, you've usually got limited time before things relying on electrical power start to fail - power steering, lights, and eventually the engine itself.

It's worth heading to a garage rather than continuing a long journey, especially if other electrical items also start behaving oddly.