The battery light means the charging system is not recharging the battery while you're driving and the battery is running down. At best you'll be able to drive up to two hours before the engine dies; less if you have the headlights on. You can start by measuring the battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If it's low, suspect the generator, but have it professionally load-tested first.