Q: Car battery dead.

Hello, it sound like your charging system is not charging the battery. The alternator which puts out 12 to 14v DC charges the battery and supplies power to run all the electrical consumers when the engine is running. With the alternator not having any output, when the engine is running and you turn on the headlamps, the headlamp puts an excessive load on the battery and cause low voltage to the ignition and fuel injection system cause a lack of power. This is something you should have looked at sooner that later of you might get stranded and have to have the vehicle towed. You might need the aid of mechanic to test the charging system and have it replaced as necessary.