Put a volt meter across the terminals of your battery. It should read about 14.5 volts when the engine is running. Anything higher and your alternater is dead. Anything lower and your battery probably isn't getting a good charge.
Check the belt to the alternater is not loose and causing slip to the alternator.
Start the car with the headlights on full beam (pointing at a close wall so you can see them) If the lights dim drastically, I'd suggest your battery isn't holding its charge, replace the battery. It isn't easy to "clean" batteries to extend their life any more
Alan, September 2007
