I think it could be your alternator that is about to go (one of the diodes could've died). My car did it when one of my diodes blew I remember now. The car was charging at 10v, had to keep the revs up so it would charge. I had it fixed, then it blew again months later. Now I have a brand new alternator, set me back $600 lol Get it checked and make sure its charging at 13v approx. If you have a meter, just start your car and put the negative to the negative and positive to positive on the battery. Should give you a reading anything under 11v is bad.