first thing I would Scrutinize is the battery terminals; are they clean of Corrosion? with today's highly computerized cars, the slightest amount of terminal corrosion would wreak havoc with the voltages the car needs; the second part I would look at is the Alternator; see if you can get your hands on a Multimeter, and check the battery voltage while the car is running with the headlights on Low Beam. I went and tested mine, which has a Known good Charging System, and it read 14.76 Volts; 12-13 volts would likely indicate a weak alternator; anything below 12 volts would indicate the alternator is not charging at all.