[T Bird]

Everything I've read indicates that a battery showing 12.6 volts is 100% charged - 12.4 volts is 75% charged - 12.2 volts is 50%- and 12.0 volts is 25% charged. At 12.87 volts after a week's rest, the battery appears to be fully charged. Admittedly, this reading is not under load.

My Optima is a yellow top deep cycle marine battery and has lived virtually its entire life (while the car is not being driven) connected to a Battery Tender. I don't think the problem is in the battery. As I understand it, the red alternator light is supposed to come on when the battery voltage exceeds the voltage output of the alternator. Since I'm measuring voltages in excess of 15 volts at the battery while the engine is on, the light should not be on.

John - I'm not sure I followed your explanation. How do I measure resistance from the battery back to the D+ connector?