issue could be the diode bridge in your alternator. When they go the car's running idling voltage (w/headlamps on) will be on the lower side, say 13v or so. Also that issue causes the alty to draw about 1.5 amps, so will pull the battery down pretty fast. If you meter has a DC ammeter -- use it to measure the battery's current draw with car off - report back on what you find. Not supposed to be more than .035 amps (35 milliamps)....