I know it seems like my car is full of problems but this issue has been bothering me for over a year. Whenever I turn on my A/C, the battery voltage drops down below the halfway point (less than 13V) when the engine is idling. When the car starts moving again, the voltage increases. Thinking the problem was with the alternator, I replaced it a few months ago and it was still doing it (the new alternator tested 140A at 2000rpms). I later replaced the battery with an Optima; no luck. As a last resort, I ordered the voltage boost mod from ZZP to eradicate it-still nothing (the voltage swings even more at idle). The A/C works-it's cold-but whenever I turn it on, the engine seems like it's lugging a trailer. I'm absolutely stumped on this one.