About 2 months ago, my 02 Navigator was draining the battery overnight. My dad would hardly drive it, so when he would try to start it, it wouldn't start. It had a red top Optima battery. I had another red top Optima in my 92 Eagle Talon Tsi AWD. Switched batteries between the cars, the Navigator now worked flawlessly, the Eagle now had trouble starting after sitting 2 days.

In my case, the battery tested good, but the voltage would drop all on it's own. Dead Cell, as usual with Optimas.