A couple of months ago my car started to lose power very quickly throughout the day. At first, I noticed while I was driving (rainy weather) the door open light would flash on and off, along with the overhead lights and eventually stop, leaving the door open light on. After that started happening, I noticed a drain in power when I turned the car off.
Car finally wouldn't start, I took the battery in for a test, but it was only a year old and they said the battery was good, just completely drained. They recharged it for me, I put it back in the car and it starts up just fine. The alternator seems to work fine because voltage levels of the running car are where they should be, and I don't lose any power during or after drives, only between them.
What I've been doing is unplug the battery every time I turn the car off, and when I plug it back in, car starts right up as if a full power. If I start the car multiple times before giving it a rest and unplugging, the power feels weaker.
My car has 166000 miles.
My car has an automatic transmission.