[MyMotor]

With my car running so poorly, I am using as little as I can inside the car. How much of a drain does having all the power in use inside cause?

[TransAm]

I think it depends on the car, but generally speaking, in my experience, keeping lights on and doors open, etc. drains the battery quite a bit. The battery is constantly being recharged by the alternator while the car is running. It wasn't designed to run anything on its own for any length of time, but rather to provide short, high voltage bursts to the starter solenoid.

[Fanbelt]

I agree with you about keeping lights on and doors open. It has ever happened to me. I didn't close the door properly hence the inside light was on. As if it was not bad enough, I parked the car and didn't touch it for two weeks or so, and the battery was completely discharged.