How Much Should A Car Battery Drain Overnight. Because of the way lead-acid batteries discharge its important that you test the battery after its been sitting for a period of time to get whats called the resting voltage. A good healthy car battery should have no fewer than 126 volts.
A typical car battery will drain in 2-3 weeks and be unable to start your car due to the parasitic draw from the cars electronics. This is where we can see the parasitic draw on the battery. Often the pull when you first connect the battery can be a lot higher than the steady pull since you might be charging some capacitors etc.
With the battery in that poor condition the regulated 144V from the car alternator will be barely enough to charge it at all.
Not sure what it should be pulling but anywhere near a whole amp is way too much and will drain the battery in no time. Note that constantly running a car battery flat will shorten its life as they are not designed for deep-discharge. I usually drain my iPhone battery between 15 and 20 overnight. The lifespan of your battery has come to an end.
