Something is on draining your battery overnight. Any light or any relay. It can also be your battery has a dead cell draining it overnight.
There is an easy way to find out what is draining the battery on your car. Disconnect the positive or negative battery terminal. Not both. Connect a test light between the post of the battery to the cable end of the battery. If your car is draining the batt ery the light will show it. To find out where the drain is coming from just pull fuses until the light goes out. Then you will know what is draining the battery by the function of the fuse you just pulled. For example, if you pulled the fuse for "radio" and the light went out you would know the radio is drawing the current off your battery.
you can avoid draining the good battery
The brake light switch is broken or out of adjustment. Park the car and pull the fuse to keep the battery from draining dead until you can get it fixed.
many things can do it suck as junk battery, junk alternator , or leaving anything on in the car
you possibly have something draining the battery when the car is shut off, with the car shut off pull the positive cable off the battery and then just touch the cable to the positive post of the battery and see if you get a spark, if you get a spark you have something electrical draining the battery.
Putting the lights on, listening to the radio or anything else that is plugged into the car.
your alternator ain't charging your battery or a bad connection draining the battery
As far as my experience is concerned, car batteries lower down either because of lack of battery water or due to not properly charging the battery. Check out whether these problems may not be occurring with your battery or consult with an auto electrician or you can know more about this from the following link. --------- You may want to check that your battery has a load on it, which is to say that something is on and draining your battery. Check to see if the glove box light,trunk light or underhood light is staying on. A simple test is to make sure the battery is fully charged, either by driving the car or charging the battery for a few hours. Then disconnect the battery overnight. If the battery is strong when you re-connect it the next day then the battery is not at fault. You have a "draw" somewhere. In other words - something is draining the battery.
If your battery is good and you know your car is draining it then yes, disconnecting your battery from your car will keep your battery charged for much longer than 3 days. Your battery should stay charged way longer than 3 days in normal conditions being hooked up to your car. You should get it checked out and figure out what is draining your battery so fast.
When your parking light stays on after you shut off the car, it is likely that the relay on that circuit stayed on. Disconnect the battery for 10 minutes or so and then reconnect it.