Either the battery has a dead cell, which will normally drain it overnight, or something is on pulling power from the battery. If the battery has a dead cell it will have to be replaced. The easiest way to determine this is to fully charge the battery and then disconnect the negative cable overnight. If the battery is dead in the morning it has a dead cell. You may not even be able to charge it with a dead cell. If the battery is still fully charged the next morning then you have another problem. Check all the lights on the vehicle. Under hood, trunk, dome, curtorsey, glove box, etc. One of them may be staying on. Also check your brake lights. If you find no lights are on then you may have a relay that is sticking. Can be power windows, locks, seat, etc.
Either a short to power or a bad fan switch.
When replacing battery, when servicing some part of the engine, or when the vehicle is not going to be started for over one month.(keeps the radio memory from draining battery)
Probably, with that age of car, it's a wrecked battery. The only last 4-5 years, on average.
Your alternator is shot.
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.
you can avoid draining the good battery
Check the dome light. These cars are notorous for having a bad door switch.
Need more info... How old is it? Do you drive short distances? Does it sit around not driven a lot? Check for a short, bad alternator, or fried battery. If you take the car to an alternator shop, they can usually tell you for free if it's the battery or the alternator.
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.
If the parking lights are on when the car is off its probably a switch issue.
many things can do it suck as junk battery, junk alternator , or leaving anything on in the car
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.