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Assuming the battery and charging system are perfectly good it's time to go back to basics. If your battery is draining overnight it'll be because something is still switched on and drawing current. Without wishing to give anyone a lesson in ohms law the time it'll take for the battery to discharge will depend on the resistance of the component through which the current is being drawn, which will determine the amount of current drawn. Amps = Voltage ÷ Resistance.
Connect an ammeter in line with the negative battery terminal and negative battery post. Have your probes or clips ready so that the circuit is never broken. This can be a bit fiddley but by no means impossible. Once the battery terminal is off and the ammeter in place the amount of current drawn will be shown on the display. Make sure your test leads are long enough to enable you to close the bonnet and still see the ammeter. Then lock the car up and wait. After a few minutes the ammeter display should show only a few milliamps which will be things that need a permanent battery supply. If it's more then you have an unacceptable drain which will discharge the battery over a period of time.
The amount of time can be calculated using the battery's amp hour (A/h) figure. If for example your battery is rated at 50A/h it means that the battery can be discharged at a rate of 50 amps for one hour, or 1 amp for 50 hours, or multiples thereof. Bear in mind that this calculation would be a based on a bettery that was in perfect condition and has a state of charge of 100%, and then be completely discharged down to 0%. In reality you will be unable to start the engine long before the battery gets anywhere near this discharged.
If you do have a drain you'll need to remove one fuse at a time until the drain disappears. When it does you'll have to isolate components on that circuit until you find the problem. A long process, and often head scratching.
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