Ford diesel loses charge every night but alternator and batteries test good
Parasitic drain on battery. Something is staying on when it's supposed to shut off.
Something is placing a drain on the battery then. Could be a short, a ground, or a component running when it shouldn't be. Old school mechanics would place a test light between the battery and the ground cable and see if it lights up when the vehicle was off, if it did something was placing a load on the battery. Now because of all the modules there's always some load on the battery so the modern version is to use a meter. 80 milliamps is about average for a load on the battery with a vehicle turned off. Anything higher is due to a faulty component or short. If you do get a higher reading you can start pulling fuses to see if the load goes away. If it does you've isolated your bad circuit. The size of the load will also give you a clue as to what it might be - a few amps could be anything but a lot of amps can only be a few things - like fuel pump, wiper arm motor, etc.