My battery keeps discharging, how can I tell whats wrong?

Automotive electrical systems can be fairly complicated, so the answer may not be evident to you without professional diagnosis. Batteries can hold a ton of power so if you aren't sure what your doing, don't. But at a low level: The engine spins the alternator which outputs electricity. The alternator has electronics inside to stabilize the electricity output and those electronics can fail. The alternator puts charge into the battery to top it off, and provides electricity to the rest of the car when running, like lights, your stereo, spark to the engine, all the sensors, etc. When you turn off the engine, the battery sits and waits patiently to be utilized with the starter motor to help the engine to start it again. The battery also continues to power a few other items like the clock in your radio, and your radio presets. It also powers the Body Control Module which can power automatic door locks primarily, or exterior camera or security systems. Typically the battery has plenty of charge to run those extra features for a good amount of time, but there will always be a limit. So if your battery tests at a parts store and passes, you either have a failing alternator and the battery is never getting a good charge. Alternators can also be tested at a parts store. Usually free but you might have to unbolt it and carry it in which isn't exactly easy to do. A simpler test with an electrical multimeter should measure 14.4v on the battery terminals when the engine is running. Or you have something drawing that charge down rapidly. Either an accessory plugged into a 12v port with constant power, or considering the age of the car, some corrosion in the wiring. A good auto shop will be able to diagnose a parasitic draw. If you have more time than money you can start googling how to troubleshoot a parasitic draw and buy a multimeter.