Car batteries will self discharge when not in use. If it is connected to your car it could also have a small load from accessories or car computers that will draw current from the battery which also aid in draining the battery when the engine is not running. In general an AGM, or lead-acid car battery self discharges at a rate of three percent per month. This rate increases when ambient temperature exceeds 20 degrees Celsius (68 degrees Fahrenheit). The more cycles a lead acid car battery has experienced will also start to reduce the capacity of the battery and eventually even slow discharging rates or small loads could cause the battery to go flat.
Bad cables causing short circuits and bad alternators which don’t effectively recharge the battery are sometimes the culprit as well. If you suspect a bad alternator you should take your car to a car battery specialist or a mechanic that can test the battery and check for parasitic drain.
