Battery won't stay charged
1. the car drain's the battery even when it is not started or the key isn't in ignicion, the car has a electrical problem, you left a light, the starter could drain it, the alternator as well, or a bad radio is draining the battery even with no key in the ignicion,
2 it's a bad battery (discharges it self by beeing counicated inside).
solucion 1. to see if the car is drainin the battery would be by disconecting a battery termina and see if discharges again, if not, then the car drain's the battery.
solucion 2. if the battery discharges even when if it's disconected from the car, then change the battery.
warning, the first cause coul'd induce to a self discharging battery. even if you changed the battery, do a litle monitoring to be sure it's not the car that is turning the battery bad. how? if you install a new battery, leave the car for a few days (i mean do not start the car) and see if the battery discharges, if so, then you have to search what is draining the battery (that could be a very costly work, time and money and complicated). or put a ''battery disconector'' so the car cannot discharge the battery.