because you have an open electrical line somewhere behind your radio.
The alternator may not be recharging the battery .
Your alternator is not recharging the battery and is defective.
The belt turning the alternator which is recharging the battery.
see this link How_can_you_tell_if_it's_your_battery_or_alternator_that's_bad
Your alternator is defective and not recharging the battery. Have the alternator checked.
It sounds like your alternator is no longer recharging your battery. You probably need a new alternator.
Possibly the alternator is defective and not recharging the battery. Or something is on, pulling power from the battery when the car is not running. Can be any light or any relay that is stuck.
My alternator stopped recharging my battery but I can't take the tension off my serpentine belt to put my alternator back on.
Suspect low battery reserve (almost dead), alternator may not be recharging battery either due to low battery level insuffucient to energize alternator field windings or alternator/wire fault probable causing low battery in first place. Battery volt meter drops because alternator not recharging, Flasher speeds up when load drops through flasher, engine stalls because insufficent reserve on battery to run engine electronics. If battery ok to start motor: then suspect fault/connection in power wire to engine bay electronics or alternator. initial starting point from my diagnostic point of view
Dead cell in the battery, defective alternator not recharging the battery, or something is on pulling power from the battery. Dome light, glove box, under hood, trunk, brake light, any light that is on. Also a stuck relay can drain the battery.
While the car is running the alternator should be recharging the battery faster than stereo can drain it. I suspect you either have a problem with the alternator or its time to get your battery replaced. Until you get it fixed, if you don't turn on the radio, it wont drain your battery :) If it is a high power model, it needs to be wired directly to the battery.
Recharging a dead battery by using jumper cables connected to another vehicle is a very bad idea. You are overworking the alternator on the donor vehicle and can very well destroy that alternator. Recharge a dead battery with a battery charger and never with another vehicle.