#2 common mistake during a stereo install
-EU (end user) forgets to hook up the ''remote'' terminal to their deck so the power amp shuts off when the car is turned off.
You may also want to invest in a capacitor for the system too, as it can hold enough charge to start your car in case the battery is dead.
I learned this the hard way too, except for the longest time i didn't know there was such a thing, and just had an on/off switch for my power amp heheh. Luckily my battery would last for 3days max if it was left on
you should have no trouble recharging the battery, just get a boost, drive around for a bit while the alternator recharges the battery fully.
(#1 common mistake is to do a shabby wiring job, leaving bare copper to come into contact with your carpeting, and thus the result is FIRE!!!! i see burnt up cars on the side of the highway all the time because of this heheheh)