I am a one ringer but...
12V installation was my profession for a few years.
Lead-acid car batteries can lose their life quite easily. Each time you discharge one to the point it needs charging(or a jump) to start your car, 25% of the capacity has been forever lost. Do that a couple more times in the same week and you can kiss that battery goodbye.
Normally, your new battery will come with anywhere from a 12 to 36 month warranty. Some may have more. Better batteries are more expensive and worth the price, but still give out when over-discharged repeatedly.
I have seen many poorly planned stereo installations with enourmous current draw. Since the alternator can only create a certain amount of power, the draw for the amp must come from somewhere...the battery. These guys can toast a brand new battery in a matter of days, sometimes hours!!!
The newer your car, the more electronic devices it has operating. These devices draw current. If you have any aftermarket electronics, that would be the first place to look. For example, an alarm system poorly planned might draw several hundred milliamps. Normally, your alarm should draw no more than 100-200 milliamps. Overnight the battery is drained severely. You come out in the morning and start up. Drive to McD's for breakfast and back home. Total trip 10 mins. The battery isn't fully recharged yet, but your alarm goes back on to drain the battery for another few hours. You can guess the end result.
If your alternator is going south, your battery will try to pick up the slack until both fail.
Sorry for the long post...hope it helps
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