Battery issue

[yukonaetahoe]

is there something i can buy to check whhy my battery is being drained?
i dont see any lights on at all and i already disconnected my music and still the battery dies, i know it could be a bad battery but id hate to go buy another yellow top for n arm n a leg and find out its not even that.
how about is there shops that actually do that? ive asked at aoutzone n kragens and ofcourse all they say they dont know shit.

 

[blueflamed03]

you can have the alt tested, and the battery to see if when given a 'load' it doesn't drop off. I know sitting voltage, it may read ok, but when given a load, like under a start, it drops to like 9v. All external stuff like amp,etc, pull from the battery lead, and get a volt meter with vehicle off, to see if it drops over time.

Example, sitting, off, no lights or anything, reads 12.5v. The 20 minutes later, see what it reads, then 20 minutes more...