battery dies 2002 focus zx3

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I have a 2002 focus zx3 and I was going to drive it to work the other day and it would not start due to a dead battery. The battery is a Motorcraft, new within the last year. I have more vehicles than just the focus and usually only drive it once a week on the weekends as my "good" car. I havent had a problem with the battery dying until now. I cant help but think that only driving it once a week has part to due with it.

Could there be something drawing battery power due to its failure?
My guess would be my stereo amplifiers somehow drawing too much battery power while sitting for a week. But why just now?

Any input would help. thank you

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