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Numbers sound about right. You neglect to say how often the car is driven. Batteries have a self discharge feature, nature of the beast, add some constant discharge features like clock, misc other items and you have a battery killer. Get a battery "maintainer" they monitor the battery and when it draws down to a specific voltage it trickles it back up, it is different than a trickle charger. I have all my vehicle batteries on maintainers, with the exception of, oddly enough, the Porsche, which is the daily driver. My last motorcycle battery lasted 10 years.
eric