We have a Toyota MR2 spyder that we drive 8-9 months out of the year. The other months it’s winter and it’s in the garage. We try to atleast start it once a month but every spring, we head back to Sears and get a replacement battery because it’s dead and won’t hold a charge. We never had a “toy” before that couldn’t be driven year round so is it the fact that it’s sitting and we’re not driving it or is something draining it?
When parking a vehicle over a period of time, it’s good idea to disconnect/remove the battery and connect a battery maintainer to the battery to keep it charged.
Tester
There is constant drain on the battery. The radio, the car’s computers, and the security system (if there is one) require constant battery power. Starting the car for a few minutes once in a while isn’t enough to recharge the battery.
Disconnecting and removing the battery is one option, as Tester suggested. Another is the use of a Battery Tender to keep the battery fully charged over the winter.
If you hooked up a Tender every few weeks and allowed it to bring the battery to full charge your Mister Two will start and the battery will last for years instead of months.
A Battery Tender Jr., which is what I use, costs less than a new battery.
This is the guide for winter storage to the extreme: http://www.miata.net/garage/winterprep.html
Do as minimum as Tester says.
The radio memory alone will draw down a battery given enough time. Staring once a month is not a good idea.
Completely discharging a lead acid battery can kill it. I take the battery out of my Miata every winter and trickle charge once a month. Year 19 and still on my second battery.
A auto battery is not meant to be totally discharged. If the battery is discharged and not FULLY recharged promptly the components inside the battery deteriorate quickly, shed material from the plates that collects in the bottom of the battery and shorts the battery out completely, then it is dead.
Starting the car once a month may not help. When you start the car you are draining the battery, since the starter on the car is the single biggest user of energy. After starting if you only run the car for a few minutes, and that is just with the motor idling, the battery is not charged up to the level it was before you started it. A month later the battery is still weaker, you start and run a minute or so and shut it down, less charged than before. By the end of the winter you have a dead battery.
While your car is parked there are active circuts in the car that use a small mount of power; clocks, security systems, etc. Over time, like months, this small current draw reduces the power in your battery. Just siting a battery will lose some of its charge. Best solution was already given, disconnect the battery and hook up a small charger to maintain the battery at full charge, one product is marketed as the “Battery Tender”.
It may work to hook up the battery tender while the battery is still connected to the car. I’ve done this with motorcycles, but they are not the same as a car, so consult the instructions that come with the product you buy.
You also could buy a standard battery charger and hook it up the car’s battery monthly for a few hours. The instructions with the charger should tell you how to determine your battery is fully charged. These devices will pay for themselves the 1st year you don’t have to buy another battery. Properly handled a battery in a vehicle like yours that is stored for a few months a year should last a normal life of 5 years or so.
Also, before you park the car for the winter add a fuel stabilzer to the gas as directed on the container. Stabil is one branded product. I usually add it as I fill up the tank with my winter gas (I have a “toy” that I don’t drive in the winter also). Then the stabilized gas is pulled from the tank into the engine as I drive home. Modern gas can start to break down in just a few months of storage and this simple step will save you from a bad gas experience.
I could see your battery still starting the car but I wonder if it still can meet the ratings it did when new.
Its an OEM battery, although not sure if its like the original AGM type. It only takes a few minutes for the charger to go green and stop charging. The car is only run for about 25 minutes at a time and mostly WOT. Don’t jinks me! 
As Tester said. Disconnect it, and I suggest also removing it from the car and bringing it to a different location with that battery tender. With the battery stored away from the car, it will make the car just a little more secure, and harder to steal.
BTW starting it like that while in storage is certainly no better than not starting it at all and likely is harder on the car.