I don't use my Mazda MX5 very much - is the lack of use enough to flatten the battery?

I have a Mazda MX5 1.8 which has done 21,000 miles since I bought it new in 2005. Last week its third battery (27 months old) was completely flat. I had driven it on country roads for about 30 miles, seven days before that. So I was surprised. On the other hand, I only use it about once a week and then only for a short journey. Do you think this relative lack of use, and the fact that the battery is only half charged most of the time, is cause enough for the short life of my batteries? The MX5 takes a small battery, as I'm sure you know. Do small batteries have shorter lives? I don't use the heated seats and seldom use the heated rear window or the fan.

Asked on 6 April 2017 by Albert Fagan

Answered by Honest John
Yes. Your use of the car has taken out of the battery more than it has put back in, ultimately reducing the capacity of the battery from its original 13.5 volts to less than 12 volts, which is not enough to start the car. If you don't use the car much you need to take it on a 20-30 mile run at least once a month to keep the battery in a reasonable state of charge, or put it on a battery conditioning float charger in your garage.
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