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.