Have you considered Gel cell type batteries, I have 2 Rods with Odyssey batteries in them, and (unfortunately) sometimes go for weeks - months without being started. One would be 10 years old & never let me down, even when you have to crank for a while to get the fuel up to the carby. The only downside I have found is they don't like a bit of drain like if you leave stereo on while stopped they got flat quick & wont charge up from car, have to be overnite charged. Maybe that's because they are only small in size. I haven't heard if people putting these in regular cars, but I cant see why not? Anyone heard of this being done?