I have a TroyBilt (MTD made) riding mower and the battery is dead
after only 4 summers. I figured I used it no more than once a week for
no more than 25 weeks a year. If I start engine twice each time I cut
the lawn, that's 50 times a year and 200 times in 4 years. Compared to
a car, it's about 3 and a half months of worth of starter usage. My
question: which is the main cause of very short battery life?

1. Lawn mower batteries are not nearly as well made (1yr warranty vs
6-8 yrs for auto batt.), or
2. The mowers don't charge batteries properly, or
3. Lack of use in winter drains and damages the battery, or
4. Something else?