The original battery in my '85 380SL lasted for 15 years. The replacement battery is going on 9 years and it was stripped out of a wrecked M-B that my FD was using for Hurst Tool practice. 15 was almost a record for me as I got 16 years out of a Sears 36 battery in another vehicle. Boat batteries are not pushed as hard - I replace the older one of the pair every three years thus never having a battery older than six years in service, since I do not want to chance a battery failure on the water.