I have noticed the battery in my tractor wasn't as strong starting the tractor recently, so I checked the battery in the only meaningful way, with a battery load tester. Here is what the battery load tester showed. It showed the battery tested fine with 12 volts while setting there but the moment I exposed the battery to a 10 second load, it showed the battery was not at full strength.
This battery is 25 months old and has about 400 hours of engine running time. With winter coming, its time for a new battery.