I have a 3 year old John Deere garden tractor and a 3 year old battery installed in it. Not having used it for 4 months it was no surprise that the battery was totally dead and so I charged it today. Within 5 minutes, the 'fully charged' light on the charger illuminated! Not a good sign but I decised to try and start it. There is now definitely some life in the battery because the lights of the tractor work but apart from a highish frequency buzzing under my seat, there was no attempt to turn over the starter. Is this a battery that has essentially died on me? I suspect that this is the case and that there are just a few cells remaining that cannot produce enough charge to turn the starter. My concern is that if I replace the battery, will I be wasting money on a battery which did not need replacing? Advice will be welcomed.
shrtrnd has great advice. Check the voltage and see what it does.
It's most likely a Lead-Acid battery, and they like to go dead like you described commonly by either drying out, or sulfating. The end result is the same. Less surface area of the plates in the battery making contact with the solution. It may still 'charge' and say 12V when you measure it resting, but as less and less surface area of the plates becomes available, it's similar to trying to start the tractor with a tiny battery xD
There are methods to 'repair' or partially recover a battery like this, but it's ill advised. If you want to prepare for a zombie apocalypse, it would be good to know. Otherwise, buying a new battery is ideal.
(You may need a friend to help you measure the voltage under stress... When the lights on the tractor turn on, you will notice the voltage drop. If it drops a LOT, then it's a dead battery. If it stays pretty stable, then try turning on more accessories on the tractor to see how it holds up. If the voltage still stays pretty stable, then you may have another problem on your hands)