In my experience, battery life is extended for 2 reasons:
1. Charging voltage of the tractor generator or alternator. If it charges at 14.8 volts, you will have shorter life. I have actually changed regulators (same brand, same model) and found that one had a higher steady charging voltage than the other. 13.8v is just about perfect for a vehicle, but chances are, your tractor will be more like 14.2v. Belarus used to have a summer/winter switch, since temperature influences how much voltage you want your battery to be charged back with. I have a menagerie of tractors, and the old Case needed to be switched off, since the "one wire" car alternator put out 15volts constantly! Fine for replenishing the starting charge, but after a few minutes, I just turned the key off, to prevent boiling out the electrolyte, since the fuel shutoff was manual anyway.
2. Maintenance voltage. If you use your tractor regularly, and it doesn't charge too high, this is no problem. For occasional users, a maintainer of some kind is needed, since wet batteries self discharge. This keeps the plates from being sulfated, which happens easier in a reduced state of charge. Heavy duty batteries, like golf cart (Trojan, Deka) or Deka 12v batteries have heavier plates that can take more, put out more, and seem to suffer from less sulfation. (2 6v golf car batteries in series would make a great battery for any tractor, but they won't usually fit). A small cheap charger will work, though, if you put a timer on it, an hour every 24, for example.
Solar charging controllers that are programmable make fantastic maintenance chargers, since you can set the exact voltage you want the battery to sit at when the sun comes out. A $50 panel of 5-15 watts plus a programmable controller set to 12.9 volts float voltage, 13.8v bulk absorbtion would give you years and years of life whether or not you ever started the tractor except for the summer. The small, cheap solar controllers (around 25-30 dollars at someplace like Harbor Freight) will generally have a cutoff voltage of at least 13.8v, but will work with a small solar cell somewhat shaded, to reduce the amount of charging.
12.7v at rest, medium temperature, one hour after charger has been disconnected, or engine shut off, is 100% charged. Alternators will 100% charge a battery, but do it by slightly overcharging, and if your maintenance charger is on 24hrs a day at a similar, high voltage (14.2v), you will lose water and have to replace it. Keeping that in mind, then, you can use almost any small trickle charger, knowing that you will have to top it off more often.
