I’m guessing your battery sits idle throughout the warmer months without any use. Batteries will only reach their designed life span if they are active.
The typical life of a snow mobile battery should be 2 -3 years if properly maintained. If the battery is routinely operating a snowmobile and has a charge running through it during the off season, then it is active and can maintain itself.
If the battery sits there for months on end, it will discharge and start to break itself down to the point of no return. A smart/ trickle charger will keep your battery from going completely dead and having to be discarded. Many of the new trickle chargers can be hooked up to the battery and forgotten.
Perfect for snowmobiles, since they are typically seasonal vehicles. A smart charger will automatically sense when to deliver a charge, shut off and turn back on in order to keep the battery maintained and at a maximum charge.
Most smart chargers also DE sulfate the battery; this keeps sulfur from building up on the internal lead plates and ending the batteries ability to maintain itself. The newer smart chargers typically come with both alligator clips and a ring adaptor with a quick disconnect. Batteries that are only partially charged or stored dead will fail much quicker than a battery that is hooked up to a charger.