One of the hazards of long-term storage of a lead-acid battery is a condition called sulfation, which can doom the battery to untimely failure.
Sulfation results from prolonged periods of battery inactivity, where the charge level slowly decays or from chronic undercharging—as when the bike’s charging system is not delivering the necessary voltage to maintain a full charge. It can destroy a battery to the point where it will take no charge and provide no energy.
In normal use, some sulfate crystals form, but this is not the problem. During prolonged undercharge conditions, however, the lead sulfate converts to a durable crystalline deposit on the negative plates in the battery. This leads to the deposition of large crystals, which reduces the battery’s active material, and as a result, its capacity.
Sulfation may be reversed if it is discovered early and a period of proper over-charge is temporarily applied. However, in cases where the battery has been in a partially discharged state for an extended period of time, such as when a battery is left in a bike during the winter months, the sulfation won’t be reversible and the damage will usually be of a kind that reduces the battery to recycling material.
Sulfation can be prevented by keeping the battery at full charge when it is not being ridden, whether the battery is left in the bike or removed and kept on a battery tender, periodically put on a charger to full charge, or even if the bike is started and runs long enough to maintain the charge with the bike’s charging system. This last option is an uncertain process since it's difficult to know for sure that full charge has been achieved and getting the bike started in cold weather may do more harm in draining the battery than running the bike can compensate for—assuming it starts. Consult the manufacturer’s specifications to determine the proper voltage for a full charge.
The accompanying images show a badly sulfated battery (left) compared to a normal, operational battery. The sulfate crystal deposits are visible as opaque, white material completely obscuring the view of the battery plates.