Batteries are like muscles- - - -if they're not exercised regularly, they atrophy. All lead-acid batteries produce current by a chemical reaction between the plate material and the acid. Negative plates are pure lead. Positive plates are lead dioxide. When they're immersed in a solution of sulfuric acid and water. and a conductor is connected between the posts, the acid combines with the plates to produce lead sulfate and an electric current. The acid reduces in strength until there is no more plate material to consume, eventually leaving nothing but water in the cells.
A DISCHARGED BATTERY IS IN DANGER OF FREEZING! Never allow a battery to run down in cold weather and leave it in sub-freezing temps- - - -it will freeze solid and ruin the cell groups.
When a current is forced back into the battery by raising the charging system voltage higher than the cells are producing, the chemical reaction reverses- - - - -the acid recombines with the water, and the lead sulfate goes back to being lead and lead dioxide. Theoretically, this reversible reaction should continue forever, and the battery would never have to be replaced. HOWEVER- - - - -in the real world, plate material detaches itself from the plate grids and falls to the bottom of the cell, where it cannot participate in the charge/discharge cycle. Vibration, like from not being bolted down, is a battery's worst enemy. Second in line of things a battery doesn't need is sitting idle, even with a maintainer in use. Unless a battery gets cycled, like turning on the headlights for an hour or so, or running the starter a few times, the cells don't go through enough of a charge/discharge cycle to stay in good condition.
When it first forms, lead sulfate is a spongy material which re-dissolves easily. If sulfate sits around a while like in a partially-discharged battery, it forms a very stable crystal which is extremely hard to dissolve. A sulfated battery will have normal open circuit voltage, but very little cranking current or reserve capacity. Does that condition sound familiar? If your truck is a "hangar queen" that doesn't get run or driven on a regular basis, you need to give the battery a little physical therapy- - - - -run it down about halfway and do a recharge with a regular battery charger, not a maintainer, or take the vehicle out for a drive and stop and start the engine a few times along the way. Also, keep an eye on the water level- - - -don't let the tops of the cell groups get exposed to air. Sulfate crystals form quickly if the plates get dry. Add clean water (never acid) to keep the cell groups full. Give that battery a little TLC and it will last a lot longer.
Jerry
