Sulfation is the mechanism for battery damage in lead-chemistry batteries during a Partial State of Charge (PSoC). Sulfation refers to the normal process of lead sulfate being deposited onto a battery's lead plates during discharge. When charged appropriately, this "soft" sulfate dissolves back into the electrolyte to begin another cycle. However, if the sulfate is allowed to stay on the plates too long or at a too-low voltage, it can become "hard" sulfation which damages battery capacity either for a long time or permanently. An analogy for sulfation is a pot used to cook oatmeal: if rinsed soon after cooking, it is easy to clean off, but if the oatmeal sits on the pot overnight it can turn into a concrete-like coating.
Rolls Battery explains the normal process of soft sulfation like this: “In normal use, battery plates are getting sulfated all the time. When a battery is being discharged the lead active material on the plates will react with the sulfate from the electrolyte forming a lead sulfate on the plates. When there is no lead active material and or sulfate from the electrolyte remaining the battery then is completely discharged. After a battery reaches this state, it must be recharged. During recharge, the lead sulfate is reconverted into lead active material and the sulfate returned to the electrolyte.[1]”
To avoid hard sulfation and dramatic damage to battery capacity, fully charge the battery to 100% SoC as often as possible. Daily is optimal.