What happens during discharge
Lead sulfate forms on the plates as a lead acid battery discharges. Normal charging reverses much of this process. Extended time at a low state of charge can encourage more persistent sulfation and reduced performance.
What you may notice
A sulfated battery may accept charge poorly, show reduced capacity, or struggle under load. Those symptoms can have other causes too, so diagnosis matters.
Can sulfation be reversed?
Some charging equipment includes reconditioning or desulfation modes intended for suitable lead acid batteries. Results depend on battery condition and design. Physical damage and advanced deterioration are different problems.
Prevention is usually easier
Use a charger suited to the battery, avoid leaving a lead acid battery deeply discharged, and follow storage guidance from the manufacturer.
For the bigger picture, see battery reconditioning basics and our battery safety guide.