A condition in a lead-acid storage battery in which both the positive and negative plates are covered with a hard deposit of lead sulfate. When a lead-acid battery discharges, the active material on both its plates changes into lead sulfate, which can be removed by recharging the battery. But if the battery is allowed to stand in a discharged state for a long period of time, the sulfate hardens and is difficult, if not impossible, to remove by normal recharging.