Here are some methods to try to recover permanently sulfated batteries:

• Light Sulfation
  • Equalize the battery.
  • Apply a constant current at 10% of the Amp Hour capacity rating for 48 to 120 hours, depending on the electrolyte temperature and capacity of the battery, at 14.4 VDC or more, depending on the battery type. Cycle (discharge to 50% and recharge) the battery a couple of times and test its capacity. You might have to increase the voltage in order to break down the hard lead sulfate crystals. If the battery gets above 45° C then stop charging and allow the battery to cool before continuing.
• Heavy Sulfation
  • Replace the old electrolyte with distilled, deionized or demineralized water, let stand for one hour, apply a constant current at 10% of the Amp Hour capacity rating at 14.4 VDC until there is no additional rise in specific gravity, remove the electrolyte, wash the sediment out, replace with fresh electrolyte (battery acid), and recharge. If the specific gravity exceeds 1.300, then remove the new electrolyte, wash the sediment out, and start over from the beginning with distilled water. You might have to increase the voltage in order to break down the hard lead sulfate crystals. If the battery gets above 45° C then stop charging and allow the battery to cool down before continuing. Cycle (discharge to 50% and recharge) the battery a couple of times and test capacity. The sulfate crystals are more soluble in water than in electrolyte. As these crystals are dissolved, the sulfate is converted back into sulfuric acid and the specific gravity rises. This procedure will only work with some batteries.