The alum reacts with lead sulphate creating soluble salt thus removing sulfation immediately. The similar method was used long time ago with a mix magnesium salt and alum. That what I learned. I think it is reasonable explanation.
http://ysuusy.com/Lead_Acid_Car_Battery_Repair.html
http://ysuusy.com/Lead_Acid_Car_Battery_Repair.html
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