Sulfation in batteries

Sulfation in lead acid batteries refers to a condition when the battery is not able to hold any charge.  It occurs when the plates of a battery get hardened with a layer of lead sulphate. 

Sulfation occurs when the battery is kept unused for long periods of time in the discharged state.  This makes the lead sulfate in the plates to get hardened.  These hardened plates prevent the battery from charging.

The lead acid battery consists of an anode made of lead oxide and a cathode made of lead.  These two electrodes are placed in an electrolyte of sulphuric acid.  When the battery is discharged, the sulphuric acid reacts with the electrodes which are transformed into lead sulphate.  As a result, the sulphuric acid in the electrolyte becomes dilute and almost becomes water.  The specific gravity of the electrolyte thus drops.

When the battery is recharged, the lead sulphate in the cathode and anode is converted into lead and lead oxide respectively.  The sulphate in the electrodes react with water to again form sulphuric acid. 

When the battery is kept unused for long periods of time in the discharged condition.  The lead sulphate which is in the electrodes solidifies into a layer which has high electrical resistance. 

When the battery is recharged using an external supply, this hardened layer of lead sulphate prevents charging.  The electrolyte too, does not become sulphuric acid.  Thus the battery is not able to absorb charge. 

Another reason for sulfation is incomplete charge and discharge, When the battery is not charged capacity and  not discharged completely, a small amount of the sulphate always remains in the electrodes.  This forms a hardened layer and diminishes battery capacity. 
Identifying Sulfation in batteries

If your battery is connected to the charger for a long time and still does not have charge.  Test the specific gravity with a hydrometer.  If the specific gravity is low, the battery probably has the problem of sulfation.

Repairing Sulfated batteries

Sulfated batteries can be repaired by applying a high charging voltage and low charging current from the battery charger. 

When batteries are to be kept unused for long periods, they can be connected to a battery minder, an electronic device which continually monitors the voltage level of the battery and prevents sulfation from occuring by firing electronic pulses into the battery to break any sulphate layer which may have formed.