Sulphation
Crystallisation of lead sulphate on the plates when the battery spends time discharged. Reduces capacity irreversibly.
Sulphation is the formation of large, hard lead sulphate (PbSO₄) crystals on the battery plates. It is the #1 cause of premature death in lead-acid batteries. During normal charge/discharge cycling, lead sulphate appears and disappears reversibly; but if the battery spends days or weeks below 50% SOC the crystals grow large and insulating — they block the active surface of the plates and permanently reduce the ability to accept current. Typical causes: leaving the car parked for months without use, motorcycle batteries in winter, systems with an undersized alternator, batteries drained by vehicle parasitic loads (alarm, GPS, keyless entry).
Prevention: trickle charger if the vehicle goes more than 3 weeks without being started. Symptoms: low SOH, normal open-circuit voltage (12.6 V) but collapsed CCA on starting. Some smart chargers offer a "desulphation" mode (high-frequency pulses) that can reverse early sulphation — it only works on light cases, not on advanced sulphation.