Why do lead-acid batteries fail after about five years of operation?
Monitoring specific gravity and electrolyte levels can lead to longer life with periodic full equalization charging.  Pulse charging can also reduce sulphation rate but not replace need to periodically equalize cells with full charge voltage. ( e.g. every 6 mos)  Ambient temperature rise greatly accelerates sulphation.

https://googlegalaxychemistry.blogspot.ca/2017/03/leadacid-battery-preventive-maintenance.html#content

http://batteryuniversity.com/learn/archive/can_the_lead_acid_battery_compete_in_modern_times