Sulfation generally starts when the battery voltage is below 12.4 volts. It is a build up that occurs when the sulphur molecules in the electrolyte (sulphuric battery acid) become so deeply discharged that they begin to coat the battery plates. Before long the plates become so coated the battery will no longer be able to be recharged or deliver current. The causes of sulfation are numerous, some examples are:
Batteries sit too long between charges, as little as 24 hours before damage can be done.
Battery is not used regularly and has no maintenance charger.
Low electrolyte level, battery plates exposed to air will sulphate.
Parasitic drain in the vehicle is constantly flattening the battery.
Alas there are many maintenance chargers on the market, just plug the charger into the mains power and attach to your battery, this will prolong the battery life considerably and can be left on permanently without damaging the battery. If there is no access to mains power another option is to use a solar trickle charger.