Mine is a AGM battery (Absorption Glass Matt) - has all the plus points of the gell battery (long life , almost indestructible deep discharge etc) but no need for special chargers (unless you have one) they also deliver a massive amount of power for their size, you'll find them under the bonnet of most new cars with Stop Start technology to take care of the constant need to run the starter motor up!!
Not cheap but VERY good unit