I've got 3, 4 and 5ah batteries. Have been using 3 and 4ah batteries on my 1/2" gun, drill and grinder for over 3 years now. Broke the drill a few weeks back (sadly over 3yrs old so out of warranty) so upgraded to the new Fuel model with 5ah batteries and fast charger. What a difference in power with the 5ah! Most noticeable when using the grinder.