All the Weller cordless irons listed now are butane powered, is yours battery operated? If so, it should have the voltage printed on it. Perhaps your iron is butane powered and you are confusing the cylinder hole with a battery hole?
Most of the small battery-operated soldering irons use a pair of 1.2v NiCd (sub "C" cells) batteries in series for an approximate 2.4 v (close to 3v) iron element.
Those battery-operated irons were always pitiful. Next to zero capability for soldering connections of any size at all and very short battery life. Isotip was the forerunner of the design and Radio Shack produced a clone for a time and also sold replacement tips.
If you need cordless soldering, butane is the way to go. Lots of heat for heavier jobs.