On my flashlight that use 3-AAA batteries, I usually modify them to use an 18500 Li-ion battery. These are a nominal 3.7V, have high capacity and are rechargeable. Be sure to use the Protected Li-ion's with the little PCB on the negative side. This is to prevent over -charge and over-discharge.
Alkaline AAA batteries have a typical capacity of 1000-1200 maH. But a 18500 has up to 1500maH capacity.
In order to perform this battery modification, a sleeve must be installed. I machine my sleeves, but paper or cardboard would serve the same function.