In this instructable i am converting a triple AAA flashlight battery power supply in one 16340 power cell.
If you ask me why, is because recharging AAA batteries takes my battery charger more than 8 hours to charge AAA batteries and their capacity (700-800mAh) is not worth the time, while 16340 is cheaper, decent in capacity, i can recharge it in one third or less of the time and of course is one and not three pieces if you want to carry a spare.
As for the voltage, three AAA alkaline batteries in series give 4.5V when they are fresh, three rechargeable AAA NiMH (or NiCd) give 3.6V and the 16340 starts at 4V so there isn't any voltage problem for the flashlight.
The triple AAA battery supply measures 20mm in diameter and 53mm in length. The Li-ion 16340 battery as it states is 16mm in diameter and 34mm in length. So in order to fit the 16340 battery in the compartment for the 3AAA power supply some modifications have to be made.
First of all i cut the one of the three plastic pieces that connecting the top and bottom of the 3AAA power supply.
Next i measured 34mm inside and trimmed using a cutter and a chisel so the 16340 fits inside.
After that i used a 3.5mm drill, drilled a hole in the negative pole and fitted inside a screw. I measured the screw and cut it so the battery can fit without force inside.
I secured the screw with a nut and put another nut at the end of the screw. I put the battery inside and turned the nut until i had good contact without stretching the plastics, and then soldered that nut.
And that is all. No more AAA batteries for the flashlight.