French researchers have announced a prototype of an 18650 sodium-ion battery ... By comparison, a typical lithium-ion battery has around 110 Wh/kg and tops out at about 1200 recharge cycles.

So [Travis] decided to upgrade it to make use of the ever-so-common, 18650 lithium-ion battery. Not looking to ... retrofitting flashlights with LEDs check out this switch-mode driver board ...