Not wrong, but "a few millimeters" means a lot more for a 14500 light than for an 18650/21700 light. While many may think the ~8mm difference in length between the D4V2 and D4K isn't much, it's enough for some folks (myself included) to sacrifice the runtime.

More importantly, it's harder to accommodate a wider range of lengths without getting some rattle on shorter cells. Not an insurmountable problem, but definitely a consideration. The range required to take protected/USB-charging cells is a bit wider; >10% of the cell length, and a much larger percentage of the travel length of the spring. Dual-spring helps by splitting that across two springs, but has limits.

[–] Zak 1 points 1 year ago

I don't think I've seen a protected 14500 over 52mm, unlike some of those 21700 abominations (78mm for the Acebeam 21700+powerbank).

2mm seems worth it to me for broad compatibility in 14500 size lights, and some popular 14500 lights like the M150 are already pretty omnivorous with batteries. The m150 accepts 52mm protected cells, protected 14430s, and flat-top 14500s.