14500 vs. D Battery

A 14500 Li-ion cell measures 14mm in diameter and 50mm in length -- the same physical size as a standard AA battery -- but delivers 3.7V nominal rather than an AA's 1.5V. Because it fits mechanically into AA-sized battery holders, mixing it up with a real AA can destroy 1.5V-only circuitry.

A D battery (IEC designation LR20, ANSI 13A) measures 34.2mm in diameter and 61.5mm in length -- the largest common cylindrical primary cell. Alkaline D cells output 1.5V nominal, with typical capacity in the range of roughly 12,000 to 20,000 mAh depending on discharge rate.