14500
Direct Answer
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.
Source: IEC 61960
Specs
| Diameter | 14.00 mm |
| Length | 50.00 mm |
| Nominal Voltage | 3.70 V |
| Chemistry | Lithium-ion |
Same Size, More Than Double the Voltage
The 14500 takes its name from the same IEC cylindrical-cell convention as the 18650 and 21700: the first two digits are diameter in millimeters, the next two are length in millimeters. At 14mm by 50mm, it's built to the exact same envelope as a standard AA cell -- which means it physically fits into any AA-sized battery holder, even though its 3.7V nominal output is more than double a real AA's 1.5V.
That overlap is a genuine hazard: a device designed only for 1.5V alkaline or NiMH AA cells has no reason to guard against overvoltage, so an accidentally inserted 14500 can damage or destroy its circuitry. As with any lithium-ion substitution, matching voltage and chemistry -- not just physical size -- is the safe rule. Published capacity figures for 14500 cells vary widely by manufacturer and are prone to inflation relative to what the cell's small volume can realistically hold, so treat any specific advertised mAh number with some skepticism.