AAA

The AAA size was introduced by the American Ever Ready Company in 1911, originally for compact devices like vanity flashlights, four years after Ever Ready introduced the larger AA size in 1907. It took until 1959 for the American National Standards Institute to formally codify the AAA format, cementing the physical dimensions -- 10.5mm diameter, 44.5mm length -- that let cells from any manufacturer interchange reliably.

The IEC uses the LR03 code for 1.5V alkaline AAA cells, alongside FR03 for lithium primary cells and HR03 for 1.2V NiMH rechargeables, all built to the same physical envelope despite their different chemistries and voltages.

Because all AAA-format cells share the same physical size, a device's battery compartment can't tell the difference between chemistries -- only voltage and current draw determine whether a substitution is safe. Standard alkaline AAA cells deliver 1.5V and typically enough capacity for low-to-moderate drain devices like remote controls and wireless mice, while rechargeable NiMH AAA cells deliver a lower 1.2V but hold up better under high-current loads like digital cameras, since alkaline cells lose a larger share of their capacity as discharge current increases.

Lithium (FR03) AAA cells also output 1.5V but perform much better in cold temperatures and high-drain devices, at a higher cost per cell than alkaline.