Why a 1.2V cell works in a 1.5V device — until it does not

An alkaline AA leaves the factory at about 1.5V and spends its life declining. A Ni-MH cell sits near 1.2V for most of its discharge and then drops sharply. Averaged across the discharge the two are closer than the labels suggest, which is why Ni-MH works in most devices designed for alkaline.

The failure case is a device that cuts off high. Some electronics stop at 1.1V per cell to protect themselves, and a Ni-MH cell spends its whole life uncomfortably close to that line. Others read voltage as a fuel gauge and will show a fresh Ni-MH cell as half empty.

A regulated 1.5V lithium cell removes both problems by holding 1.5V until it stops — which is also its own quirk, because the device gets no warning before it dies. Pick according to which behaviour your product can live with.