Alkaline disposable AAs generally have a starting voltage of 1.5V. In contrast, the nickel-metal hydride (NiMH) batteries we test start off at 1.2V. But as they discharge the NiMH batteries hold steady at 1.2V until they’re fully-drained, while alkaline batteries quickly lose voltage, and within about one third of their lifespan they’ve fallen below 1.2V.
Some devices, like high power LED flashlights, can benefit from the initial kick of 1.5V from disposable batteries (a flashlight might be brighter at first), but for the bulk of their lifespan their performance will be worse than rechargeable NiMH batteries. Most portable electronics are designed to function well at 1.2V, and you shouldn’t notice any difference in performance between rechargeables and alkalines. But if you’re concerned about the lower initial voltage of rechargeable batteries, we recommend buying a small pack of quality pre-charged NiMH batteries to see whether your devices will be affected by their lower operating voltage.