Specialty Batteries
Rechargeable cells that do not fit a tool: 9V packs for multimeters and smoke detectors, AA and AAA lithium cells for remotes, mice and small fans, and replacement packs for robot and stick vacuums. Capacities range from an 800mAh Ni-MH AA cell up to a 5500mAh 9V pack with a USB port built into the case.
These are the cells people forget until a device dies at the wrong moment. Lithium AA and AAA cells hold 1.5V where a NiMH cell sags to 1.2V, which matters in devices that read voltage to guess their charge level. A 9V with built-in USB removes the dock from the equation entirely, which is the difference between a battery you charge and a battery you forget.
The 9V 5500mAh USB pack is the one most people land on. Start with the 9V shortlist if you are replacing smoke detector cells. Vacuum pack replacement covers the connector and mount traps, and 18650 cell selection is the deeper read if you build your own packs.
