I bought a cheap little air quality meter recently. It has a power socket and battery compartment on the back. The manual says "DO NOT PUT BATTERIES IN THE BATT

I bought a cheap little air quality meter recently. It has a power socket and battery compartment on the back. The manual says "DO NOT PUT BATTERIES IN THE BATTERY COMPARTMENT, THEY WILL NOT WORK". I would guess they reused the case from a previous product that supported battery power, in order to save tooling costs.