Rechargeable Fans and USB Gadgets

Reading mAh, Wh and runtime

Battery capacity appears as mAh on some listings and Wh on others. Milliamp-hours compare fairly only at the same voltage, while watt-hours equal volts times amp-hours and work across voltages. Divide watt-hours by the gadget's watts for a best-case runtime: a 5 W fan on a 5 Wh battery lasts about an hour, and real life is shorter. Output ratings like 5 V at 3 A mean 15 W available, though a device draws only what it needs.

Power supplies and alarms: match voltage and connector

Many listings here are accessories rather than fans: power supplies, cable kits, splitters and detectors. A supply must match the device's voltage; 5 V gadgets take USB power, and a higher voltage can destroy them. For computer boards, the amps matter too, and a 5 V 3 A USB-C supply is typical for a Raspberry Pi 4B. Check connector names such as 24-pin, RJ11 or Lightning. Smoke and CO alarms have a replacement date printed on them.

Questions about Rechargeable Fans and USB Gadgets

What is the difference between mAh and Wh?

Milliamp-hours measure charge, so they only compare fairly between batteries of the same voltage. Watt-hours measure energy and work across voltages. Divide the watt-hours by the device's watts to estimate runtime.

Can I charge a 5 V gadget with any USB charger?

Usually yes if the charger outputs 5 V, since the device draws only what it needs. The charger's amps should meet or exceed the device's requirement. Never use a higher-voltage supply unless the device states that it supports it.

How should I look after a combined smoke and CO alarm?

Test it monthly with the test button, change batteries when the unit says so, and keep vents clear of dust. Replace the whole alarm by the date printed on it, because the sensors wear out.