this post was submitted on 02 Apr 2025
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Powerbanks are where it's most problematic. They're usually reporting the capacity of the battery cells in mAh. Those cells will be at 2.8-4.2V during operation, but the powerbank outputs 5V, or in modern powerbanks some higher number. 5000 mAh at the 3.6V average of the cells during discharge is certainly not 5000 mAh at the 9V it's giving to my phone.
It's not going to give my phone 2000 mAh @ 9V or 18 Wh as the math would suggest either because it's well below 100% efficient. I'm not sure what's reasonable to demand in terms of advertising here since efficiency will vary with output voltage and output wattage.
Yeah, and my newer powerbanks all do PowerDelivery for 5, 9, 12, and 20 V.
I'm assuming watt-hours would be universal for them all (watts are watts, as the saying goes).
Well... sort of.
Batteries perform differently under load. A battery that delivers 10Wh under a 1W load will probably deliver less (and get warmer) under a 10W load. Power supplies also perform differently under load, and DC-DC switching power supplies perform differently based on the output voltage. Generally, a larger voltage conversion and/or a higher load is less efficient. There's also going to be some base power consumption in the circuit, so the most output power is probably achieved at some sort of medium load.
To make things more fun, batteries are usually tested under constant current, not constant power. The increasing current as the battery drains of a constant power load will result in less total power, and constant output power often means increasing input power as the battery drains.
In short, the real world is complicated. Giving best and worst case Watt-hours could be a reasonable approach.