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Comment by amluto | Hacker News Reader

> Pre-“USB-PD”, it still kind of made sense to use mAh because we knew it was always 5v but that too has changed.

Really? How many of those devices actually had 5V batteries inside? (Are there even credible 5V Li-poly battery banks?)

Original USB (based on a quick search) specified 5V +- 5%. You’re not getting that by direct connection to a battery, and you also can’t charge the internal battery by direct connection to any common supply. Somewhere in the power brick is a switching converter.

So maybe a 5000mAh USB-A power brick can supply 5000mAh output before conking out, but I doubt it.

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> How many of those devices actually had 5V batteries inside? (Are there even credible 5V Li-poly battery banks)?

That’s a good point and revealed that I conflated battery capacity with source-able/usable power. You are correct (as far as I am aware as well) that you cannot use a lithium ion/polymer based battery chemistry and have between 4.75 and 5.25 volts output.

Thinking about it some more, the battery packs seem to represent a special case where the function of the device is to provide power and, as per your point about squeezing out maximum power, we probably need to have two separate power ratings: one for the battery and one for the usable output.

But most consumers probably don't care about the internal pack voltages or whatever, all of that is just implementation details. All they care about is how much more time that battery is going to give their device power. Which if they know how many watts their device uses and how many watthours their battery is good for, they'll roughly know the answer.

For a lot of people, all they really need to know is the watts and watthours. Everything else will just line up in the implementation. If I know my phone uses ~1W on average, and my battery is 18Wh, then I know I'll get roughly 18 hours of life on that battery. If I want to charge the 18Wh battery in two hours, I'll need one that can supply at least 10W after charging losses.

There is no 5V Li-po batteries, LiFePO4 is 3.2V. Nominal per-cell voltage of a battery depends on anode/cathode pairs. Can be tweaked, state of charge dependent, but isn't an adjustable parameter.

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