@stfn Ah Is an equally stupid unit. If I have a 6v 6Ah battery, and a 12v 6Ah battery, I have very different quantities of energy stored in them.

@quixoticgeek Actually, that is very true. I recently came across this issue when I wanted to calculate the difference between a 5v 20Ah powerbank and a 12V 22Ah lead acid battery.

@stfn I just put 20Ah of energy at 5v into a 26800mAh battery pack. Which has a nominal internal voltage of 3.7v. How many of the 26800 nominal mAh did I use ?

@quixoticgeek Yes.

@stfn exactly. If this was done in terms of Wh... It would make things usefully comparable

@quixoticgeek let's see how it turns out:

22Ah battery at 12V: 22A h* 12V = 264Wh
26Ah powerbank at 5V: 26Ah * 5V = 130Wh

Ok, a bit of a difference. But now I am starting to think that actually we should not compare 12v cells to 5v cells, it's apples and oranges.

@stfn except if your power bank outputs 5v usb, it probably has 3.7v lithium ion batteries on side. So that means that a 10000mAh 3.7v battery is actually 37Wh. Where as if the battery was actually 5v, it would be 50Wh. This is also why "my phone draws 1000mA, this is a 10000mAh pack, I should be able to run it for 10 hours" suddenly fails.

Comparison between a 12v and a 5v source can be useful "I can get a 500Wh 12v battery for half the cost of a 5v one, how much do I lose in conversion?"

@quixoticgeek so the bottom line: it's hard to say what is the actual capacity of a battery

@quixoticgeek@social.v.st @stfn@fosstodon.org This is why I tend to think of my USB battery packs in terms of their (measured) capacity at 5V. Except that falls down with the ones that support higher-voltage charging.

TBH, if you're not actually designing things with batteries, watt-hours are more generally useful.

@kim @quixoticgeek actually I am right now building my own power bank, tomorrow there should be a blog post ready on it :)