> when you're building something and you control all the variables, its easy to say the voltage is fixed
No, my point is I don't control that variable. I can't get 14.4V microcontrollers, nor 6.5V batteries. Neither can you. I am obliged to power it with 3.280V/0.1-150mA regulated power from 4.2/3.7/2.8 max/typ/min aluminized pouches of wet wipes. Wattage figures therefore won't help me.
USB-PD sure runs at 5/9/12/15/20V. Off of 3.7V li-ion cells, to charge 3.7V li-ion cells. So it's clear how many times more capacity a 20100mAh power bank battery has over 5000mAh phone battery, or a series 6-cell 18650 laptop battery pack(with nominal 2600mAh capacity per cell: 9x2600mAh of 3.7V equivalent capacity).
> And circling around, you can't be certain how many hours my 10,000mAh battery pack will run a 10W load.
37Wh? 3.7 hours, _ish_. Give or take one full hour. I don't even know what capacity of an advertised 10000mAh battery at ~0.3C might be. Battery capacity depends on discharge current. And here again it's current, not discharge energy.
So after assuming a value that's not even printed on it we're not even sure within +-25%. What a great unit of measure for battery storage. When if it was just rated in Wh we'd actually know.
I'd much rather a battery say how many Wh its good for, its nominal voltage, and its peak amps. Its incredibly easy to reason how many watts a phone uses, a laptop uses, a game console uses, a TV uses, a car uses, a blender uses, a kettle uses, etc. Then its easy for me to know how long can this battery potentially run X.
> When if it was just rated in Wh we'd actually know.
No.
The capacity literally changes depending on how fast you draw.
This is especially a problem on lead acids, so they often have couple "n-hour rate capacity" figures to make estimates for your particular use cases. It's less of a problem for li-ion, where C-rates(not coulomb but alphabet cee) is used to plan for cell counts, runtime, current limits and required wire thickness(because wire thickness depends on current, yet again, not total energy through it). It's a non-issue for dry cells because it's among equipment designer's responsibilities to account for.
And that has little to do with Wh vs mAh. It'll be easier to guesstimate more precisely from mAh using experiences and implications, if anything.
> So it's clear how many times more capacity a 20100mAh power bank battery has over 5000mAh phone battery
We don't, because we don't know the pack's battery voltage nor do we know the phone's battery voltage. You're assuming they're both 3.7V, but its not listed so we don't actually know unless we tear it open. In fact, my phone is rated for 3.85V, if the pack was rated at like 3.6V or less that's a decent bit of difference. Over 20,000mAh with that voltage difference we're talking about a discrepancy of 5Wh, which for a device using about a watt means five hours of difference in run time from that tiny discrepancy. Which, as mentioned, would be a good bit closer to a realistic answer if it just listed it in Wh, because personally I don't care what its internal pack voltage is I just care about how long it'll keep my 1W device alive. And even then, theoretically this battery pack can charge my laptop as well, that laptop's battery is definitely not even close to the same voltage as that battery pack. So comparing Ah to Ah there is pretty meaningless.
I've got a battery pack here rated for 5,000mAh. Roughly the same size as my phone, huh. Oh wait, its 56V. So roughly how many times can I charge my phone's battery off this pack? Going by Ah, once! But if I just knew my phone battery was ~17Wh, and we knew the battery was 280Wh, then we can guess we'll get about 16ish charges off of it.
Would you honestly state for understanding the capacity of the above device it would be more useful if it gave some rating in purely Ah? We've got no info on its internal pack voltage, that's practically useless to us. Its input is 24V, but is that what its internal pack actually is? Who knows. Clearly 3.7, because that's what we'll assume every pack is! Then its running things at all kinds of voltages: 12V, probably half a dozen profiles of USB-PD, and 110V.
I want to charge my car and I'd like to know how long it'll take. I'll do the back conversions to go away from the useful units here so we're talking just purely amps here, and its a 170Ah battery pack. I'll be pulling 40A from the wall. So roughly four and a quarter hours to charge, right? Nope, we didn't normalize for voltage. Well, we can normalize for voltage if we're just looking at watts. So its 240V 40A = 9,600W. Its a 68,000Wh battery pack. So the answer is a bit over 7 hours.
And sure, you're absolutely right about c-rates. The total useful capacity of the battery is going to be different depending on the current you draw on it, fully agreed. But that same logic still applies if you're talking about runtime and you're doing your math purely in Ah. Running at half or double the amps doesn't necessarily mean you'll get twice or half the life, your unit of measure doesn't change that, so I don't get why you're bringing up concepts like wire thickness when discussing which is the better unit to measure battery capacity in a general sense.
Sure, if we're talking purely about you building a circuit and wanting a battery matched to the circuit exactly, matching the voltage is useful. And when you actually do have voltage fixed, Ah becomes relevant. But in the real world, voltages are all over the place. You don't know what my battery pack here is rated for, you only have a guess its 3.7V. That assumed 3.7V is different from the 3.85 my phone's battery is rated for and is way off what my laptop is, so its not a straight 1 to 1 comparison.
Oh, so you don't know. Battery voltage is an S-shaped function of state of charge. For Li-ions except LiFePO4, it's 3.7 +/-0.5V. Charging circuit takes care of that, and designers take care of current draw so as not to exceed discharge current. No, there's no "3.85V battery". No hidden 5Wh either. Yeah EVs and laptop batteries use Wh because it makes sense there. That's irrelevant(as yet).
Y'all are just bunch of arrogant dicks crying you can't divide mAh with mAh and the whole world has to do your homework just for you. No, you can, and that covers 90% of your needs. That is to say, you don't get more precise than that anyway from just sticker figures.
Now, go back to Reddit. This is more than enough Reddit-speak for a week.
I dont understand why do you supposedly know yet pretend to think there is mysterious 5Wh situation. Just makes no sense. Only one of those can be true at the same time. Same for "3.85V battery". If you know you know.
Because it's not a 3.7V pack, it's a 3.85V pack. I'm at like 78% state of charge and it's still 4.26V. It's obviously not a 3.7V pack.
You yourself even mentioned there were 3.6V packs on the market for a while. So if it is a 3.6V pack, a 10,000mAh pack would be 36Wh. This battery is a 3.85V 4410mAh 16.97Wh. If we ignored the voltage, 10 / 4.41 = 2.26x the energy. But 36 / 16.97 = 2.12x. If that 10,000 was the actual same 3.85V that's 38.5Wh. So with that, there's a 2.5Wh difference between what it would have been if the voltage did match. If it's a 20,000mAh pack, that becomes 5Wh of difference for a pack rated 3.6V versus one rated 3.85.
Which, as I've shown, it is rated for 3.85V. And you've even agreed, there were packs on the market with an internal pack voltage of 3.6V!
> Yeah EVs and laptop batteries use Wh because it makes sense there.
And I reiterate, why does it make sense there? Because you're going to juggle a lot of different voltages and amperages when talking charging and we're relying on charging circuits dealing with it. None of what I've been talking about was about getting a little 3.7V pouch to run a small microcontroller or whatever, I'm talking about what actual consumers will really experience in the wild. So I've got phone with a 4410mAh battery and a wall charger rated for 25W. About how long will it take to charge? Need more info! I've got a battery rated 16.97Wh and a 25W charger. Roughly how long will it take to charge? 17 / 25 = .68 so like about 45min. That's the kind of question most consumers are likely to encounter.
Just a whole lot of nonsense. If you want to splice some 25W charger into your 3.8754568351V battery and blow it up, go do it over on Reddit. Somewhere far enough away from here. You've done nothing meaningful to move the Chesterton's Fence.
Since you seriously seem to doubt that 3.85V batteries can possibly exist, here's some additional reading for you. But I don't know if I can change your mind with any materials, since I've shown you the actual pack for my phone, you can see the nominal rating, but you refuse it. 3.6V is a thing, 3.85V is a thing. I don't understand why someone who clearly does know a bit about batteries seems to refuse the existence of a 3.85V battery.
And sorry, didn't realize it but it's actually a 30W charger I'm plugged into right now. I didn't have to splice anything, it's what came from the factory. Or did you think all phone chargers were also 3.7V?
No, my point is I don't control that variable. I can't get 14.4V microcontrollers, nor 6.5V batteries. Neither can you. I am obliged to power it with 3.280V/0.1-150mA regulated power from 4.2/3.7/2.8 max/typ/min aluminized pouches of wet wipes. Wattage figures therefore won't help me.
USB-PD sure runs at 5/9/12/15/20V. Off of 3.7V li-ion cells, to charge 3.7V li-ion cells. So it's clear how many times more capacity a 20100mAh power bank battery has over 5000mAh phone battery, or a series 6-cell 18650 laptop battery pack(with nominal 2600mAh capacity per cell: 9x2600mAh of 3.7V equivalent capacity).
> And circling around, you can't be certain how many hours my 10,000mAh battery pack will run a 10W load.
37Wh? 3.7 hours, _ish_. Give or take one full hour. I don't even know what capacity of an advertised 10000mAh battery at ~0.3C might be. Battery capacity depends on discharge current. And here again it's current, not discharge energy.