4 ms·
...your explanation would work way better if the unit dimensions actually worked out. And they don't, since W/h is not a unit of energy, and mAh is not a unit o
by Joker_vD 3y ago
...your explanation would work way better if the unit dimensions actually worked out. And they don't, since W/h is not a unit of energy, and mAh is not a unit of "current per hour".
And your second paragraph is bogus anyhow: the voltage would be labelled on the batteries despite whether the other metrics is charge (which is what mAh is unit of) or energy (which is what Wh is unit of).
P.S. It is still beyond me why people in general don't comprehend that both "amper" and "watt" are already units of "speed", no need to divide them by time any further. Maybe we should've given km/h its own name, to ease people into such concept? /rant
- vel0city 3y agoJoules are a lot harder to reason about than watthours. I've got a 5W device that I run for three hours. How much energy did it use? Easy, 5W * 3 hours = 15Wh. How many joules did it use? 1 J = 1 wattsecond, so 5 * 60 * 60 * 3 = 54,000J = 54kJ.
- Joker_vD 3y agoStop dividing watts by hours or even seconds, please. 1 Joule is 1 watt sustained for 1 second, that is, 1 watt * 1 second.
- numpad0 3y agoPeople want runtime estimates. Often for fixed voltages. There comes Ah handy. Stop complaining people do "stupid" things before even Googling what it's for.
- vel0city 3y agoBut Wh would be just as handy, and then wouldn't need voltages to be fixed in the comparison. For example, I've got a battery pack on my desk that says its a 10,000mAh battery pack. No additional documentation on it. It has two USB ports for output and one USB port for input. Is that mAh rated at the 5V for USB? Is that mAh rated at the probably 3.6V of the lithium pack? Is it using some other voltage internally? Is it a 36Wh battery or a 50Wh battery or something else entirely? How long is it going to run my 5W load? 10 hours? 7.2 hours? Less? That's a nearly 30% swing in runtime that I don't truly know unless I go measure it, when they could have just stamped how many Wh its good for and I'd know.
- numpad0 3y ago> then wouldn't need voltages to be fixed in the comparison. > How long is it going to run my 5W load? 10 hours? Voltages are fixed. Fixed, passive. Amperage is designer's discretion. You can't make a computer run at 7V because you want to. The computer draws 12V/1A, and you have bunch of 3.7V/2600mAh cells around, so you join 4 in series to get 14.4V, shave off 2.4V through LDO, and get 2.6h runtime(is that 2.4W excess heat there? ew). Wh? Not handy at all. I don't control what many Watts my device draws. I know the CPU runs at precise 3.3V +/- 200mV and draws up to 20mA at full speed. Not "50 mW at up to 3.3V", no. I know my phone charges at up to 5V/2A or 10W, so what. An airline approved XL sized battery should roughly run the phone for 10 hours give or take few with just the battery and that's accurate as it gets from advertised data. > Is that mAh rated at the 5V for USB? It's practically ALWAYS rated at 3.7V because every "Chinese" brands - not always from there or with associated ethnicity - wants figures to be based on maximum indisputable advertisable numbers. That had already eliminated ambiguities years ago. I've seen batteries rated for a mAh capacity on 3.6V advertised nominal voltages as well as those in 5V, those don't exist anymore. All in all, battery capacity in Wh, not so practical. Maybe it could make some sense for consumer variable voltage power supplies. Definitely not for cells.
- vel0city 3y ago> Voltages are fixed. Fixed, passive. And yet this thing has some internal pack voltage, its putting it out at something like 5V, and then my phone is then turning it into something other than 5V for whatever its internal pack voltage is, which then once again gets changed for whatever its internal circuitry runs at. Doesn't seem very fixed to me. USB-PD has tons of different voltages it can operate at, all vastly different from whatever pack voltage my phone or laptop run at, vastly different from what the CPU runs at. Sure, when you're building something and you control all the variables, its easy to say the voltage is fixed. But most consumers aren't in that environment, they're plugging their phone into whatever charger supporting a myriad of voltages and amperages. And circling around, you can't be certain how many hours my 10,000mAh battery pack will run a 10W load. The only voltage assumed here is USB, 5V, so why would anyone assume 3.7? Why not 2.5? Lets just make up whatever numbers for the internal pack voltage. And you can't tell me about how many times that 10,000mAh battery pack can recharge my phone's battery pack, since we don't know either pack's voltage. But if we knew how many watthours each pack was, we could reason that.
- vel0city 3y agoI didn't in the actual math. I just wrote with the slash to make it easier to read than wattsecond. You'll see I did do 1 watt * 1 second. Please stop being so pedantic. I edited it for you though. Either way, I (and most other humans) will probably continue to use watthours instead of joules for most things, as most people will probably agree 5W for 3 hours makes more sense as 15Wh instead of 54kJ. Joules are too small a measurement for a lot of practical applications. Needing to multiply by 60 or 3600 to get to minutes and hours gets frustrating.
- Joker_vD 3y ago...can you please point me at the place in my original comment where I am arguing for using joules instead of watt-hours?
- vel0city 3y agoI took your original comment as a complaint about using watthours in general, as watts are already a unit of speed. Implying you'd prefer to not use watthours. I guess I was wrong on reading it that way.
- trashtester 3y agoDividing watts per hours makes perfect sense when making a statement about how fast a lignite coal plant can meet transients in load. ;)