Battery capacity in mWh instead of mAh

Question

I know that the capacity of a battery is expressed in mAh, i.e. A/s (international system of units). Today I used a command in PowerShell (*) in order to get the battery capacity, but the unit of measure is mWh. Why and how can I express it in mAh?

I know from here that:

$$ Wh = V \times A \times hours $$

(*) powercfg /batteryreport /output "C:\battery-report.html"

I think you answered your own question. You just need to know the voltage rating of your laptop battery. Googling the model number tells me it's 15 V, so you can do the math. — TypeIA, Dec 22 '21 at 19:26
Hello @TypeIA on my AC adapter there is 19 V as output. Where did you find 15 V? Has the voltage I found a different meaning? — Gennaro Arguzzi, Dec 22 '21 at 19:29
I Googled the model number in your post and found "replacement batteries" spec'd at 15 V. The battery voltage is not the same as the AC adapter voltage. — TypeIA, Dec 22 '21 at 19:30
The voltage from teh AC adapter is different (usually higher, as here) than the battery voltage (which should be on the battery label). Makes charging circuitry easier. — , Dec 22 '21 at 19:31
As an aside, mWh can be a more useful unit than mAh in situations like this where (a) a switch-mode power supply is in use and (b) the battery voltage changes significantly during discharge. A switch mode supply will tend to draw the same power for a given load even when the battery voltage changes substantially, so as the battery voltage drops the current drawn will increase. In the case of lithium batteries the voltage drops from about 4.2V per cell to 3.0V, so accurate prediction if the battery life needs to take this into account. — Frog, Dec 22 '21 at 20:12

Christianidis Vasileios · Answer 1 · 2021-12-22T20:01:05.200

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you will need to know the output Voltage of the battery you are using. If the Vout is 20V, then:

$$ Wh=V*A*hours $$ $$ <=>40.6mWh=20*A*1 $$ $$ <=>A=40.6mWh/20=2.03mAh $$

Hmm This 2.03 mAh current sounds verry little. I was expecting something like 1.5AmpHours.

EDIT:

Thanks to the comments, I see that "." was used as thousands separator. So the right answer is 2.03A/h (plus or minus the voltage of the battery swinging from 19 to 21 maybe)

edited Dec 22 '21 at 20:01

answered Dec 22 '21 at 19:28

Christianidis Vasileios

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The numbers don't make sense for a laptop computer. If the laptop requires even 10 W (and most come with 60-150 W chargers/power bricks), then these batteries wouldn't last even a minute. – The Photon Dec 22 '21 at 19:35
oh yes, thats why I ended up with such a small number, he gives me 40mWh, thats too small indeed. I took the number he/she gave as granted – Christianidis Vasileios Dec 22 '21 at 19:36
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Come on, many countries use a "." as a thousands separator. It's simply 40600mWh, which makes a lot more sense. – StarCat Dec 22 '21 at 19:36
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@StarCat heh I use commas (,) to separate parameters of functions. so Like foo(192.4 , 89.0) – Christianidis Vasileios Dec 22 '21 at 19:41
I do that too, but what does that have to do with thousands separators? – StarCat Dec 22 '21 at 19:43
Well, if they were thousands separators, this would make it 2.03kA/h, which still does not make sense. (did I calculate something wrong?) – Christianidis Vasileios Dec 22 '21 at 19:48
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@ChristianidisVasileios, 2.03 mA hr x 1000 is 2.03 A hr, not 2.03 kA hr. – The Photon Dec 22 '21 at 19:49
2.03Ah is the right answer I think, @ChristianidisVasileios, you could revise your answer. – StarCat Dec 22 '21 at 19:54
@StarCat Yes, I edited my answer. – Christianidis Vasileios Dec 22 '21 at 20:01

Battery capacity in mWh instead of mAh

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