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I'm shopping for a AC Power brick like. Sometimes they're rated in mAh like this one and other times they're rated in mWh like this one

To try to estimate the how long I can run a 10 watt LED wall lamp I'm doing this:

mAh Formula

mAh_of_battery_capacity / ((watt_of_lamp/120v)*1000)

So the calculation would be 60000 / ((10/120)*1000) or 720 hours. That seems really high. (and yes I know batteries don't go to zero so I should have a .9 in there). But on a high-level is this correct?

mWh Formula

Is it just ((mWh_of_battery_capacity / 1000) / watt_of_device ??

Or (58830/1000) / 10 = 5.8 hours. That seems really low. Their ad says they can run a MacBook Pro for 7.9 hours which takes 10-30 watts of power.

Appreciate any help correcting these formulas.

cbizy
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  • Apples and oranges, one is DC only in mAh, other has an inverter for AC – Tony Stewart EE75 Feb 24 '21 at 03:29
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    A 1 watt hour battery will run a 1 watt device for 1 hour. If the battery capacity you have is in amp hours, remember that power = voltage * current and then convert to watts. – user1850479 Feb 24 '21 at 03:30
  • You need to consider the current through the *battery*, not throuhg the load. – Hearth Feb 24 '21 at 04:06
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    The first linked one is rated "60000mAh/222Wh" from which you can discern the voltage at which it delivers 60Ah. 222Wh/60Ah is, not surprisingly, 3.7V Its capacity at 120Vrms will NOT be 60Ah! Use the Wh rating with your lamp's demand in W to give hours. Yes they do this deliberately to mislead unwary customers. –  Feb 24 '21 at 13:08

1 Answers1

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The 6Ah is going to be the battery capacity, probably at 3.7V or so.

So your first number is off by a factor of about 40, if you allow 80% efficiency. So 18 hours-ish.

If they are being honest on battery capacity, which is not at all a sure thing.

The second number is closer, 58-ish W\$\cdot\$h. But they probably don't account for efficiency of the conversion and they may also be "optimistic" on battery capacity so you might get rather less.

The first item also gives the capacity in W\$\cdot\$h at 222W-h, so the claimed ratio is 222/58 or almost 4:1.

My guess on actual performance would be roughly 16-17 hours for the first one to 4-5 hours for the second, at room temperature when new.

Expect to pay a significant penalty in currency and weight to get that higher capacity, and be suspicious if it seems to good to be true.

Spehro Pefhany
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