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I was under the impression that
Wh = Ah * V

I have a 20000mAh battery charging device for my phone but am confused about the information provided by it on the side.

It claims to have a 5V output (3.1A max), and a capacity of "20000mAh/74Wh". But following the above equation, 100Wh = 20Ah * 5V. Where does this discrepancy come from?

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    \$\begingroup\$ The battery voltage is 3.7 V. It has a step up converter to 5 V after it. You are trading current for voltage. Recalculate for 3.7 V instead. On the outside you will see (3.7/5) times less Ah, minus efficiency losses. \$\endgroup\$ – winny Jul 13 '16 at 17:42
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    \$\begingroup\$ this discrepancy is still probably smaller than the "advertised power vs real power" discrepancy \$\endgroup\$ – Wesley Lee Jul 13 '16 at 18:12
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    \$\begingroup\$ As @winny says, it's the 3.7 V Ah capacity and not the 5 V. This is one good reason to use Wh. It's a measure of the energy available. \$\endgroup\$ – Transistor Jul 13 '16 at 18:22
  • \$\begingroup\$ @winny You should make this an answer. \$\endgroup\$ – DoxyLover Jul 14 '16 at 5:54
  • \$\begingroup\$ @DoxyLover It will just be downvoted and/or removed. \$\endgroup\$ – winny Jul 14 '16 at 7:45
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This answer was originally by @winny in comments. He isn't interested in making it an answer so I'm doing so as I feel it is a valid answer and should be posted as such.

The internal battery is most likely Lithium-Ion, producing 3.7 volts (nominal), which is fed into a boost-regulator to produce 5 volts. The 20 Ah rating is on the battery, not the 5 volt output. Thus, 20Ah * 3.7V = 74Wh.

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