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At first thought, it's a good idea to prevent any short circuiting via contact with external hazards such as water or coins, but how often do that happen?

If the battery is already in contact with water, then it's going to get fried anyways. And it can't be so often that metal is in direct contact of the ports (on the battery) to warrant a dedicated power button.

Are there other reasons for including a power button on external batteries that I may be missing? Is it OK for them to not have power buttons?

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  • \$\begingroup\$ CAn be done without. Usually gets a better result easier with oje. \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 17, 2015 at 9:52

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To save power when not in use...

A power bank's job is to be available to charge a dead phone when needed -- so it spends most of it's life "asleep". It needs to wake up only when a device is plugged in and needs power, but USB doesn't provide a mechanical means of detecting cable insertion. The only way to detect a new device attached to the USB "power" port is to process the USB data traffic and look for the device "announcing" itself by triggering a bus enumeration. This requires that you run a USB host controller and that takes a lot of power that is pure waste.

...so we design them with a button. Basically, we just ask you, the user, to tell us when you've attached something... and thanks for letting us know! :)

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    \$\begingroup\$ And to produce 5 V from 3.3 V, the power bank needs to run a DC to DC converter. Even with no load on the USB, this would drain the battery in a day or two. \$\endgroup\$
    – tomnexus
    Commented Jan 17, 2015 at 8:31
  • \$\begingroup\$ and why would the power bank have a 3v3 battery inside? There are plenty of possibilities, a single cell LiIon is just one of them. \$\endgroup\$
    – Vladimir Cravero
    Commented Jan 17, 2015 at 13:39
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    \$\begingroup\$ I'm pretty sure he meant 3.7V, just a thinko. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Jan 17, 2015 at 14:41
  • \$\begingroup\$ Yes, 3.7, more likely. A USB LiPo power bank is most common now but there are ones which run on 2xAA or a lead acid battery. Still, I don't think you'll find a battery to USB power device with a linear regulator. I guess you could detect a plugged in device in a low power way, and save the user the trouble. So now I also wonder, why not. \$\endgroup\$
    – tomnexus
    Commented Jan 17, 2015 at 15:21
  • \$\begingroup\$ Probably because the switch is cheaper than the bump up to an IC that can perform the detection. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Jan 17, 2015 at 16:03

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