Power banks

One hot gift this season is the “power bank,” an external battery that can power cell phones, and in some cases notebook computers, when there’s no power available at the wall socket.

I recently went on a shopping expedition for power banks. I’ll tell you what I encountered.

Power banks are rated in milliamp-hours (mAh; more on that below). Most seem to range in price from $10 to $160. A power bank isn’t something you use and throw out, it’s something that you recharge and keep around.

The power banks that I’ve seen all feature USB ports for output. For input (to recharge the power bank itself), the small and medium ones are charged via USB ports as well.

By contrast, the larger power banks, designed to charge notebook computers as well as cell phones, usually require AC power from a converter (in other words, grid power from the wall socket) to recharge themselves. As for output, in addition to USB ports, they’ll have other flavors such as 20 volts DC or even 110 volts AC. As you might guess, these large models are the more expensive end of the spectrum.

In rough, guesstimate terms, a typical cell phone battery (at 6.5 watt-hours of capacity; an assumption that may or may not apply to your case) could theoretically be charged the following amount of times:

• Small, 2600 mAh power bank: 1.5 times

• Medium, 10000 mAh power bank: 6 times

• Large, 50000 mAh power bank: 28 times

As regards physical size, a small power bank can be as small as a roll of quarters. A medium one is, very roughly, the size of a smartphone, or perhaps two smartphones pancaked together. A large one can be as big as a hardcover book.

If cell phones are your only concern, I’d suggest starting as small and cheap as possible. You can then get familiar with the various factors at your leisure and adjust your sights from there. By contrast, if you want to drive a notebook computer, you’ll want to do as much research as possible so you can avoid making an expensive mistake.

That’s all I’ve really got to say about power banks. However, if you like to geek-out with numbers, then pull up a beach chair, fire up a cigarette, and let’s get rolling.

For reasons that elude me, power banks, as noted, advertise their ratings in milliamp-hours (mAh). This is like selling milk by the foot. A more useful measurement is the watt-hour (Wh), which is sort of like a gallon of electric power. The math is easy: If you multiply one volt by one amp, you get one watt. Run something at one watt for one hour, and you’ve got a watt-hour. Also, while we’re messing with terminology here, and as you might have already surmised, a milliamp is one 1000th of an amp.

Power banks usually run on 3.7 volt (well, 3.6 volts to 3.8 volts from what I’ve seen) battery packs, an assumption I used for the above charging estimates. Using 3.7 volts as a generic assumption, a small, 2600 mAh power bank will, therefore, hold about 9.6 watt-hours of juice; a 50000 mAh monster will hold 185 watt-hours.

If you’re going to crunch numbers, it’s worth noting that the battery pack within a power bank is stepping up its voltage for its output; going, for example, to 5 volts via the USB port (USB ports are a standard 5 volts). If you base your capacity calculations on the output voltage, instead of the voltage of the bank’s underlying battery pack, you’ll get messed-up results.

This threw me for a loop when I was crunching numbers for my first power bank. Although it did have the watt-hour specification stamped on the device (but not on the packaging), I wasn’t able to figure out what was going on under the hood.

I decided to get a real expert on the case. I reached out to Isidor Buchmann, CEO of

Cadex Electronics Inc. in Canada. Cadex is also behind the highly-informative website BatteryUniversity.com.

Of my power bank, Mr. Buchmann noted, “Your power bank likely includes a charger that takes a 5V supply and provides a 5V conditioned output by a buck-boost circuit.” And that’s certainly the case here. Charging power goes into the power bank through the input USB port at 5 volts, is reduced to charge the battery in the power bank (a battery rated at 3.6 volts, not 3.7 volts, in this particular case; hey, nothing is ever simple), and then the output of that battery is pushed back up to 5 volts for the output USB port. That’s pretty fancy footwork for something that’s the size of a roll of quarters.

My capacity and charging estimates are just simple math that doesn’t take into account the rigors of real-world conditions. The actual performance we get out of a power bank will be less than these idealized estimates. How much less? I have no idea.

On that note, I’m still testing things to see what happens. Hey, it’s the holidays: Let the sparks fly!