USB power bank

USB power banks are relatively small, so they are generally very portable. And all of the input(s) and outputs are usually USB based. They are low priced compared to solar generators. Solar generators usually have USB outputs, but also 12V outputs and one or more higher voltage AC outlet(s). Solar generators usually need about 12V up to a higher maximum voltage to charge, while almost all USB power banks charge via a 5V USB.

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Some uses:

  • Recharge cellphone.
  • Recharge a flashlight that is USB rechargeable.
  • Power and/or recharge a portable USB fan.

USB inputs versus outputs:

  • Input – For charging.
  • Output – For powering a load (discharging).
  • In/Out – Some USB C ports can accept a charge, or power a load.
  • USB power banks are not usually intended for data sharing. USB is designed with pins for communicating data. But, most power banks and many cables aren’t wired for exchanging data, just for delivering power.

Common USB ports:

  • USB A
    • Rectangle shaped. Male and female plugs are designed to only connect if
    • 2A to 3A is common.
  • USB C
    • somewhat oval shaped but wide and flat on the top and bottom, with narrow rounded sides.
    • Either side of the plug can be the top or the bottom. There is no wrong to plug it in.
    • Can handle more power than USB A.
    • PD = Power delivery. A special input/output that can provide up to 100W if the other device communicates that it can exchange that much power.
  • Micro USB
    • Usually just for charging the USB power bank. Especially older USB power banks.
      • Semi oval, flat and wide bottom half, with flat and narrow top half. It simply won’t plug in if upside down.
      • 2A is common.

Amp hour (battery):

Amp hour refers to how much current the battery can provide if you want to discharge it in 1 hour. USB power banks usually have a 3.6V LiPo (lithium polymer) battery. Li ion (also 3.6V) is also common. Other chemistries are possible, but not likely to be used for a USB power bank.

  • 10Ah or 10,000mAh x 3.6V = 36Wh
  • 20Ah or 20,000mAh x 3.6V = 72Wh
  • 30Ah or 30,000mAh x 3.6V = 108Wh
  • 40Ah or 40,000mAh x 3.6V = 144Wh
  • 50Ah or 50,000mAh x 3.6V = 180Wh

Ah to expect from 5V output(s):

USB inputs and outputs are usually 5V. USB power banks have a circuit board that converts the 5V USB input to the lithium polymer/ion battery 3.6V nominal voltage while charging. Then the same, or another circuit board, converts the voltage of the battery to the 5V output while providing power.

Note: There is always power losses during power (voltage and current) conversion. You can expect less Amp hours than the following unless the battery actually has a bit more capacity than advertised. Also keep in mind that some sellers overstate the capacity of the battery. Capacity also goes down (slowly) with use and with age. 

  • 36Wh/5V = 7.2Ah
  • 72Wh/5V = 14.4Ah
  • 108Wh/5V = 21.6Ah
  • 144Wh/5V = 28.8Ah
  • 180Wh/5V = 36Ah

Probably not good for powering low power circuits:

It is tempting to use the 5V output of a USB power bank, to power a 5V low power circuit. Unfortunately, the units tend to turn off if there isn’t at least hundreds of milliamps of current being drawn from it.

  • I have 1W LEDs that will keep some USB power banks on while there is a lower power circuit that is parallel to the 1W LED. The LED can be flashed to save energy. You just need to turn the LED on often enough and for long enough to prevent that particular unit from shutting off. The less time the LED is on, the more energy is conserved.

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