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It seems appealing to power my small electronics projects with a 5 V USB power bank, however, it starts with 2 or 3 V, not 5. How come, and how to get a reliable 5 V output?

I tested with 2 digital voltmeters (DVM) to be sure these are not at fault.

I also own a small usb tester, with volt and current display.

I used two old USB cables, cut open, to use the red and black wires.

I have two older small usb power banks of about 2200 mAh. Also a large brand new usb-pd model of 20 Ah, Verico PowerPro PD V2.

What I observe:

When plugging in the cable with just the DVM, and no load, each power bank measures something like 2.5 or 3.3 V.

When instead plugging in the usb tester, it shows 4.9 or 5.1 V.

When plugging in the cable in the USB tester I measure about 5 V.

What is going on here? Is there a protocol to 'turn on' a powerbank? Something like resistors on the D1/D2 wires?

I should note that the electronics projects are very low power, so the power bank may perhaps think that there is no phone to be charged.

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  • \$\begingroup\$ You measured this because that is the battery voltage. When it detects a load, it turns on the DC/DC converter mosfet and then you read 5V. \$\endgroup\$
    – Codebeat
    May 1, 2023 at 7:16
  • \$\begingroup\$ @Codebeat Yeah, I already wrote about that in my own answer :-) Clever design, though, simple and effective \$\endgroup\$
    – Roland
    May 2, 2023 at 12:22

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If the power banks have no button to turn them on, they may periodically turn on to see if a load is connected or not, and stay on only if there is a load that consumes enough current. Even if they have a button to turn them on, they may anyway turn off when current is below some set level.

So sure the powerbanks can be handy, but if they are designed to simply charge a phone or something like that, they are not general purpose 5V supplies then.

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  • \$\begingroup\$ I guess I will have to present a load of a few milli amps with a resistor to turn the the powerbank on. Tomorrow I will try this \$\endgroup\$
    – Roland
    Apr 29, 2023 at 23:02
  • \$\begingroup\$ Thanks for your hypothesis on periodic sensing. \$\endgroup\$
    – Roland
    Apr 30, 2023 at 20:36
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With a little experimentation it turned out that a load as small as 1 mA is enough to get the USB powerbank to turn on and output the nominal voltage of 5 V.

What I measured with the digital voltmeter, representing a load of less than a microampere, apparently was a kind of sensing voltage, possibly high impedance, intended to detect a real load like a mobile phone. I guess that this voltage comes straight from the Li-Ion battery with just a big resistor, but smaller than the input impedance of a digital voltmeter. So you can measure the actual state of the internal battery. When hooking up a resistor of, say, 4.7 kΩ, the output will go to zero, and the internals will decide to turn on the boost converter.

With the oscilloscope I saw on the 5 V output a ripple of about 50 mV at 10 kHz. This is of course from the switching boost converter to turn the 3 to 4 V from the Li-Ion battery into 5 V for the USB output.

This ripple is no problem when charging a battery, but for powering your electronic project you might want to add a couple 100 μF to smooth that out.

With all that in mind a USB powerbank can be a fine power supply for small electronics projects. Especially so with USB-PD that, with a separate protocol board, can provide more output voltages up to 20 V.

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    \$\begingroup\$ You can't draw that conclusion by making a one project with one powerbank. It is statistically irrelevant. Internet is full of projects that others try to replicate and when asked why it does not work, the reply just is that it worked for me, once. Couple hundred microfarads may also be too much and it may not work because too much capacitance is added. Official USB device limit is max 10uF. \$\endgroup\$
    – Justme
    Apr 30, 2023 at 20:24
  • \$\begingroup\$ @Justme I tested with 3 powerbanks, of which one modern usb-pd type \$\endgroup\$
    – Roland
    Apr 30, 2023 at 20:34
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    \$\begingroup\$ Type-A (generally) does not support PD. Did you use Type-C connector, and did your project communicate over PD? And there are powerbanks that work differently to yours, requiring a lot more current to stay on, as they are intended to turn off when phone charging current is dropped below certain level. \$\endgroup\$
    – Justme
    Apr 30, 2023 at 20:37
  • \$\begingroup\$ @Justme no i just need 5 v on a cut up usb cable. The point is that i don't hook up a supply to my project without first verifying that the voltage and polarity is right. Now i know about minimum load behavior of powerbanks. \$\endgroup\$
    – Roland
    Apr 30, 2023 at 20:45

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