I am trying to run a 5 Volt 4 Amp Item off of a USB Power, and something went wrong today that I wouldn't have expected.

For reference, I am using a USB cable capable of sending 2.4 Amps, and either one of these items: enter image description here

So, what I am confused about is: the device runs perfectly fine on the 2.4 Amp, 5v USB Output Wall Plug, as shown in the top, but doesn't run at all on the 2.4 AMP Output, 5v Battery pack (the gold stick). The battery pack if fully charged, and the device tries to start, the item glows/spins for a brief second, then shuts off.

I don't really know what to think of this. The battery and the wall charger run at the same current and voltage, and on the wall charger the item runs and on the battery pack the item doesn't run even though the are the same exact voltage, current, even the same USB line.

If I try and use a lower current item, like a 5v .5A fan and connect to the USB and then battery it runs perfectly, but not the item I want to run as explained.

Thanks guys, thanks a lot.


closed as off-topic by Chris Stratton, PeterJ, brhans, Brian Carlton, Turbo J Sep 10 '17 at 9:00

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Chris Stratton, PeterJ, brhans, Brian Carlton, Turbo J
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  • 1
    \$\begingroup\$ Your numbers are likely mistaken, for example you may be confusing a capacity rating in amp hours for a current rating. But it should be obvious that trying to get 4 amps from sources and wires allegedly rated in the 2 amp range is a bad idea. \$\endgroup\$ – Chris Stratton Sep 9 '17 at 23:13
  • \$\begingroup\$ @ChrisStratton Hey thanks again for your help, say I am actually trying to hook up a motor rated at 5V 4A to a USB battery pack. I thought that even if its max rating is 4A and is capable of running at lower currents/voltages it would be okay because the item says the max current input is 4A and does not require 4A maximum to run. \$\endgroup\$ – Omar Sumadi Sep 10 '17 at 0:10

The usb wall wart says 2.4 Amp per output, but this is rarely limited. In my experience, all of the outputs are tied together and you can easily pull more than 2.4A from a single port. Any limiting tends to be done via the usb data resistors that are used to signal what type of port it is.

The battery pack is likely not meant to allow that much current. If you measure the voltage while in use, it likely has dropped down well below 5V, due to the excessive current draw. The internal boost circuit and the lithium cell inside likely cannot keep up with what you are drawing. That type usb power bank rarely does more than 1A at best.


The output ratings are what the devices are designed to be capable of. They may be able to provide more power, but this isn't guaranteed.

If your device will draw 4A, and your power supplies are only rated at 2.4A, then in either case you are exceeding the specified current that the devices are designed to provide.

It sounds like you got lucky with the AC-powered supply. Even so, you shouldn't do this. You never know when a failure is about to happen. For example, perhaps an internal trace in the AC-powered supply is slowly getting hot enough to melt.

  • \$\begingroup\$ I thought that since the max output the USB cable would be able to draw is 2.4 AMP everything would be limited to that 2.4 AMp...HMM? \$\endgroup\$ – Omar Sumadi Sep 10 '17 at 0:53
  • \$\begingroup\$ @OmarSumadi Sorry, no. The current requirement is determined by the load. If you starve the load by giving it an insufficient power supply, this manifests by a collapse of the supply's output voltage. Depending on the motor type, this might simply mean it doesn't spin as fast, but it may also cause the windings to heat up and/or the motor to stall. \$\endgroup\$ – bitsmack Sep 10 '17 at 1:07

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