I have turned my raspberry pi B+ into a wifi repeater (i.e running 2 usb wifi keys). It runs on a portable power bank now. So I am limited by the lifetime of the battery, approx. 3 hours.

I already have a 3 meters USB extension cable and couple of others of 1 meter, but none of the combination I have tried have allowed me to power up the raspi at no less than 1.5 meter from the power plug.

What's wrong with my cables ? What are the required spec for this to work at 3 or 4 meters ?

My USB charger outputs 5V/2A

  • This has little to do with the Pi. A lot of USB leads have flimsy wires which lose a lot of voltage over short distances. Get a lead with big chunky wires or make your own.
    – joan
    Jul 10, 2015 at 21:03

1 Answer 1


The longer the wire and the smaller the wire the more voltage you loose. That is a plain and simple fact. There are basically three ways to compensate for this:

  1. increase the source voltage: if you lose .4v over the wire increase your source voltage to 5.4 volts so that you get 5v at the point of use. You must measure your exact voltage loss.

  2. increase conductor diameter.

  3. decrease cable length. This is easy enough, if you use an ac extension power cord you can place the power supply close to the rpi.

  • Yes of course. 3. is the obvious answer and I didn't think about it. For 2. it is not easy to find out about the USB cable diameter when you buy online. Most of the time, this is not even mentionnned in the spec. You have to guess from the picture...
    – solsTiCe
    Jul 11, 2015 at 10:02

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