I have an 8-cell NiMH battery pack (10V) connected to a UBEC voltage regulator, so that it provides a constant 5V as the batteries discharge. This is wired to the micro-USB connector that's connected to the Pi.

The voltage at the end of the USB connector is 5V, but the voltage at the test points on the Pi itself is only around 4V. The pPi turns on but won't activate the webcam or the wireless, and soon gets a kernel panic. What's causing this drop in voltage, and why do they even differ? I thought the test points were wired directly to the input connector?

The problem turned out to be the cable - the resistance of the micro usb connector that goes into the pi was 2 Ohms

  • The fuse. der der dummmmmmmmmmmmmmmmmmmmmmmmmmmm Probably not the most useful answer and/or comment, but I fear it may be correct. Can you measure the current being supplied? Commented Aug 21, 2012 at 12:36
  • Yep, it initially draws 120mA and after a few seconds increases to 420mA. Then after a few more seconds it decreases to 120mA again, and continues doing this until I turn it off.
    – Matt
    Commented Aug 21, 2012 at 12:41
  • You could connect 10 Ohm resistor (so that you get 500mA of current on it) to your connector and see what voltage you will get on it. Is it 5V? Commented Aug 21, 2012 at 12:52
  • The lowest resistance I can get to is 17.7 Ohms. This draws 275mA and the voltage is 4.94V. I'm not really sure that this proves anything, but I don't have a 10 Ohm resistor and only did this by connecting all 10 of my 180 Ohm resistors in parallel.
    – Matt
    Commented Aug 21, 2012 at 13:16
  • 1
    Just realised we've all missed the obvious question... Does it work when connected to a normal adapter? Commented Aug 22, 2012 at 7:24

2 Answers 2


Throwing my hat into the ring... I think it is a dodgy fuse.

Dodgy Fuse

You need to take at least 2 measurements to confirm or deny this. Please measure (have fun...)

  1. The current flowing through the Raspberry Pi.
  2. The potential difference over the fuse.

Calculate the resistance. It should be low under 1A.

  • 1
    @Matt Accepted... Oooo what happened? Commented Aug 22, 2012 at 8:14
  • Well it wasn't the fuse but your answer made me check the pd across it, and it was low - meaning there must be something wrong with my cable. Turns out the resistance of the tiny connector bit was 2 Ohms! Replacing it with a different connector fixed the problem.. I feel so dumb now :P
    – Matt
    Commented Aug 22, 2012 at 14:45

The voltage drop is a result of the current draw of the RPi. It's the reason why Adafruit offers a 5.25v power supply for the RPi, so that the voltage drop will make it become a true 5v.

  • If you look at the comments below question, OP did a test with resistors making 0.95A of load (way more than RPi do) and voltage dropped only to 4.85V. Dropping voltage from 5V to 4V on such a low load that RPi does is not really expected behaviour. Commented Aug 22, 2012 at 7:02

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