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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 end of the USB connector is 5V, but the voltage at the test points on the pi itself is only around 4V. The pi turns on but wont activate webcam and 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?

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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? – Alex Chamberlain Aug 21 '12 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 Aug 21 '12 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? – Krzysztof Adamski Aug 21 '12 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 Aug 21 '12 at 13:16
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Just realised we've all missed the obvious question... Does it work when connected to a normal adapter? – Alex Chamberlain Aug 22 '12 at 7:24
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3 Answers

up vote 4 down vote accepted

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.

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@Matt Accepted... Oooo what happened? – Alex Chamberlain Aug 22 '12 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 Aug 22 '12 at 14:45
up vote -1 down vote

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.

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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. – Krzysztof Adamski Aug 22 '12 at 7:02
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okay if the power seems inadequate, how would you know whether it is okay or inadequate? symptoms? it would seem okay but not when GUI loads, sd card errors at startup also indicate low power! voltage after the power supplies (connected on the board after the the micro-usb) should be 5V, a higher current is needed, depending on the board you are using whether it is A or B? for the A unit 300mA 500mA min is required and as for unit B you need 700mA min! i don't know the rating of the batteries you are using! then again providing 5 volts at the micro-usb inlet isn't sufficient enough, even a higher voltage source be connected to the micro-USB, check the current at the Pi board power points, test voltage at the points labelled TP1, TP2, it should be between 4.75V to 5.25V lower or higher values would create problem! multimeter helps a lot in studying the connections and the ratings! see the power rating, as well as discharge rate and %age, with load connected

the power supply design isn't direct 5 V, is the requirement, but you need a higher voltage input 6-20V, because board uses switch mode power supplies that can generate 5V, but input of 5V isn't supported, your Pi Unit is powering up due to the high current that NiMH batteries provide! refer to this: http://elinux.org/images/6/6f/PSU_7805_v02.jpg more on microUSB and power supplies: http://www.raspberrypi.org/archives/260

thanks!

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Hi port, welcome to Stack Exchange. Sorry for the down vote, but I found your answer hard to read. Furthermore, the OP has tested using a multimeter. – Alex Chamberlain Aug 21 '12 at 19:49
ah okay, 5V reading on multimeter should be on the insides after the power supplies after the microUSB connector! – port Aug 21 '12 at 19:56
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The micro-USB connection should be supplied 5V and it should read 5V across the Test Points. – Alex Chamberlain Aug 21 '12 at 20:33
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5V/1A is needed make sure you have that! raspberrypi.org/wp-content/uploads/2011/12/psu.png – port Aug 21 '12 at 21:08
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Please format your answer better. It like a blob of text and hard to read. Please read this for more info – ppumkin Aug 22 '12 at 7:44
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