Reducing Raspberry Pi power consumption

I’m currently working a project that will put my Raspberry Pi 3 B to work as an embedded system running primarily on battery power. Naturally one of my goals became to reduce the power consumption of the Raspberry Pi as much as possible to make the most of battery life. I saw tips for disabling the activity LEDs, switching off HDMI, and a few other things, but none seemed to move the needle at all on power consumption. It still used about 140 mA (0.7 W) at idle.

Then I checked if you could disable the USB hub since I don’t need it. After disabling the USB hub the power consumption dropped to 20 mA (0.1 W). That’s a huge power savings.

One caveat with this approach is that it also disables the Ethernet controller. So if you need wired LAN, don’t do this.

The quick way to disable the USB hub:

echo '1-1' | sudo tee /sys/bus/usb/drivers/usb/unbind

This won’t persist across reboots, so I wrote a script to do it for me and set it to run on startup via /etc/rc.local. In case I need to do some manual fixes on the device with a keyboard and/or Ethernet, I have the script check if any USB devices are plugged-in or if the Ethernet cable is connected. Only if nothing is connected will it disable the USB hub. So if I need a keyboard, I just plug it in and power cycle the Pi!

#!/bin/bash

# With no devices plugged-in, there will only be "devices" at "1" and "1.1"
# If any devices beyond that are detected, such as 1.2, don't disable USB.
cat /sys/bus/usb/devices/**/devpath | grep -Eq '^[1-9]+\.[2-9]'

if [ $? -eq 0 ]; then
  echo "USB devices attached. Aborting."
  exit 1
fi

# Disabling the USB controller also disables Ethernet, so don't do that
# if there's an Ethernet cable plugged-in.
if [ `cat /sys/class/net/eth0/carrier` != "0" ]; then
  echo "Ethernet cable is plugged-in. Aborting."
  exit 2
fi

echo "Disabling USB interface"
echo '1-1' > /sys/bus/usb/drivers/usb/unbind

Posted 12 Mar 2019