How to Build a DIY Raspberry Pi UPS Battery Backup System

Raspberry Pi's are vulnerable to power cuts which corrupt the SD card. Build a DIY Uninterruptible Power Supply (UPS) to protect your Pi.

Raspberry Pi's are low-cost computers, great for education, and projects and ideal for home servers. They are however vulnerable to power cuts which corrupt the SD card. This simple DIY project allows you to build a DIY Uninterruptible Power Supply (UPS) from a mobile phone power bank and a few lines of code.

An Uninterruptible Power Supply (UPS) is a peripheral which provides emergency battery power to servers when the input power source or mains power fails. While most UPS devices are quite large and heavy, smaller devices are used to protect individual PCs. These smaller devices are more affordable with prices around £100, however, they are overkill for a Raspberry Pi. In this DIY project, we will use a mobile phone power block to provide a battery backup for the Raspberry Pi which is both low-cost and small in size. If you have a spare power bank lying around not being used, this project may not even cost a penny.

A mobile phone power bank is ideal as it already has a USB charger in and typically a micro USB out so in the most basic form all that is required is to plug the mains adaptor into the power bank and the power bank into the Raspberry Pi. Depending on the capacity of the power bank you may get a few minutes to several hours of use from the battery. Some of the newer ultra-high batteries may even last days, but I'd recommend anything over 4,000 mA.

To be fully protected, you need to have the Raspberry Pi automatically shut down when the power is cut. For this, we need to have the Raspberry Pi connected to an ethernet hub which is not on the protected circuit. The plan is that when the power is cut, the hub loses power. We can then detect this from the Raspberry Pi and have a shutdown command issued. Of course, this will also cause the Raspberry Pi to shut down should the cable be disconnected as well, but for most home server applications, this isn't an issue.

We will be using a small bash script which will allow us to shut down the Raspberry Pi during a power outage and log the events to the system log.

sudo mkdir /opt/check_lan
sudo pico /opt/check_lan.sh

Copy in these lines for the script

#!/bin/sh

# cron script for checking lan connectivity
# change 192.168.1.1 to whatever IP you want to check.
IP_FOR_TEST="192.168.1.1"
PING_COUNT=1

PING="/bin/ping"
IFUP="/sbin/ifup"
IFDOWN="/sbin/ifdown --force"

INTERFACE="eth0"

FFLAG="/opt/check_lan/stuck.fflg"

# ping test
$PING -c $PING_COUNT $IP_FOR_TEST > /dev/null 2> /dev/null
if [ $? -ge 1 ]
then
    logger "$INTERFACE seems to be down, trying to bring it up..."
        if [ -e $FFLAG ]
        then
                logger "$INTERFACE is still down, shutting down..."
                rm -f $FFLAG 2>/dev/null
                sudo shutdown -h -t 0
        else
                touch $FFLAG
                logger $(sudo $IFDOWN $INTERFACE)
                sleep 10
                logger $(sudo $IFUP $INTERFACE)
        fi
else
#    logger "$INTERFACE is up"
    rm -f $FFLAG 2>/dev/null
fi

Next, we need to make this script executable:

sudo chmod 700 /opt/check_lan.sh

Now we need to edit the crontab:

sudo pico /etc/crontab

And add this line so that our script runs every two minutes

*/2 * * * * root /opt/check_lan.sh >/dev/null

That should be all good to go.

What this is going to do is create a scheduled task which runs every two minutes. This task will call a bash script to check the LAN connection. It will first ping the IP address defined at the start of the script, in this case, my router. If the ping is unsuccessful it will write a temp file stuck.fflg, drop the ethernet connection and restart it. The script then ends. Two minutes after, it checks the ping. If it still doesn't work it removes the temp file and shuts down the Raspberry Pi. If the ping does work, it removes the temp file and carries on as normal.

The total time between first checking the connection and the shutdown is 4 minutes. You can change this value to anything you want, just make sure that your battery lasts at least this long because if the battery runs out before the script runs you will risk damaging the Raspberry Pi.