May 30th 2016
it's even worse for a machine like mine with a E3D all metal head as it requires active cooling to stop damage to the head mount or prevent a right mess of molten filament inside the heatbreak.
See below for instructions on setting up an APC UPS so that it can send a command to octopi to abort the print and start cooling the head before the batteries in the UPS are exhausted.
I used a APC BackUPS Pro 550, which seems to be about the minimum spec I can get away with, on my printer this gives me approximately 5 minutes of print time without power, or 40 minutes of the printer powered but idle, other UPS's would work but APC is the only type tested with these instructions
Test this throughly and make sure you have enough runtime to cool the head before the batteries are exhausted, the only way to do this properly is to set up a test print and pull the power.
Once you have installed the power leads to and from the UPS and got the printer powered through it (not forgetting the Rpi or whatever you have running octoprint also needs power...mine is powered via the printer PSU ) You need to install acpupsd, it's in the default repo for raspian so just install it with apt.
sudo apt-get install apcupsd
Now we need to tweak apcupsd's configuration a bit
Edit the apcupsd configuration as follows, you can find it at /etc/apcupsd/apcupsd.conf, just use your favourite editor.
Find and change the following lines
DEVICE (this should be blank)
You might need to tweak BATTERYLEVEL and MINUTES for your printer and UPS. this is the percentage of power left before the shutdown will trigger or the minutes of runtime, whichever one happens first
Remember this is minutes as calculated whilst the printer is still running. Once the print is stopped the runtime will be longer as the heaters will be off, so setting 5 minutes here would in my case give me 20 minutes of runtime once the print has aborted for the hot-end to cool
Plug the USB cable from the UPS into a spare port on the Rpi
Now activate the service by editing /etc/default/apcupsd and changing the following line
Now start the service, it will start by itself on the next boot
sudo service apcupsd start
If all is well typing acpaccess at the prompt should get you some stats from the UPS, battery level etc
If that's all good then apcupsd is configured, now for the script that aborts your print
First go into the octoprint settings from the web interface, make sure API access is turned on and record the API key carefully
Back on the rpi go to the home directory
Now download my custom shutdown script with wget
sudo cp doshutdown /etc/apcupsd
Set the permissions so the script can run
chmod 755 doshutdown
Don't be tempted to rename the file, leave it as this name
Now edit the script and change the variable at the top API_KEY to the API key you got from your copy of octoprint earlier
That should be it, the script does 3 things when the power fails and the battery goes below one of the trigger points
Prints a warning on the printer's LCD screen
Records the current printer status and print file position to a file in /home/pi, so that maybe you can work out how to slice the reminder of the model and save the print
Aborts the print
This hasn't had a massive amount of testing and there are a few bugs, if you have a really big layer going on when the power goes you might not have enough power to make it to the end, octoprint only aborts at specific points in the print, same if you are at the first stages and are heating the bed, octoprint will wait until the bed is up to temp before running the next command (abort).
The sleep at the end of the script stops the rpi from shutting down, we need to wait here and make sure the printer has taken the abort command before killing the pi so that's an unknown amount of time so I leave it running by sleeping indefinitely here
If I get time I will make a proper octoprint plugin for all this