A traditional UPS is rather inefficient (12 or 24VDC battery string, stepped up to 120VAC, just to go back down to 24VDC to run the printer) and doesn't provide enough runtime during a power failure.  I already have allot of extra tool batteries (which I'm very likely not using during a power outage) … so why not utilize these to run the 3D printer during an outage!

The circuit design can be found in the pictures.  Basically, we are taking 40VDC from two Ryobi packs in series, using an efficient buck converter to get down to the 24VDC that the printer needs.  The printer's normal 24VDC supply keeps a relay picked, which leaves the batteries disconnected.  When the AC supply goes out, the relay switches to the batteries.  A buffer (capacitor bank) is required to bridge the time the relay takes to switch over, and provides protection against brown-outs and multiple brief power failures.  I am using two buffers in parallel (as pictured) but only because I am running multiple printers off this system, if you have only one printer, you only need one buffer.  The buffer is in VIN-1 mode. The cost of the buffer is the most expensive part, but still less than a quality UPS, which wouldn't come close in terms of runtime.

While I have Anderson Powerpole connectors between many of the components, this is not necessary, this was more of a convenience for my testing; being able to swap different components or insert an inline watt meter.

Note this setup does NOT recharge the battery packs.  In the event of a power failure you will need to swap in fresh packs, this process is made easy by the hot-plug design, and can even be done while printing if you have multiple packs in parallel (as I have, 3 pictured)

I tested a print on the MINI, using PETG, with 2hrs 16min of runtime on a single pack (the pack was two Ryobi 6Ah PBP007 batteries in series, 40v out) before the on-battery meter started flashing 1 bar.   If you string multiple of these series packs in parallel (as I have, 3 pictured) the runtime is increased even further.

Note that I also tested another configuration, two Ryobi 6Ah PBP007 batteries in parallel (18v out) into a 24v step-up boost converter and this was slightly less efficient with only 2hrs of runtime.

The print files in this model listing are for stacked Powerpole connectors (which are optional) the rest of the files you need to grab from these other model listings…

“Dual 18v side-by-side” found here…
[https://www.printables.com/model/212523-ryobi-dual-18v-to-40v-battery-adapter/files](https://www.printables.com/model/212523-ryobi-dual-18v-to-40v-battery-adapter/files)

“Dual 18v plug in mount rev2” found here…
[https://www.printables.com/model/357455-ryobi-dual-18v-battery-plug-in-mount/files](https://www.printables.com/model/357455-ryobi-dual-18v-battery-plug-in-mount/files)

“Ryobi 18v dual holder handle” found here…
[https://www.printables.com/model/323786-ryobi-dual-18v-battery-carrying-handle/files](https://www.printables.com/model/323786-ryobi-dual-18v-battery-carrying-handle/files)

"Din rail mount" found here…
[https://www.printables.com/model/357461-din-rail-mount](https://www.printables.com/model/357461-din-rail-mount)

Links to the other parts needed…

14AWG hookup wire
[https://www.amazon.com/gp/product/B07RQP6HHN](https://www.amazon.com/gp/product/B07RQP6HHN)

Omron relay socket
[https://www.amazon.com/gp/product/B005T67OFE](https://www.amazon.com/gp/product/B005T67OFE)

Omron relay (24v coil, strongly suggested to use a quality relay like Omron)
[https://www.amazon.com/gp/product/B005MIAQOA](https://www.amazon.com/gp/product/B005MIAQOA)

Buffer TDK-LAMBDA DBM20
[https://www.masterelectronics.com/en/tdk-lambda/dbm20-55847070.html](https://www.masterelectronics.com/en/tdk-lambda/dbm20-55847070.html)

12SQ45 (12A blocking diode, adequate if you are only running 1 printer, use 1 before the buffer, and 1 between each battery pack)
[https://www.amazon.com/gp/product/B0859R7MQT](https://www.amazon.com/gp/product/B0859R7MQT)

1N4004 (diodes for relay voltage drop forces relay to UN-energize quicker, use 6 in series)
[https://www.amazon.com/NTE-Electronics-1N4004-Standard-Rectifier/dp/B008UG14R4](https://www.amazon.com/NTE-Electronics-1N4004-Standard-Rectifier/dp/B008UG14R4)

36 to 40v input, to 24v output buck converter
[https://www.amazon.com/gp/product/B09KGCDDW5](https://www.amazon.com/gp/product/B09KGCDDW5)

Anderson Powerpole connectors, if you like…
[https://powerwerx.com/anderson-power-powerpole-sb-connectors](https://powerwerx.com/anderson-power-powerpole-sb-connectors)

Let me know how you like this solution in the comments!