Doug Gaede's PriUPS

Doug has provided a nice write-up describing his PriUPS system.  He's tried several UPS units and has some valuable recommendations.

Here is a picture of my current Priups configuration in action – complete with obligatory shop light.  The small plug on the left is the tap from the Prius' battery, connected to the rear of the UPS.  Several years ago I purchased a Liebert UPStation S, which worked well and was compatible with the Prius’ traction battery without modification.  I ran a 14-hour test with that UPS, using 2.8 gallons or about 0.2 gallons per hour.  I accidentally left the cabin heat on, set to 72 with an outside low of 65 that night, so I expect that figure can be improved slightly.  The UPStation line is old, very large, and can’t do a cold/black startup so I searched for a more modern replacement.   

I initially purchased an APC SURT10000XLT since the entire SURT line uses an external battery pack rated for 192 volts nominal.  Your readers can benefit by my learning curve.  I discovered that units above 6000 VA use 384 volts nominal by connecting two battery strings in series – units between 3000 and 6000 VA connect the two external strings in parallel.  I now have a SURT6000XLT, like the UPStation it works without modification.  You can connect it to a Prius using either the internal battery connection (left pin on the left brown plug, right pin on the right brown plug), or the external connection (both pins of either white plug).  For reference, the following models should all be compatible:  SURT3000, SURTD3000, SURT5000, SURTD5000, SURT6000, SURTD6000.  There are many sub-models such as the SURT6000XLT and most only output single-phase 208 volts (or 240 with reprogramming, which I recommend, easily done with a laptop). There are 2 rare models which will output 120/240 natively, but don't expect to get a cheap deal on Ebay.  You see a second black box in the photo below, which is a step-down transformer (APC SURT003).  It accepts 208 or 240 volts and outputs 120/240 split-phase via the all-black twist-lock outlet, perfect for direct connection to a sub-panel.  It also has several 120 volt outlets for field use.  It weighs over 100 pounds so don’t expect to just throw it in the trunk on a whim – I have these semi-permanently installed in my garage and just run an extension cord from the Prius battery tap like you do.  I haven’t run any long-term tests with this UPS yet, I hope to do this soon and this time I’ll be sure to turn the A/C system off!

An engineering note - the traction battery voltage range I found with my 2008 Prius was slightly different than yours – 200 to 244 volts in each cycle, with a brief drop to 190 volts when the engine started. 

And an interesting update from Doug - 04 April 2012:

I have some additional info you can add. First, I’ve confirmed with APC that only the SURT line at and above 3000 va use the RBC44 battery (which is 192 volts nominal). I was hoping that they would have some smaller models that are directly compatible, even if they output modified sine wave. I also ran another overnight test with my current configuration. It ran for twelve hours with moderate loading for 1/3 of that time and baseline loading for the rest (2 fridges, 1 freezer, various minor loads) and according to the gas pump the next day it only used 1.6 gallons. That’s about 0.13 gallons per hour, which puts it in the same ballpark as the 1 & 2 kw Honda/Yamaha inverter gensets! At that rate it should run continuously for 2.5 to 3 days on one tank of gas, longer if I took some steps to conserve.