PriUPS Update 31 Dec 2007


Tempting Fate on New Year's Eve

My last "update" was in November of 2006, and I haven't had all that much to say since then, at least not about PriUPS.  I've had entirely too much to say about other subjects, but that is what my blog is for.  I did want to mention two recent developments, however.  The first is a prospective one.  I just purchased a Ford Escape Hybrid to replace the aging and dangerous Audi wagon.  I live on a hill in snowy country, so having an AWD vehicle is a necessity.  I had originally planned to buy a Toyota Highlander Hybrid, but at least in its early days of availability the price as I wanted it was ridic.  Here's a comparison between the Prius electrical system and the Ford.


Toyota Prius Ford Escape Hybrid
Motor type Permanent magnet AC synchronous motor Permanent Magnet AC synchronous motor
Power output 67 hp @ 1200-1540 rpm (50 kW @ 1200-1540 rpm) 94 hp (70 kW) @ 5,000 rpm
Torque 295 lb.-ft. @ 0-1200 rpm (400 Nm @ 0-1200 rpm)  
Voltage 500V maximum  
Traction Battery    
Type Sealed Nickel-Metal Hydride (Ni-MH) Nickel-Metal Hydride (Ni-MH)
Power output 28 hp (21 kW)  
Voltage 201.6V 330V
Hybrid System Net Power 110 hp (82 kW) 155 hp

The specifications imply that the Ford should be able to provide somewhat more power than the Prius.  Of course that remains to be seen.  I did a lot of testing on the Prius to confirm that the software was compatible and that it wouldn't shut down or otherwise damage itself when several kilowatts were drawn from the traction battery.  Similar testing remains to be done on the Ford.  If I confirm that it can supply power in the same manner as the Prius, I have a strategy for dealing with the disparate battery voltages.  I'll go into that in a future update.

The Second Development

I've had one nagging worry about the PriUPS system as constructed.  Although the UPS battery voltage and the Prius battery voltage are close enough to be interoperable, I was never happy that the UPS battery discharged as much as it did during extended operation.  Ideally, the Prius would maintain the UPS batteries at their normal state of charge.  Unfortunately, there is no way to control the Prius computer to do this; it does as it is programmed.  This past summer I decided to build a small booster module for the Prius.  It would simply add 12VDC in series with the traction battery to provide a slightly higher voltage for the UPS batteries, thus keeping them in better shape.  I had the opportunity to test this arrangement yesterday, when I had what I hope will be the last power failure of 2007.  (I am writing this on the morning of 31 December.  Care to wager?)

As is customary, the yellow line represents the battery voltage, the red line the DC from the Prius.  (Click on either graph for full details)  When the power failed (blue line), the battery voltage immediately dropped from its float-charge value to its nominal voltage while delivering power, and dropped from there as it discharged.  I first connected the Prius, and then a few minutes later, added the "booster" in the circuit which increased the Prius output voltage by 12VDC.  This power failure lasted somewhat over an hour, and power was actually delivered by the Prius for about 35 minutes.  The lowest point reached by the UPS battery pack was about 225V, a satisfactory value.

I consider this booster module to be a worthwhile addition to the project.  Here is information on how it was constructed and tested.


If you're reading this, perhaps you're working on a PriUPS of your own.  If you get a system operating, please send info and pictures, and join the others who have documented their project on this site

Although I declared this project to be "finished" last year, you can see that it really isn't and maybe never will be.  So, see you in 2008, which I hope will be a fun and prosperous one for us all.

rcf 31 December 2007