Troubleshooting a red triangle

The UPS runs fine, and powers the ten circuits of my transfer switch (fridge; furnace; several rooms worth of lights), whether connected to the Prius or not. Without the Prius, the UPS only runs for an hour or two before the batteries run down too low to continue supplying AC power. With the Prius, it can run indefinitely (assuming you go fill the gas tank every day or two).

The problem was that the first time I connected Prius output to UPS, the red triangle (and the red car with an exclamation point over it) came on.

Tried it again today. Started the car, measured the HV output with a voltmeter (238 volts DC). Started my UPS and added load to it. Plugged in the Prius to the UPS batteries. About two minutes later, the red triangle, the red car with an exclamation point over it (on the LCD display), the brake symbol (yellow circle with (!)), and the yellow VSC light all came on.

The Prius stayed in READY mode, supplying about 220-230 volts output, with the engine coming on and off as needed. It is delivering power to my UPS, keeping my lights fridge and furnace on.

I just need to figure out what is causing the red triangle. At least five or ten other people have done this before, and have posted pictures and commentary about it at Son of PriUPS but I did not see any of them mention this problem.

I think one commenter yesterday suggested that the wire connecting the Prius to the UPS might be too small. For the wire, I used an extension cord 100 feet long. I considered using 10 gauge wire, but when I saw that several other PriUPS owners had used 12 gauge, I went with that. The voltage drop is only a couple volts over that distance, so it appears (based on their experience) that this is sufficient.

 

From this comment it appears you haven't read the description of the PriUPS system, and therefore don't understand how it works. It uses voltage from the Prius to keep the string of batteries in the UPS charged.

The DC wire is 12-gauge extension cord and it is 100 feet long. At first this seems like too small a gauge for this load; but this was determined to be sufficient based on the calculations made by other PriUPS owners, two of whom are electrical engineers. It has been shown to be sufficient based on their experience powering their house dirng which showed that this gauge was sufficient in this length.

Please tell me how to monitor Prius battery temps, and what temps I should be looking for.

Actually, all the R&D was done by Richard Factor, the inventor of the PriUPS system. I am far from the first person to install this system in their house. You can see pictures and some text by several of the PriUPS owners at Son of PriUPS