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Two Questions

Discussion in 'Gen 2 Prius Fuel Economy' started by M8s, Jul 28, 2011.

  1. M8s

    M8s Retired and Lovin' It

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    We took a ride in the Rockies yesterday. Naturally, it included going over a number of mountain passes - some well over 10,000 feet. This led to a few questions that I wanted to raise with you.

    First, let's say you go 25 miles from Buena Vista to the top of Cottonwood Pass and average 20 mpg for the trip. Returning the way you came, you go 25 miles coasting the entire time. (This is very nearly what happened yesterday.) Let's say that coasting is equivalent to 300 mpg (I've heard the Prius uses 300 mpg for coasting when it calculates mpg). Does that mean you've averaged 160 mpg for the 50 mile trip? I only ask because it doesn't make sense to me that you'd get better mpgs in that kind of driving than you do in around-town driving.

    Second, we went up Independence Pass (over 12,000 feet). We stopped for 20 minutes or so at the top, then went a about a mile down the other side and took some pictures. While returning to the top, the traction battery fan started to run. It continued to run for about 20 minutes even though for about 17 minutes of that time we were coasting back down the pass with the windows down. It was 57 degrees at the top and 64 degrees near the bottom when the fan turned off. Is this normal traction battery fan behavior?
     
    #1 M8s, Jul 28, 2011
  2. seilerts

    seilerts Battery Curmudgeon

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    First, to make it easier, suppose you average 20 mpg for a 20 mile uphill trip. That means you have burned 1 gallon. Next, you coast all the way back, 20 miles down hill, and burn no gas at all. Now you have covered 40 miles on exactly 1 gallon of gas -- for net 40 mpg -- rather than half of infinity mpg.

    Second, if your traction battery is full and stays that way, this tends to happen. As NiMH nears full charge, it begins to heat. This heat continues for some time. If the car was turned off for some period, then the fan would need to run at full blast for a time, especially if you are still going downhill and keeping it at full SOC. It could be an indication of a weak cell/overcharged module as well.
     
    #2 seilerts, Jul 28, 2011
  3. M8s

    M8s Retired and Lovin' It

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    That's it. I was using Miles per average MPG instead of Miles per Gallon. I knew I was doing something wrong but it just escaped me for some reason. Thanks!

    The battery was way low while the fan was running. But that battery had gotten a lot of use yesterday. In order, it was depleted going up Kenosha Pass, then nearly full by the bottom; depleted going up Cottonwood Pass (about 90 minutes later), then full way before the bottom (with the engine spinning to burn off electrons); depleted going up Independence Pass (another 60 minutes later), and then full by the bottom. It was on Independence Pass that the fan started running.

    After Indendepdence Pass, we did Freemont Pass (another 60 minutes later) and Loveland Pass (another 120 minutes later) but the traction battery fan only ran while coasting down Independence Pass with a very low battery.
     
    #3 M8s, Jul 28, 2011
  4. F8L

    F8L Protecting Habitat & AG Lands

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    If coasting with the engine off you will have infinite mpg. The ScangaugeII reads 9999mpg while gliding. This is assuming you are not using battery power that needs to be regenerated later via the engine. :)
     
    #4 F8L, Jul 28, 2011
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