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Fan noise coming down long grade

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by MrMischief, May 30, 2016.

  1. MrMischief

    MrMischief Active Member

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    Over the weekend I took a bit of a road trip with the Prius that involved climbing my way up into the mountains of Colorado then coming back. For those interested I went from Denver to Colorado Springs and went up 24 to Florissant. No big deal on the way there, car held the speed limit (45 - 55) without issue. On the way back I think I started the steepest bit of the descent with somewhere around 3/4 battery in it. I left it in D and I let the radar cruise do its thing. I had cruise set at the 45 mph speed limit, temperature was in the 70s I believe, and for the most part everyone was driving faster than me so the car didn't slow down off that 45 much. Anyways; I of course quickly filled the battery, and it seemed as though the car kept using the regenerative braking system. I didn't think too much of it until near the bottom of the hill I hear a fan kick on. It sounded as though it was behind the dash which wasn't what I was expecting.
    The only real hands on experience I have with hybrids prior to the Prius is maintaining Hitachi's mining trucks (like the EH5000AC, think 326 ton payload capacity dump truck used in a gold mine). Those things use the same basic principle as a Prius for braking, reverse the electric motor so the kinetic energy becomes electrical energy.... But the mining trucks I worked on didn't really have batteries it charged. Instead that electrical energy was transferred to a giant toaster with huge fans on it. So I figure the Prius does a very similar thing, when the battery is full take the excess electrical energy, sends it to a grid and runs a fan over That fan is what I suspect I was hearing.
    So my question is... how wrong am I? If I'm wrong, how does the braking work when the battery is full? If I'm right, where is this toaster grid and fan?
     
    #1 MrMischief, May 30, 2016
  2. CR94

    CR94 Senior Member

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    Nope, there's no "giant toaster" in your Prius. Perhaps what you heard was the engine, which the system spins up in order to throw away some of the excess energy that would otherwise have to go into the friction brakes when the battery is "full."
     
    #2 CR94, May 31, 2016
    Last edited: Jun 1, 2016
    pakitt likes this.
  3. eric1234

    eric1234 Active Member

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    Could it have been the A/C condenser fan? (Or, maybe the sound from the Battery fan was bounced around the cabin such that it sounded like it was from up front?) Or, maybe the fan came on to cool the inverter coolant loop & radiator? Just 3 guesses...
     
    #3 eric1234, May 31, 2016
  4. Trollbait

    Trollbait It's a D&D thing

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    The 'toaster' are called dynamic brakes, and are also used on locomotives. With no battery, the electricity produced from braking is converted to heat in a bank of resistors.

    I think the previous posts covered the most likely possibles.
     
    #4 Trollbait, May 31, 2016
  5. MrMischief

    MrMischief Active Member

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    Interesting, we always called the actual toaster bit as a resistor grid, but we also called the act of reversing the motors to generate braking as "retarding." Probably isn't PC anymore... Anyways, it could handle a continuous load of something like 3,500 kW (4,700 hp) and is capable of bringing the entire loaded truck to a stop on grade without a friction brake. I imagine the ones on a locomotive are much more impressive.
     
    #5 MrMischief, May 31, 2016
  6. Trollbait

    Trollbait It's a D&D thing

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    Dynamic probably comes from the fact that the propulsion system is being used to stop the vehicle. On a locomotive, the control for acceleration and braking can just be a dial, and it is like one pedal operation on a BEV.
     
    #6 Trollbait, May 31, 2016
  7. bwilson4web

    bwilson4web BMW i3 and Model 3

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    You heard the traction battery fan under the rear seats and it is 'not good.' Charging the NiMH and probably the LiON battery is exothermic, it generates heat. As you found, even modest hills can fully charge the car but 'heat is the enemy.'

    The recommended practice is to use "B", engine brake, so the excess energy is used to spin the engine that is a basic heat engine and well equipped to handle the load. Certainly better than the traction battery.

    The only risk is the 2010-15 models would kick off their limited cruise control so I often would shift between "B" and "D" to maintain a safe descend profile.

    GOOD LUCK!
    Bob Wilson
     
    #7 bwilson4web, May 31, 2016
  8. krmcg

    krmcg Lowered Blizzard Pearl Beauty

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    I do the same.
     
    #8 krmcg, May 31, 2016
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