Which features use 12V battery / HV battery / Engine?

Discussion in 'Gen 2 Prius Fuel Economy' started by Jasonb13, Aug 17, 2010.

  1. Jasonb13

    Jasonb13 New Member

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    Hi there,

    Just got an 2008 Prius last week, a car I've wanted for a few years now! I live in Ireland, but I guess it's basically a T4 Prius ( i.e. it's got everything but Sat Nav / Reverse Sensors / Reserve Camera ).

    Anyhow, I love it so far, am getting about 57Mpg ( Imperial - that's about 48Mpg US ) at the moment and that was with some motorway driving.

    However, I'm trying to figure out which features ( AC / Fans / Radio etc. ) use what power sources.

    For example, I'm pretty certain that the Radio / CD player uses the 12V Battery, just like any other car. Is that right? In other words, the Radio doesn't affect Mpg at all.

    I've heard that AC can really affect Mpg, but I've also heard that it runs off the battery. I presume this is the HV Battery? If that's the case, I'm guessing that the AC being on only affects your Mpg if your HV battery runs low, so the engine has to kick in? In other words, using AC for a few minutes ( assuming your HV battery is charged ), doesn't affect Mpg at all? Is this the same for the air heater ( in the cold months )?

    Also, what about the internal power supply ( in the Central Console ). Does that run off the 12v battery or the HV Battery? And how about other features, like the lights ( internal and external ), windows etc.

    I guess, like most owners, I'd like to max my Mpg. I've already started learning gliding / coasting and not doing short journeys where possible, but would like to have a better idea of what electrics will affect Mpg. Any advice would be appreciated, thanks!

    J.
     
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  2. BAllanJ

    BAllanJ Active Member

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    welcome

    The 12volt system provides most of the electrical systems and is powered when the car is on by a dc dc converter from the higher voltage supplied from the high v battery or the motor generators. The high v powers the motor generators and the dc dc converter and that's about it I think ... I may be wrong about the ac. Important to note that the small 12 volt battery does not start the engine just the electronics at startup. So it's much smaller than that in other cars and can be easily depleated if the car isn't on. So don't sit around much running things like the radio without the car being on or leave the interior lights on when you park. The tiny battery is a pain when dead.
     
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  3. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    The 12 volt battery is recharged by the HV Battery, so a drain on the 12 volt battery is a drain on the HV Battery. All the power for the 12 volt battery comes from the HV battery.

    The HV battery is recharged by the Motor-Generators, so a drain on the HV Battery is a drain on the Motor-Generators. Power for the Motor-Generators comes from regenerative braking and the Internal Combustion Engine.

    The Internal Combustion Engine is powered by Gasoline. (petrol) Ultimately, all energy in a Prius comes from Gasoline. Any energy you use reduces your MPG, often by minuscule amounts. Play the radio, it is good for your soul.

    (The 12 volt battery is only recharged in READY, so be wary of running the radio without the car in READY)
     
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  4. dogfriend

    dogfriend Human - Animal Hybrid

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    The AC compressor is powered by the HV battery with its own connection to the inverter. The AC compressor is a variable speed, 3 phase scroll compressor that is more like a home AC compressor than a typical auto compressor. The rest of the AC system (controls, blower motor, etc.) is powered by the 12v battery. As mentioned above, the 12v battery gets charged by the HV battery through a DC to DC converter and all of the energy ultimately comes from the gas (petrol) that you buy for the car.
     
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  5. timtim2008

    timtim2008 Member

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    i always wondered this as-well
     
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  6. Jasonb13

    Jasonb13 New Member

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    Thanks for all those replies! That makes a lot of sense, of course all the energy comes from the ICE really, though I'd guess some of it is to all intents and purposes 'free'. By this I mean, say you use the AC for a couple of mins, and it uses the HV Battery for those couple of mins, and the battery was pretty full anyhow, and then it's recharged 'cos you're going downhill. Effectively that AC use hasn't affected your Mpg at all, has it?

    Anyhow, thanks for all the info...

    J.
     
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  7. dogfriend

    dogfriend Human - Animal Hybrid

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    Well, not really. The energy that is used for the AC is no longer available for other purposes (e.g. powering MG2 to move the car at low speed in a parking lot). The energy that is used for AC will run down the HV battery more quickly which will mean that the ICE (internal combustion engine) will need to come on sooner to help recharge the battery which means that you will use a little more fuel which means that you can't go quite as far on a gallon of petrol (i.e. lower mpg).
     
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  8. Bob64

    Bob64 Sapphire of the Blue Sky

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    Energy is never free. You spent energy to go up the hill. If you recharge the batteries/convert the energy into electrical energy instead of mechanical energy (motion) then obviously your going to convert some of that potential energy and lower your speed when you reach the bottom of the hill.

    If you wanna be perfectly efficient, then you'd coast as long as possible without braking and reconvert all that potential energy into mechanical energy in the form of speed and use that momentum to climb the next hill or overcome rolling resistance as long as possible.
     
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  9. f16bmathis

    f16bmathis New Member

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    I read somewhere that regening is more wastefull than just coasting. Makes sense. Why slow down and charge a battery at 90% efficiency, to then accelerate and discharge it at 90% efficiency (guessing on those numbers).

    I installed the EV mode button, and found if I push the button while driving, it wont engage the EV mode, but it will cause the engine to turn instantly off as soon as you let go of the gas pedal (if the car is warmed up and your going less than 45mph). It increased my MPG from around 52 to just above 75mpg (not on the highway). Its great cause now instead of wishing the car can see a red light ahead or going down a hill, I'll just let go of the gas and coast with the engine off!
     
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  10. PRIUS NUBE

    PRIUS NUBE New Member

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    I finally have an answer as to how the A/C compressor is powered. Thank you one and all.
     
  11. peter hughes

    peter hughes New Member

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    Hi everyone. I know this thread is an age old but it is helpful for me at the moment and if possible I'd like to clarify something...
    I have a 2009 gen 3 Prius. I think I'm having trouble with my 12 volt battery but I wanted to clarify something with you... I've read that the 12v boots up the car... various electrical trickeries, but I also read that it "runs a few pumps..." I'm guessing the inverter coolant pump etc. My question is... Once the car has booted up and the HV battery is on line and continuously charging the 12v does the 12v run the usual systems (pumps, lights etc like a normal car and the HV just keeps it topped up as you go along so it can do that) or does the HV take over fully and the 12v just goes to sleep and gets charged, until it's needed when you next start the car?
    Hope this makes sense.
    Regards Pete
     
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  12. NortTexSalv04Prius

    NortTexSalv04Prius Active Member

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    Just from personal experience with my Gen2 Prius.... after start up and then "Ready" mode ….besides charging and checking 12V aux voltage level …….the engine will continue to run so that catalytic converter will heat up and reach O2 sensor threshold.

    try this link for further research Hobbit's techie-rants
     
  13. ChapmanF

    ChapmanF Senior Member

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    There's nothing special happening to "take over" or "go to sleep", it's all just a matter of looking at voltages.

    What's called a "12 volt" battery doesn't produce exactly 12 volts all the time. It can be down around 10.5 to 11 at the last of its useful capacity, and up nearly to 12.8 when fully charged.

    When the car is in READY and the DC/DC converter is on line, all of the "12 volt" accessories and wiring are shared connections to both the battery and the converter. But the converter's output is a volt or two higher, 13.8 to 14.7 or so.

    So whenever the converter is operating, the direction of power flow is from it, into the battery, therefore the battery is charging. All the power to your other 12 volt devices also is coming out of that, on its way from the converter to the battery. Only when you take the car out of READY is the power flow direction again out of the battery.
     
  14. peter hughes

    peter hughes New Member

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    Thank you very much for your replies.
    Regards Pete
     
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