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Does the 12v battery have anything to do with MPG?

Discussion in 'Gen 2 Prius Fuel Economy' started by F8L, Dec 22, 2007.

  1. F8L

    F8L Protecting Habitat & AG Lands

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    I've asked this before and I never got a real repsonse. Those that I did recieve seemed to conclude that a constant drain on the 12v battery from a large stereo system would not have an effect.

    A week ago I saw a thread where the electrical draws from various factory items (fog lights, headlights, tailights, heater, etc.) were shown and it was implied that these could lead to worse gas milage if all were used often.

    I'm still toying with maximizing my gas milage and the stereo system has me confused. I know the weight of the subwoofer boxes will reduce my milage but will the energy consumption from my rather large amplifier drain the 12v enough to cause a problem?

    The amplifier is rated at 1500watts RMS at 1ohm (this is how I run it) and has 4 40amp fuses on the amp. I.E. this is not a cheap fleamarket special. lol
     
    #1 F8L, Dec 22, 2007
  2. dogfriend

    dogfriend Human - Animal Hybrid

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    In purely theoretical terms, if you have a drain on the 12v system, the DC to DC converter will have to supply that energy and that comes from the traction battery which will need to be charged from the ICE, so yes, it theoretically should lower your MPG. You are using energy which could be used to power MG2. Theoretically.

    In reality, its probably less than the effect of the electric A/C compressor unless you have one monster of an audio system.
     
    #2 dogfriend, Dec 22, 2007
  3. F8L

    F8L Protecting Habitat & AG Lands

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    From a physics standpoint is indeed how I was looking at it. :)

    I do not know the charge characteristics of our cars and I was not sure if there was enough current being supplied under freeway driving conditions to power my system AND charge the AUX battery so that I don't end up with a constant low charge condition.

    The system is not huge in my terms but at a true 1500 watts it's not tiny either. I was contemplating putting my other amp in to run the mids/highs but that would add another 400 watts (a large, very high-end, inefficient amplifier). I want to get a better understanding of how this may effect my milage before I do it though. If it is going to increase my consumption in a negative manner then I cannot do it.
     
    #3 F8L, Dec 22, 2007
  4. jdenenberg

    jdenenberg EE Professor

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    Your amp at 1500 watts is equivalent to 2 hp. Since your car ICE output at highway cruise averages about 20 hp, you are adding a 10% electrical load so expect about a 10% drop in MPG (actually a little worse taking electrical efficiencies into account).

    Another factor is that the Prius inverter has a fuse in its 13.8 volt output (maintains the 12v battery and supplies all 12v accessories when in ready mode. You are pushing it to (perhaps beyond) its limit if you indeed are running at 1500 watts (more than 100 amps, divide power by voltage to get current).

    JeffD

    ps. The 12v battery has limited capacity. Don't run ANY heavy electrical load except in "Ready" mode.
     
    #4 jdenenberg, Dec 22, 2007
  5. icarus

    icarus Senior Member

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    A 1500 watt amp doesn't draw 1500 watts all the time, if ever. Look into RMS watts and how it relates to draw watts/ amps.

    Icarus
     
    #5 icarus, Dec 22, 2007
  6. qbee42

    qbee42 My other car is a boat

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    At peak power, a 1500 watt amp will draw more than 1500 watts, do to the inefficiency of the amplifier. To put out 1500 watts, it has to draw more than that. However, no one ever runs an amp continuously at full power; in fact most amplifiers can't come close to that without frying. You need to look at the average draw of your amp and see how that fits into your power budget. Any power use will cause some drop in mileage, but often it's very small. Another issue is the 12V capacity of the Prius. It's 12V system is not designed to run any heavy loads, so some people have had chronic battery problems from running a big audio system.

    Tom
     
    #6 qbee42, Dec 22, 2007
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