Can I tap 12volts off the Gen 3 Prius battery for big stereo?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Flagmaster, Dec 4, 2018.

  1. Flagmaster

    Flagmaster Junior Member

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    Im kinda thinking I should be able to pull between 75 and 90 amps without any problems if I do this? The Main 12 volt fuse is 100, and I figure the car gets 25 for all the pumps, lights, defroster, etc.

    BUT, I tried it, and all I end up with is a dead or dying 12volt battery every time. Put in a brand new one, was giving me problems (dim internal lights at power down, poor starts (had to try a few times). Took out the stereo, and it recovered.

    Is having a big stereo is a Prius just not possible?
     
  2. bisco

    bisco cookie crumbler

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    nope, the battery is way too small
     
  3. Flagmaster

    Flagmaster Junior Member

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    I never see any errors on the dash (other then when it fails to start and every light comes on). What the best way to read error codes, what do I need to get or buy?
     
  4. Flagmaster

    Flagmaster Junior Member

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    If the inverter puts out 14.6-15 volts even with the stereo on, then dont that mean the battery is seeing that too? Why wont it charge?
     
  5. bisco

    bisco cookie crumbler

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    get tech stream from amazon and a compatible windows laptop.

    look through audio threads for detailed advice on large stereos. most end up adding a second battery.

    apparently, you're pulling more amps than it is charging? not sure.
     
  6. Flagmaster

    Flagmaster Junior Member

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    Thanks, I did try the 2nd battery thing and it works for a while, but then that battery along with the factory AGM battery go flat. Is so crazy, if Im getting >14 volts out that big fat white cable from the inverter, and my stereo rocks with no issues, no errors, no lights dimming while pounding subs, then why the **** won't the battery keep a charge? Im trying something very radical now, complex as hell, using 120v power inverter and batter charger, and relays that "lift" the car battery off the car after its started, and then gets it changer from the external smart charger, so far, 3rd day, very good results. But I know for sure Im adding MEGA stress to the car's inverter's 12 volt side, probably getting close to that 85 amp point. Ill try to get a real measurement soon. No errors tho, at least none that show up in the dash.
     
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  7. bisco

    bisco cookie crumbler

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    would be interesting to measure the output vs the input with everything pounding
     
  8. VFerdman

    VFerdman Senior Member

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    Electric power is not just the Volts. It's the Volts times the Amps. You'll see 14.5V coming out of the inverter into the battery, but how many net amps are you pumping in? That would be amps from the inverter minus amps from your stereo. If the number is negative (as I suspect it is), then no battery will be big enough because your charging is not keeping up with your discharging. It's like if you start with $1000 in the bank and get paid $100 per week but are spending $150 per week you will run out of money in the bank at some point. If you start with $2000 in the bank and nothing else changes you will still run out of money, only a little later.
     
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  9. edthefox5

    edthefox5 Senior Member

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    A car stereo that pulls 90 amps.
     
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  10. VFerdman

    VFerdman Senior Member

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    Yeah! Well, some of us really are hard of hearing, I guess. For me 15 Watts per channel is more than enough. That'll be 60 Watts (2 front, 2 rear) at 12V is 5 Amps. Even if we double to 30 Watts per channel it's still under what a cigarette lighter socket provides and should destroy most people's eardrums if the amplifier is a good one.
     
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  11. Patrick Wong

    Patrick Wong DIY Enthusiast

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    If the audio system is producing 60W RMS audio output, the system will require ~150W of DC power input. At best, the output audio amp is 50% efficient... and then it is necessary to power other components besides the output amp.

    The OP is quite a bit off in thinking that the car only needs 25A total, but he could measure the required current using a clamp DC ammeter. For example if exterior halogen lights are on that will take around 10A. If the heater is on that will take 40A for the PTC heater elements plus more current to run the fan. We haven’t even discussed the current to run the many ECUs which probably is another 20A or more.
     
    #11 Patrick Wong, Dec 8, 2018
    Last edited: Dec 8, 2018
  12. audiodave

    audiodave Active Member

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    I use a Alpine V9 5 channel class D amp. 100 by 4 and 500 by 1. The Prius charging system doesn't keep up so I just trickle charge it once or twice a week. But least this amp is more efficient at what it draws to what it puts out.

    Posted via the PriusChat mobile app.
     
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  13. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Yes, I was thinking of a Class AB amplifier. You are right that Class D is much more efficient, 85% or so.
     
  14. audiodave

    audiodave Active Member

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    The Alpines sound excellent, are very small foot print. I remember when I swapped out my old Alpine class AB for the first series PDX. Midrange sound quality came to life and amp was half the size.

    Posted via the PriusChat mobile app.
     
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  15. audiodave

    audiodave Active Member

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  16. sam spade 2

    sam spade 2 Active Member

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    Certainly not practical with the stock equipment.

    The inverter that keeps the 12 V battery charged and runs all of the 12 V equipment was NOT designed for loads anywhere near what you are putting on the system.

    And yes, it is quite common for a power supply to maintain it's rated voltage under a heavy load......as the current available goes down and DOWN as the load goes up. Once you hit the rated wattage, something has to give.
     
    #16 sam spade 2, Dec 8, 2018
    Last edited: Dec 8, 2018
  17. sam spade 2

    sam spade 2 Active Member

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    You can't do analog (audio) with a class D amp.
     
  18. ChapmanF

    ChapmanF Senior Member

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  19. Patrick Wong

    Patrick Wong DIY Enthusiast

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  20. audiodave

    audiodave Active Member

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    It's analog in the end. Alpine been using this design for over 10 years with great success. I'm friends with the guy who was apart of it's design.
    Only way they'd squeeze that kinda power out of such a small package. Alpine PDX-F9.

    Posted via the PriusChat mobile app.
     
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