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Current Maximum on 12V system

Discussion in 'Model S: Battery & Charging' started by neko, May 21, 2014.

  1. neko

    neko Member

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    I would like to use the 12V circuit to run an external device. How much current can the 12V circuit handle? I realize the battery is relatively small, but I presume that as soon as it starts to discharge, the inverter that charges the 12V battery from the main battery will turn on. So the question is how much current can the inverter supply? And if there is enough current available for my purposes (I need about 20A), where is a good place to connect? The cigarette lighter outlet is very limited.

    Thanks,

    David
     
  2. trils0n

    trils0n 2013 P85

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    I seem to remember that the 12v outlet is 15A. Anything higher will blow a fuse. That is just off the top of my head though -- not 100% sure.
     
  3. MikeC

    MikeC Active Member

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  4. mknox

    mknox Well-Known Member

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    Good to know. I heard elsewhere that inverters cause some sort of electrical problems with the Model S. I never really understood why since I've used them in other cars. I have used my tire air compressor on my Model S with no issues.
     
  5. mgboyes

    mgboyes Member

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    The 12V socket is rated 150W. There are probably higher capacity fuses in the system (rear heated screen; headlights, etc?) but tapping directly into the 12V supply to get more current is probably a bad idea.

    Any concern about inverters and the Model S was about using a big one to charge the car, since they tend to produce very noisy non-sinusoidal AC which the chargers might not like very much.
     
  6. cinergi

    cinergi Active Member

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    This is true of the Roadster, but not the Model S.

    - - - Updated - - -

    I have a large inverter that I've been meaning to permanently install in the car, but I've been unable to get an answer from Tesla about how much power I can safely draw from the 12v system.
     
  7. green1

    green1 Active Member

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    If someone is adventurous, they could probably dig down to the DC-DC converter and it likely has a label on it with the capacity... (now of course you still need to guess at how much of that is "spare" but it would give an idea)
    I wouldn't hesitate for a moment to connect something directly to the 12V battery, but I would be curious what the capacity is (I too have high current accessories I would like to run)

    That said, if someone is even more adventurous, I wonder if it wouldn't be possible to add your own DC-DC converter to power whatever you need....
     
  8. siai47

    siai47 Member

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    I don't see any label on the DC-DC converter as to output. However, judging from the size of the low voltage battery cables connected to it, it looks like you could pull some serious current from it. It is located (on the newer cars) on the firewall just toward the drivers side of center. You will see the large cables going to the black and red terminals--most likely "hot" at all times. Easier to get to might be the terminals used for jump starting the car behind the nose. I don't know what limit on power they would have or if they are protected by a diode or something else to prevent power from being drawn from them. It is also possible that the car would be monitoring 12 volt battery levels and wake up to charge it with the converter if an external load was being used.
     
  9. Bugeater

    Bugeater Member

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    Before my trip a year ago I explicitly asked Tesla about the max drain on the 12V battery. The answer was 150 watts. No more. I sometimes plug in a 100 watt inverter to speed charge my cell phone. It works great!
     
  10. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    That is the limit for the Cigarette Lighter Port. I am sure that the DC-DC converter can do more, the question is how much more. It should be pretty easy to connect a larger inverter in the Frunk and see how much power can be drawn before the Voltage droops, and the battery starts providing some of the current. When I am back near my MS, and have some time, I will give this a try.

    I had a 2005 Prius and put a 1 kW pure sine inverter in the trunk next to the little Prius 12 Volt battery. Although the DC-DC converter in the 2005 Prius can put out 100 Amps, the wire from the DC-DC converter in the front to the battery and inverter in the back limited the total draw to about 500 Watts AC. I had always planned to put in another, larger wire in parallel with the existing 12 Volt wire, but never got around to it before I gave the car to my daughter. The inverter is still installed in that Prius, still available for backup power, has about the same efficiency as a Honda Inverter/Generator, but with a much larger gas tank!
     
  11. wycolo

    wycolo Active Member

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    You could place a new 12v 35AH U-1 AGM UB-12350 battery (just an example) in the Frunk and connected thru a 10A mini-breaker to the 'jumper terminals' in the Nose. This would allow instantaneous draw of almost any amperage you might need with replenishment limited to the 10A breaker rating. Thus MS system would not see much of an additional load at any moment. After doing a heavy draw-down the breaker would cycle the recharge of this 2nd battery until it levels off under 10A.
    --
     
  12. green1

    green1 Active Member

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    150w? That's it? That's pathetic. My normal inverter I use in every other vehicle is 350w and I know for a fact that 150w is not enough for my purposes. I guess it's plan b, add a second dc-dc converter.
     
  13. siai47

    siai47 Member

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    I had the trunk liner out for general cleaning and took a look at the DC-DC converter. I found a tag on it with the specifications. This should satisfy anybody's needs. Input is 220-430 Volts DC, Output is 9-16 Volts DC. 2500 watts maximum. If it was running at 13.5 volts output (common for battery charging) it would translate into 185 amps total available to the vehicle. So it seems like there is quite a bit of reserve beyond the vehicle needs in the converter. BTW--made in Thailand by Delta Electronics, INC.
     
  14. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Thanks for the info. That means that it should be possible to run an inverter like a 1000 Watt PURE Sine Power Inverter from the car. Where exactly is the DC-DC converter? Because the 12V currents can get high, it's good to locate the inverter near the power source (the DC-DC converter) and use good size cables (probably a #4 AWG).
     
  15. green1

    green1 Active Member

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    2500 watts? wow... that's more than I would have hoped for... now I'd assume all the other 12v systems are actually pretty hungry, but I still can't imagine even 1000w for the car's onboard systems, leaving a lot to work with for the owner. Though that said, I should ask, does anyone know if the heat and AC are 12V or if they run straight off the main pack? those are two things that really could eat up a lot of that capacity.
     
  16. siai47

    siai47 Member

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    The DC-DC converter is mounted to the firewall just slightly toward the drivers side of the car. You can access it by removing the rear section of the frunk liner. The water heater (mounted directly above the converter) and the A/C compressor are both run off the high voltage DC. All of the rest of the systems run off the low voltage DC. Biggest users would be the power steering system, vacuum pump, air suspension compressor (if you have one) coolant pumps and blower motors. Most run intermittently so the chance of everything being on at once is pretty low. Throw a clamp-on current meter on one of the cables and take a look.
     
  17. siai47

    siai47 Member

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    A picture is worth a 1000 words--so I am going to try to post one. The DC-DC converter is the box to the right of the picture. You can see the red bushing near the bottom left of the box which is the positive terminal. The negative terminal is just behind it. Above the converter is the electric hot water heater. The box to the left of the converter is the high voltage junction box with wires going to the converter, heater and A/C compressor. It would be easy to attach a pig tail with an Anderson style connecter at the converter. Be aware that the cables will have battery power on at all times so it needs to be disconnected before installing anything at the converter.
     

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  18. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Great info! Thanks! I would probably pull the pigtail with #4 AWG wire into the Frunk and terminate with the appropriate size Anderson connector; they are quite good for applications like this. Because of the "alway hot" issue, and even though the Anderson connectors are well protected, I would probably make an insulating, dirt-repelling "condom" for the Anderson connector/pigtail setup with velcro securing straps; the "condom" would be a great place to put some high current warnings.
     
  19. mknox

    mknox Well-Known Member

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    If I'm not mistaken, the DC-DC converter on older cars like mine is located in the passenger side wheel well, not in the Frunk cubby.
     
  20. siai47

    siai47 Member

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    You are correct and that needs to be pointed out---It used to be closer to the battery on the far right side of the car. I am not sure where the change took place in production.
     

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