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How many amps can you get from the 12V system for emergency situations?

Discussion in 'Model 3: Battery & Charging' started by marcmerlin, Dec 16, 2019.

  1. marcmerlin

    marcmerlin Member

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    #41 marcmerlin, Jan 7, 2020
    Last edited: Jan 7, 2020
    Here's what I did:
    I used this
    2000W pure sine wave inverter: https://www.amazon.com/SUDOKEJI-Inverter-2000W-Display-Outlets/dp/B07RWHN22W/ref=sr_1_1
    200A 12V relay: https://www.amazon.com/gp/product/B01MYPTVJD/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
    100W 1.8Ohm: https://www.amazon.com/gp/product/B01N0XZN1F/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
    100A energy meter: https://www.amazon.com/gp/product/B013PKYILS/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

    - battery (or car) connected through relay to the inverter
    - relay 12V control input connected on the inverter side (this is important, that way it only activates after enough current has travelled to the inverter to fill the massive capacitor)
    - 1.8Ohm resistor connected across the relay to conduct when the relay is not activated. This stops the current inrush when the inverter powers on and fills its capacitor at over 200A, causing a massive spark if you are connecting the cable (and that's unsafe since it could set off hydrogen sitting on top of a battery).

    What happens is:
    1) battery connected
    2) resistor limits current and starts charging the capacitor in the inverter
    3) voltage across the inverter starts rising until 10V or whatever
    4) once it's enough, the relay is energized and bypasses the resistor
    5) now 200A can go to the inverter and no spark/current inrush was created

    Stopping the inrush is super important because it does disable the DC-DC converter in the car, and that converter is non trivial to reset once it's gone in safety shutoff mode.

    I tested a 1300W microwave which worked without issues, so did my fridge and other devices. The pure sine wave inverter is very important for a microwave, but a cheaper inverter worked well enough for my fridge and basic loads.

    [​IMG]

    Car side, it looks like this. Only downside is that the inverter is "big" .
    Testing so far seems to show that the car goes to sleep ok, inverter dies, and when the car powers back on, the inverter comes back online without issues. In the case of a power outage, camper mode would be used to keep the car awake

    [​IMG]

    [​IMG]

    Let's talk about voltage drop for 100A (1300W) use:
    0.7V was lost in total:
    - 0.25V lost in each of the two 10AWG cables (good quality)
    - Extra wire from relay to inverter lost 0.06V
    - 200A relay only lost 0.06V
    - 100A shunt for power meter lost 0.1V (and gets warm)

    Given that the car outputs 13.5 to 14V, this drop is not big enough to matter since the inverter works all the way down to 10V or so.
     
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  2. miimura

    miimura Well-Known Member

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    The bold part is demonstrably false. Evidence below.

    Chart 2018-06-30_23-44-24_000.jpg

    The part that is restricted is that you cannot charge from the grid during Off-Peak. You also cannot export more than your solar production.
     
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  3. marcmerlin

    marcmerlin Member

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    Sorry, what I wrote was indeed wrong in the details. They do allow you to store solar and use it at night, but it's stupid to do price-wise (i.e. you're losing money by not sending back solar to the grid), and by "you can't do TOU arbitration", I did mean that you can't charge your powerwall off peak and discharge it during peak to actually help the grid and make a bit of money to recoup your equipment investment.
    This seems like the stupidest restriction because doing so would help clean the grid.
     
  4. miimura

    miimura Well-Known Member

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    Tesla has implemented "Grid Services" in the Powerwall system, but it does not have any agreements or regulatory framework to use it in California. I believe it is implemented in New York and Vermont. Whether you charge from the grid or not depends on whether you plan to or did already take the ITC Federal Tax Credit. The IRS specifically stipulates that batteries must be charged from on-site renewable power to qualify for the credit. However, you could earn extra credits by feeding your solar charged battery energy into the grid for extra compensation from the utility. I'm pretty sure Green Mountain Power subsidizes the Powerwalls so you don't have to take the ITC and can therefore charge from the grid during Off-Peak when you're on their plan. They even reserve some of the capacity for the utility's use and the utility commands the discharge. People in Arizona on SRP's demand metered plan have also gained the ability to charge from the gird because of a special arrangement with SRP.
     
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  5. marcmerlin

    marcmerlin Member

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    So, I have an half unsolved question: what is the most battery efficient way to keep the DC-DC converter running?
    Camper mode with AC off, a low temperature, and fan on 1, seems to be the best I found so far.
    Ideally there should be a way to keep the DC-DC on while having the computer screen off (which can happen if the internal 12V battery needs recharging, the car is smart enough to turn on DC-DC but leave the computers off)
    Is there a way to do that on demand?
     
  6. Randy Spencer

    Randy Spencer Active Member

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    That is confusing, you are hooked to a terminal that comes off the DC-DC, somewhere upstream of that is the car 12v battery. When the car goes to sleep, the DC-DC stops providing you power and the inverter turns off, is there another part of this circuit that disconnects the battery when the DC-DC turns off? Otherwise, the battery should continue to feed your inverter, no?
     
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  7. marcmerlin

    marcmerlin Member

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  8. marcmerlin

    marcmerlin Member

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    If you are connected to the DC-DC system, you aren't connected to the battery (which is good). When the car goes to sleep, the DC-DC feed under the back seat, turns off.
    My solution works as long as you can keep the DC-DC on, which can be done with camper mode or other ways. I'm just interested in the most battery efficient way to keep it active without wasting extra energy in other ways you don't need.
     
  9. Randy Spencer

    Randy Spencer Active Member

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    Isn't the whole point of the DC-DC to charge the battery? How is the DC-DC not hooked to the battery? (Gads I hope it's not obvious, cause it isn't to me)
     
  10. marcmerlin

    marcmerlin Member

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    it is not directly connected to the battery in one direction (it can feed it via a trickle charger), but battery cannot feed back to the points I connected to.
    I have verified voltage drops to 0 when the car shuts down, which is good even that I would not want my inverter to ever be powered by the battery. It would empty it in 1mn or less at full power.
     
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  11. n2mb_racing

    n2mb_racing Active Member

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    Not sure if this was mentioned, but in your blog post you mentioned connecting the inverters to your house to power things. I've looked into this once and you need to be careful. Many DC / AC inverters have a 60V sine wave on the neutral and another 60V sine wave on the hot, 180 degrees out of phase. The result is the same for the appliances, 120V AC. But, if you connect them to a home power system that has the neutral shorted to ground, it could blow up your inverter. You'd need to verify that the voltage on your inverter neutral is 0V relative to ground before attempting this.

    Or you can connect the inverter to your home through a 120V to 240V transformer with a center tap. Or get a 240V inverter with two 120V phases, if you can find such a thing....
     
  12. marcmerlin

    marcmerlin Member

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    Oh, thanks
    Oh, thanks for that warning. To be honest, I wasn't seriously considering backfeeding my house due to how many sharp edges there are in doing this, but very interesting to know that inverters would do a weird things like putting neutral at a different potential than ground.
    Is that even legal? I thought neutral and ground were supposed to be at about the same potential.
     
  13. n2mb_racing

    n2mb_racing Active Member

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    It's not legal for house wiring, but it's fine for inverters. Here's what I found:


    The UL standard for this type of inverters- UL458 does not have a requirement for a bonded neutral on the output of inverters. As long as the installation requirement of grounding the chassis of the inverter has been accomplished, loads that are plugged in will have their chassis held at the same ground potential as the chassis of the inverter and the house or RV. The only difference is that the neutral slot of the receptacle has approximately 60V on it instead of the usual 0V. The impact of this is minimal, since parts of wiring and equipment that are connected to the neutral side of the circuit are required by safety standards to be treated as if they were at 120VAC, since there are many receptacles that are wired backwards or 2-prong plugs that are not polarized. Therefore, a voltage of approximately 60VAC of the Neutral slot is not accessible to the user, and any shock hazard presented is mitigated by lack of access. The main safety agencies, CSA, UL, and ETL, have all approved inverters with this half-voltage on the neutral scheme.​
    Home


    There are some very expensive inverters that are designed for neutral bonding, so it is possible.
     
  14. Randy Spencer

    Randy Spencer Active Member

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    #55 Randy Spencer, Feb 6, 2020
    Last edited: Feb 6, 2020
    Basically ground and neutral ARE the same, but only at ONE point in your house, the grounding rod. If you have two of those (like a lazy solar installer wants to save ground wire and drives a second ground rod) there is potential to develop current between them which is dangerous. In an RV the rubber tires prevent grounding, so all ground plugs are neutral plugs but that can still work for GFCI. The transfer switch will bond ground to neutral when power is coming from an inverter, and keep them separate when on shore power.

    There are actual Grid Tie inverters, but they are designed to be used WHILE the grid is connected. If you charged your car during the day from solar and wanted to use that at night these are good as any extra power your house needed would come from the grid. They often come with current sensors that allow them to only put out enough power to provide what is currently in use w/o sending power BACK to the grid, up to the limit of the inverter itself. But if you hooked them to your house when grid power was off they would hit their limit very easily as your house has lots of loads.

    It's best to pick what you want to run from the inverter and hook those things directly to it
     
  15. n2mb_racing

    n2mb_racing Active Member

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    Yeah, I think a grid tie inverter would work, but I haven't found any that will start up without the grid being present...
     
  16. Randy Spencer

    Randy Spencer Active Member

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    Grid-Tie inverters are less useful for emergencies, but regular inverters don't hook to the whole house so well. That's why I recommended hooking them to individual appliances directly. There are generator hook-up kits you can buy that add a bunch of breakers in a subpanel that can be fed either by the grid thru the main panel or a generator (or a car and an inverter) that could be used to easily isolate the things you think need power for an emergency. They are pretty cheap and easy to wire, though I imagine most places require you get to a permit to do it.
     
  17. Bernard_S

    Bernard_S Member

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    I probably missed it but why does the car need this 12V output to be capable of supplying 200A ?
     
  18. n2mb_racing

    n2mb_racing Active Member

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    There's a lot of stuff powered by 12V. I think power steering, seat heaters, window defrosters, the computers, displays, lighting, etc...
     
  19. unkel

    unkel New Member

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    I've resigned myself to the limitations of the much easier setup of using the terminals behind the nose cone. I'm worried about blowing the 50A fuse though, as I don't fancy having to replace it myself. So here is my question:

    If I connect a 100A short automotive cable to the positive terminal, and connect that to a mega size 40A fuse and then the fuse to the positive terminal of my inverter, I should be safe, right? If I overdraw, my local fuse will blow and the 50A Tesla fuse near the 12V battery stays good?
     
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