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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 Active 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 Active 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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