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Using a transfer switch to turn your EV car into a small backup generator

Discussion in 'Electric Vehicles' started by mattack4000, Nov 4, 2019.

  1. mattack4000

    mattack4000 Active Member

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    Please consider installing a 110v manual generator transfer switch so you can turn your EV car into a small backup generator during PG&E's PSPS. Once you have the switch setup, you can simply connect your car to the switch via an extension cord + inverter and you will be able to power 4 circuits to the house. While the power you can put out is limited, but you can make the house last a long time with the big batteries that are in the vehicle. Obviously the cheapest way is to install an inverter off the 12v directly, but you could also go through something like a Goal Zero portable battery to limit the risk of any damage. You would end up creating a setup where you are charging the Goal Zero with your EV while the Goal Zero is powering the house. Depending on what you are pulling, a parked Model S charged with 80% can probably make an energy star rated full size fridge for a week!
     
  2. Ampster

    Ampster Active Member

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    I am thinking about doing something similar. I was going to use an Anderson connector wire to the jump charging terminals. I was going to use a 50 Amp fuse but might consider a circuit breaker instead. I will have to confirm the capacity of the DC to DC converter.

    I did look at my Energy Star refrigerator and it draws 8 kWhrs per week so lasting a week would not be a problem. The overhead of the inverter I would need to calculate so that draw would not be a problem. However the startup Amps would be 100 Amps at 12V. I will have to check what my inverter can output and whether it is full sine wsve. I have heard that motors in refrigerators dont do well if they don't have pure sine wave.
     
  3. ajdelange

    ajdelange Banned

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    Please consult someone who has at least a minimal understanding of electricity before making suggestions like this.
     
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  4. Ampster

    Ampster Active Member

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    I presume you are talking about the original poster? The purpose of my post was to test the concept which does appear to be viable with the appropriate limitations.
     
  5. ajdelange

    ajdelange Banned

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    Yes.

    Your refrigerator's draw of 8 kWh/wk implies an average draw of only 48 W which is 4 A or 96 Ah per day. If the duty cycle is 50% the peak draw would be 8A but if it is 10%, 40A. 100A at 12 V is 1200 W but it only lasts, typically, for a fraction of a second. Are you sure this refrigerator's startup load is 1200W? I assume it is an RV or truck fridge designed to be super efficient and therefore to have an ECM motor which motors start pretty gently. Also, ECM motors first rectify the AC to make DC which they then invert back to AC at the frequency they need so pure sine waves aren't necessary in the AC supply. With an induction motor clean AC isn't really needed but the motors run less efficiently (and thus hotter) when current content is high. But don't listen to me - go with what the equipment manufacturer says.

    As you may have gathered my opinion in general is that these cars are not intended as emergency power sources and Tesla is quite specific about that. In fact using the main battery as a power source voids the warranty. I realize that you are not intending to do that but instead tap the 12V battery and rely on the HV battery to charge it. The 12V batteries in these cars are quite puny at about 23 A/H. I wouldn't feel comfortable using mine as a backup power source but I understand the situation out there is dicey and I sympathize as we have been without power for 3 days now in Quebec with little chance of restoration for several days at least. For a refrigerator, TV and a light or two I would greatly prefer one of those little Honda camping gnerators.
     
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  6. ewoodrick

    ewoodrick Active Member

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    Simply put, this won't work.
     
  7. mattack4000

    mattack4000 Active Member

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    Huh? I did it for a day last week already , too late.

    I don’t recommend just a straight inverter as they do fail, but you can use a goal zero battery pack and have your Ev car charge it to basically provide an unlimited supply of electricity, well at least for a few days.

    You guys need to invest in a new fridge. My 4 years old full size fridge pulls 300w on average, the starting surge might be 1000w.
     
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  8. Ampster

    Ampster Active Member

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    I understand. I actually have a 4kW hybrid inverter powered by Nissan Leaf modules. I was going through the math to illustrate some of the questions that need to be resolved in order to do that safely, but the warranty issue should not be ignored.
    I have had 7 EVs including two Teslas and in 3 cases the 12v battery had become depleted. It has never happened in either of my Teslas but I do now carry a small 12v Lithium kit design to at least fire up my car if the 12v battery does die. I did have the 12v battery in a Model X replaced when the software sent a warning that it was getting low. My MX is out of warranty with 89k miles on it.
    I am considering adding some Anderson connectors to make jump starting easier and as a phrophalactic to increase the odds that I will never need to do that. During the recent power outage I was doing some construction at my sisters house using battery powered tools and did attempt to charge those batteries using a small inverter powered from the 12v auxiliary source. I was frustrated since that source powered down after a few minutes and I realized that would be another application to use those Anderson connectors. I searched this forum and discovered that the DD to DC converter is always on and is capable of at least 50 Amps continuous. I still dont have the correct process for powering down the 12v system before making the 12v connections. That is as far as my thinking has advanced.

    Incidentally the data about my refrigerator cam from a Sense whole house monitor and that is how I determined the 1200 Watt start up spike. Nominally it appears to draw 90 Watts when running.
     
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  9. Ampster

    Ampster Active Member

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    My nephew's F 150 has a 120v receptacle on the dash, and reportedly the Rivian and Tesla will have some form of AC auxiliary power. I guess Elon and others never got your message. LOL
     
  10. Watts_Up

    Watts_Up Active Member

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    #10 Watts_Up, Nov 4, 2019
    Last edited: Nov 4, 2019
    This 1200 Watt starting spike force you to have a quite beef up system compare to the 90 W running consumption.

    I mean, on the 120 V side it's only 10 A, but on the 12 V battery side and the converter it's 100 A.

    And if you want to be more cautious, using the 80% load, you need to build a 1500 W back up system.

    This is about the Amperage that an ICE starter would require, but quite a strain on a battery for a fridge starting every 20 minutes or so.

    Would a capacitor plugged in parallel with the fridge plug socket would help to absorb some of the start up spike.
     
  11. ewoodrick

    ewoodrick Active Member

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    No, that's not what I said. No where near it. And trust me, the F-150 wouldn't do what the original poster suggested.

    Using a 120V inverter from 12V, if the inverter supports 1200W operation (which is a LOT), then it's going to be pulling more than 100A from that 12V battery. A standard car battery may be around 50 Ah (not CCA, that's another number) of usage. So that 1200W appliance will drain the battery in 30 minutes. For argument, let's even say you were to get 300Ah (a CCA rating) that's 3 hours for one circuit.

    The original poster suggested 4 circuits, that's 6000W or at 12V, 500A. On the wire gauge chart that I looked at, it doesn't even go that high.

    Go get a space heater and plug into your nephew's F-150. Odds are is that it will trip and nothing will happen. If it works, leave it on, see how long it runs. You can even crank the engine, so that the alternator is running, and the battery is still going to die.

    If you have a inverter, look at it and see how much it is rate for.

    The original poster mentioned a Goal Zero device. Their largest device will indeed do 1500W. But only for 2 hours. And it costs $3,000. That's 1 circuit, 2 hours.


    Again, it just doesn't work that way.
     
  12. ajdelange

    ajdelange Banned

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    1200 W startup for a 90 W running is a factor of 13 which is huge. Three or 4 times is more normal.

    No, a capacitor across the fridge socket wouldn't help. In only stores energy for a half cycle (8 ms) before returning it to the line.

    The Rivian and Tesla trucks will, apparently, both have 120 (and I think maybe 240 in the Telsa) VAC available. There is no way they would power the associated inverters from the 12 V battery. The inverter would be fed from a bucking DC/DC converter tied to the main battery. Actually I'm guessing there would simply be an H-bridge of PWM gated SiC transistors since that technology is already present in the Teslas. This would give "pure" sinewave output with high efficiency.

    I used to run around with a 1200W inverter in a Toyota land cruiser. It was tied directly to the battery terminals with multiple pieces of pretty hefty wire. Of course I never pulled 1200W from it even LRA for a fraction of a second.
     
  13. Watts_Up

    Watts_Up Active Member

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    I was was wondering which Goal Zero Portable Power device you are using?

    It's an interesting compact system, similar to the UPS backup protection for computer, but now using Litium batteries, which them make pricey.

    The UPS I used for my computer were never working when I needed them, I guess there was a small 12v motorcycle battery inside not lasting.
     
  14. mattack4000

    mattack4000 Active Member

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    Why are some of you arguing with someone who has done it? It’s already installed and tested. I have 4 circuits hooked up doesn’t mean I am running 4 welders at the same time. There is a cheap Hacky way to do it or you can spend money to put enough failed safe in the system to get you by.

    Honestly who the hell is wasting precious power to run space heater during an outage. Get a propane heater or gas heater if you are that cold

    And buy an energy efficient fridge, what fridge runs 1200w. That’s dumb. My fridge pulls 200-300w constantly, it might spike to 600-700 at the most.
     
  15. Randy Spencer

    Randy Spencer Active Member

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    #15 Randy Spencer, Nov 4, 2019
    Last edited: Nov 4, 2019
    Looking at the 48 amp charger circuit board on the Tesla Model 3 there is an AC -> DC -> AC -> DC setup. This takes the 60 Hz coming in from the wall, changes it to DC and then inverts it at a higher frequency and then transforms it to a higher voltage and then flattens it out to DC again for charging the battery. This technique allows them to use smaller transformers than the lower frequency of 60 Hz would.
    Image 9-8-19 at 4.25 PM.jpg
    There are actually three of these circuits in parallel in the actual charger.
    Probably only two of these circuits on the 32-amp charger on the Standard Range TM3

    But if you had software that allowed it, you COULD take the DC from the traction pack, feed it into the circuit backward as it is an exact mirror of itself going from DC and going to DC. With the correct timings applied to the MOSFETs the lower voltage could be inverted into AC, you could even build a box that plugs into the charge port on the car and presents AC outlets to plug-in for easily 30 amps of power.

    It's almost like they designed it that way, figuring someday they could change the software and create a plug to act as an outlet on the charge port of the car.
     
  16. ajdelange

    ajdelange Banned

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    Yes, but there is no need for most of that stuff for on board AC. All you need is one H-bridge and a low pass filter. By gating the transistors to low duty cycle an AC voltage of any desired amplitude (or frequency or phase) can be produced. But generally speaking interconversion between AC, DC, frequency, voltage is possible and the circuit posted is a good example of the kind of flexibility available.
     
  17. mspohr

    mspohr Well-Known Member

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    Motor soft starter - Wikipedia
     
  18. Ampster

    Ampster Active Member

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    #18 Ampster, Nov 4, 2019
    Last edited: Nov 4, 2019
    Perhaps you didn't catch the LOL at the end of my post. I was trying to inject a little humor into the conversation.

    Furthermore, there is no need to reinvent the wheel with regard to any of this because Tesla are already on it as far as utility vehicles are concerned. I for one, would never try to run any household loads off either of my Teslas.
     
  19. mspohr

    mspohr Well-Known Member

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  20. Watts_Up

    Watts_Up Active Member

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    I was was wondering which Goal Zero Portable Power device you are using?
     

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