Model S

DC to Dc charging from Powerwall

edited November -1 in Model S
Will you be able to charge DC to Dc from Powerwall? Seems silly to have to use an inverter to conveet to AC only to have the charger in the car convert back to DC. Not to mention the loses from doing so.


  • edited November -1
  • edited November -1
    So Powerwall isnt compatible with Model S? Yes I know its only 10kw but if it could only charge it at 2kw it would be useless to store energy for Model S
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    No. and +1 to tes-s

    The PowerWall is rated at a continuous draw of 2KW.

    PowerWall is an energy source which supplements the grid during the time that the PowerWall is "powering". Think of it as a 2nd garden-hose watering your electronic garden of appliances. The demand from the grid can be reduced by 2KW during peak times of the day for a few hours. That works to cut down on your electric bills in places like California. Charge it at night, deplete it during the day in the afternoon.
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    You can charge off the PowerWall but only at the same rate you charge from a regular outlet (~3m/hr). And only for 5 hours. So you could only get 15 miles out of the PowerWall.

    But that's not what it's for. BTW, you typically will use 50KWH per day in your home--AC, dishwasher, lights, computers, etc. So you'd need 5 of these to break even at least here in the midwest in summer. In California--with that culture--you might be able to get by but just barely.

    The idea is still sound it's just that we're such power hogs in this country (me too) it's just not going to be practical for most people. It just takes the edge off the power bill by shifting some of the load to night time.
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    Why is it such a ridiculous question? If one wants to go completely off grid and has a Tesla, they need a way to charge their car. Presumably the cells would share similar technology. Is transferring the power from one battery to another directly DC to DC plausible? I'd like to hear from someone who could explain why or why not.

    Anyone have anything productive to add? Bonaire's and Tes-s's comments added nothing to the discussion.
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    how does 10KWh equate to only 15 miles? If the 70 KWh battery int the 70 is good for over 200 miles shouldn't that mean 10 KWh is good for approx 30 miles? Is there that much loss in charging?

    People have asked in other threads about DC charging, and I think the answer in most all cases is that it can't be done at home.
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    The batteries can't be connected directly to each other. There has to be a charge controller so the current flow doesn't exceed the maximum charge value. The battery supplying the power has to be at a higher voltage than the one being charged or no current will flow.
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    I have a household of two. We use on average 14 to 15kwh per day. On my commute to work I use 10kwh in my Model S so theoretically, 3 Powerwalls could meet my needs off the panels on my roof and I could go off grid.

    So would it be possible with a charge controller? You can charge from 120V or 240V AC which is below the voltage in the MS. Why wouldn't you be able to charge from a lower DC? I know it's been asked before but that was before Tesla sold a battery. I'm not asking if you can do it now. I know the answer to that. I'm asking if it is possible or reasonable.

    And 10KWH is about 25miles in my P85D
  • edited November -1
    Here is my guess, based on old, partially remembered electrical engineering college courses.

    Current flows from high voltage to low voltage. If you're connecting a DC battery to another DC battery, the charging battery has to be a higher voltage so the current flows in the correct direction.

    So how does a 120 V AC line charge a higher voltage DC battery? I am guessing its done with a transformer. Its been a while, but from what I remember, transformers work by running an alternating current through a coil of wire, which creates as oscillating magnetic field. Since change in magnetic flux through a closed loop creates a voltage in that loop, a neighboring coil of wire has a voltage induced by the magnetic field in the coil. The voltage produced in the second coil is dependent on the number of windings and the geometry, and can be higher than the voltage driving the first coil.

    This lets AC circuits step voltages up (or down), whereas I don't know if there is a way to directly step up a DC voltage. This is why AC won out over DC power, even though it is generally more dangerous; low voltage can be stepped up to high voltage for long distance power transmission, then stepped back down to something that is safe to use, while the current in the transmission wire is kept low because its not part of a physical closed circuit with the powerplant or the end user (its circuit is coupled through the transformer circuits via induction). That lets AC power transmit much further with minimal losses than DC power, which allowed for larger, more efficient and centralized powerplants to be developed, which is why all our stuff runs off AC even though DC is usually easier to work with and safer.

    My guess is that AC works to charge because there is a transformer to turn the 120 V AC into something higher voltage than the onboard battery so that it current flows correctly, but that a 120 DC voltage just wouldn't do anything. You would have to step the voltage up, which I am guessing requires turning it to AC, running it through a transformer, then turning it back into DC. Hence the need for an inverter.
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    @7thGate - Great answer. Right on target.

    Changing AC at 60 Hz to other voltages requires a very large and heavy transformer. One trick used today is to electronically up the AC frequency from 60 Hz to 15-20kHz. This allows the use of a relatively small and light transformer to change the incoming AC voltage to a different voltage. Other techniques are used to set the AC output voltage as needed, and its quite easy and efficient to convert high-frequency AC to DC.

    To charge a battery without shortening it's life, it's critical to adjust the power. You'll see that occur while charging as the voltage and current changes as it gets near capacity. You can't just wire in a Solar panel or another battery to charge another battery (i.e. the MS) - it needs to be tightly controlled. A solar panel system will have an output from 0 to as much as 600 volts DC depending on the sunlight.
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    TytanX - you are a low kWh user. You can go off grid but it actually doesn't make a lot of sense to do so in these times. The "talk" on the tv and the articles about people wanting to go off grid are "leading articles" meant to cause people to think what is possible. However, I don't see the grid as a big-bad-beast. it has high value. In some areas, like California, state regulations about nuclear and other forms of generation have caused high daytime peak rates.

    Now, if you have a tesla and want to go off grid. Talk to Solar City. They can spec out an off-grid system. You actually can use or may not want to use a tesla wall battery. There are other ways to go off grid with solar and an off-grid battery system sized properly. It will depend on your area you live in and the solar hours per day there to size the array, the battery, the discharge rate and so on. Most likely, you want to be gentle on such a battery and use a 240V J-1772 ESVE at home to recharge at a rate of maybe 15A. You don't want to draw an off-grid storage system too hard because you are not going to be as efficient in the draw by pulling higher amps.

    I suspect you would need approximately 50 kWh of off-grid battery, 5-6 KW of solar, if not slightly more. I am sure you can work with Solar City or some other vendor to set you up. people have been doing this for decades in their mountain homes.
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    @7th gate - I suspect the tesla PowerWall will allow for 240V charging. The reason is that the higher voltage is standard in Asia and Europe. Why not charge in the USA using two legs as well? The battery specs on the teslaenergy web site show a voltage similar to the car @ 350-450 volts. Transformation needed going either way, but it sounds like they are building a high voltage battery system. Don't crack it open because high voltage circuits are a bit dangerous. Then again, most solar pv arrays operate at high voltages as well and we're all ok using them.
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    DC to DC Charging straight from a solar controller would be nice.. this way we dont have the efficiency loss. Oh well.. Looks like we just have to live with the efficiency loss.. We live totally off the grid using Tesla car batteries, we have two at the moment. Its talking serious money to build up the system big enough to charge up the living and the Tesla.. We cant do both at the moment, but we will be able to shortly.. We use about 7kwh a day for household average.. We have 3000 watts of solar maxing out our midnite controller, so we need at least 10 more 300 watt panels and another solar controller.. Our inverters can operate split phase 240 volts, but will still limit the Tesla charging to 3k watts which will be 12.5 amps per leg which should be able to keep up with its use.. Now when winter comes along in the NW, we will have to use a 240v generator automated. we may still have enough solar for the house though. Im thinking the efficiency loss isnt worth the extra hassle of a dedicated solar controller, wiring, etc..
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