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Thread: Variable charging rate?

  1. #1

    Variable charging rate?

    Is there any way to set the amp draw to adjust over the course of a charge?

    Say I want to charge at work for 8 or 9 hours. Is it possible to do 40A for half an hour, then automatically switch to 16A for half an hour (or even off), then back again? Rinse. Repeat.

    I'm asking because as it has started to heat up here (Nebraska) this spring, with multiple AC units running, an office full of computers and factory equipment, we (ok, I) maxed the circuits a couple times with the car drawing 40A for an extended period and blacked out a third of the building.

  2. #2
    Roadster 919, S 2006 Doug_G's Avatar
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    You could use a Tesla Tattler to remotely control the charge current. But it wouldn't be automatic - you'd have to send the commands. Unless you can figure out a way to write a program to send SMS commands, of course.

  3. #3
    That's a pretty slick gadget.

    It's not that big of a deal for me to just charge at a lower draw at work, so I was kind of hoping I could find a free band-aid for the office until I get a charger at the condo.

  4. #4
    Senior Member markwj's Avatar
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    Possible with Tattler, or OVMS, remote control - but you would need to control it somehow.

    Surely half the time at 40amp and half at 16amp, it would be easier to set to 28amp and leave it there? The car can do this approximately, or both OVMS and Tattler have charge current limits in 1amp increments.

    I guess with 1/3rd of the lights in the factory going out, you don't need charge interruption notification - you've got a pretty good notification system in place already
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  5. #5
    EU Model S P-37 VolkerP's Avatar
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    Fuzzylogic built a DIY EVSE that varies the charging rate by modulating the pilot signal. The Roadster obeys within 5 seconds. This is short enough a period that fuses will tolerate moderate overload.
    Here's how I understand it works:
    1. A smart meter in your mains breaker panel gives a live signal of current amp usage.
    2. The EVSE detects overload if other electric loads kick in, exceeding the amperage of your service.
    3. The EVSE modulates the pilot to signal "fewer amps available".
    4. The Roadster honors variation of the pilot signal and lowers the amps it draws.
    5. total current returns to allowed value.


    It's all automatic, and the Roadster returns to maximum amps set to charge as soon as the smart meter signals "enough juice availabe".

    Lots of details in this thread: Charging in the Netherlands. Enjoy!

    Another way to tackle the problem is to add rooftop solar photovoltaic to your company building. It can offset a fair amount of the current drawn by AC. PV production should correlate with AC usage (=both run when the sun shines).
    Last edited by VolkerP; 2012-04-25 at 02:28 AM. Reason: Giving credit to FuzzyLogic

  6. #6
    Model S R231 EU widodh's Avatar
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    Quote Originally Posted by VolkerP View Post
    Fuzzylogic built a DIY EVSE that varies the charging rate by modulating the pilot signal. The Roadster obeys within 5 seconds. This is short enough a period that fuses will tolerate moderate overload.
    Here's how I understand it works:
    1. A smart meter in your mains breaker panel gives a live signal of current amp usage.
    2. The EVSE detects overload if other electric loads kick in, exceeding the amperage of your service.
    3. The EVSE modulates the pilot to signal "fewer amps available".
    4. The Roadster honors variation of the pilot signal and lowers the amps it draws.
    5. total current returns to allowed value.


    It's all automatic, and the Roadster returns to maximum amps set to charge as soon as the smart meter signals "enough juice availabe".

    Lots of details in this thread: Charging in the Netherlands. Enjoy!

    Another way to tackle the problem is to add rooftop solar photovoltaic to your company building. It can offset a fair amount of the current drawn by AC. PV production should correlate with AC usage (=both run when the sun shines).
    FuzzyLogic inspired me to create the same thing.

    I'm thinking about using the OpenEVSE project for this, but right now I have this in place: http://energy.widodh.nl/

    Check out the technical details as well: http://energy.widodh.nl/technical.html

    I want to use this data to control my EVSE and change the current going to the car based on what I'm consuming in my house.

  7. #7
    EU Sport 359 & S94
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    @VolkerP
    Yes that's exactly how i did it. I can post my code online for anyone who wants to build it, but right now it's fixed to 26A, and uses a Kwh counter with 1000 pulses per Kwh.
    Coded in PIC assembly it's difficult to change.

    But as the Model S will support 3 Phase charging, i have to redesign it anyway. Then not one phase determines the Max current the car get's but i'll have to monitor all three phases.
    I'm thinking of a good and easy way to implement this.
    Maybe i can use current transformers on all phases, they just click on the wires. But the signals need to be sent to the EVSE somehow, which can be far away. Not ideal.
    Widodh's setup with a M-bus Kwh meter might be better.

    But as i also have solar panels feeding in on one phase, i wonder if the M-bus Kwh meter can measure currents going in the opposite direction..

    @ Widodh
    Looking good btw!

  8. #8
    Quote Originally Posted by markwj View Post
    Surely half the time at 40amp and half at 16amp, it would be easier to set to 28amp and leave it there?

    I guess with 1/3rd of the lights in the factory going out, you don't need charge interruption notification - you've got a pretty good notification system in place already
    The issue is we don't have a constant draw in the factory. If we had a steady draw from 6a to 6p it would be easy enough to determine what rate to charge at. But sometime we run heavy equipment, sometimes compactors, sometimes water pumps. Sometimes all three. Sometimes none. Its the random times when we are running a lot of things simultaneously that causes the problems, and theres no real way to schedule around it other than giving the circuit a break periodically.

    Anyway, it's not the end of the world right now. But might be a neat something for tesla to add in a software update later.
    Last edited by Eric from NE; 2012-04-25 at 03:49 PM.

  9. #9
    Model S Sig Perf #698 Sig698's Avatar
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    How long is your commute? Shouldn't 8-9 hours at 16A be enough to top you off?

  10. #10
    80 miles round trip, but that's assuming I only drive to / from work with no side trips. On most days I can get away with a 16A for most of the day and then top off with 40A, but not always.

    I do seem to have figured out a routine that keeps me from blacking out the building until I can get a charger installed at both ends of the trip.

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