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voltage sag while charging

Discussion in 'Model S: Battery & Charging' started by mckemie, Apr 10, 2013.

  1. mckemie

    mckemie Member

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    I was thrilled when I found my new Tesla S reports ac line voltage. Then, I was surprised at how much sag I have.
    I have examined two charge sites so far:
    1) An RV panel that is supplied from the main panel by about 60' of 6ga wire
    2) A J1772 EVSE, used also by a Leaf, supplied by about 30' of 10ga wire

    Both have sag of 10+ volts when used at full current, 40 amps for the RV panel and 30 amps for the J1772. Experiments indicate the J1772 can be used at around 15 amps with minimal sag.

    My understanding is that the sag represents lost energy. That is, a sag of 15 volts represents maybe 15/242 fraction of lost energy, something over 5%. Where no current voltage is 242 which is typical around my place. So, my strategy is to use as low a current as possible to get the charge job done in the alloted time. Can anyone refute or support that strategy?

    I wonder if a significant portion of that sag comes from the seemingly small wire sizes in the "to the car" portion of the wiring?

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    Where MY current voltage is 242
     
  2. mitch672

    mitch672 Active Member

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    Mine sags from 238-240 Volts to 218-219 Volts, under a 75A load (75A J-1772 OpenEVSE I built), I have called my power company to investigate and see if the feeder to my house is the correct size, and if perhaps their transformer might need to be "upsized"
     
  3. dhrivnak

    dhrivnak Active Member

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    With my Roadster i see only a 2-4 volt sag at home. So i suspect you may have less than ideal connections. Is there any warmth? If so it may need looked at.
     
  4. MichaelS

    MichaelS Member

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    Regretfully, AC voltage sag in a distribution system is a fact of life. The power losses are due to the resistances of the wire, circuit breakers and connections in the line. This lost energy is converted to heat.

    In my opinion, 60' of wire on a 60A circuit is too small. I would use #4 wire.

    I really doubt that the utility can do much as the losses are after the service entrance. I think if you measure the voltage during charging you will find it will only drop a couple of volts at the most.

    Running at a lower current is an excellent way to avoid these loses.

    Another thing to think about is that you have these same loses in the car. When you draw a lot of power from the pack, there is resistance in all the connections between the battery and motor. So driving gently improves your efficiency there.
     
  5. mitch672

    mitch672 Active Member

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    MichaelS, it depends on where the sag is occurring, if it present at the service entrance, it's the power company's issue, not your wiring or breakers.
     
  6. MichaelS

    MichaelS Member

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    Mitch672,
    You are right, but typically the service feed is pretty stiff because the grid impedance is quite low. But if there are other houses on the same transformer drawing a lot of power, that will make drop your service entrance voltage. So having the utility check it is a good idea. I know a lot of the older ple transformers are lower power and will sag more.

    If you can do it, I would check the service input voltage myself.
     
  7. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    No need.

    On a 50A circuit, #6 is just fine to a few hundred feet without suffering significant voltage loss.

    It is incorrect that the losses are after service entrance. Losses start at the transformer that steps down primary distribution (7.2 or 14.4 kV, typically) to 240V single-phase. The transformer's load, the service feeder cable, the service entrance cable, and the circuit size from service panel all determine the loss. In many cases, if there is a service feed of 100' or more, you'll see it in voltage drop.
     
  8. mitch672

    mitch672 Active Member

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    Seems to be the case for me, my house is setback 200' from the Main Street, then the transformer is down the street another 150-200'. National Grid is investigating... I have a newly upgraded 200A main service, feeding about 40' of #1AWG copper on a 125A subfeed from the main outdoor CSED (Combinded Service Entrance/Disconnect), that runs to the garage panel, which then has a 100A breaker on about 15' of #3 copper THHN wire to the 75A J-1772 OpenEVSE. My voltage sag can be seen at the main panel, getting the power company to do anything about it, that's another matter. Of course they did come quickly when I told them my meter seal was missing still from the service upgrade.
     
  9. mknox

    mknox Well-Known Member

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    Not necessarily. I've investigated many voltage complaints over the years, and a significant percentage are due to wiring issues beyond the service entrance. There are utility guidelines (CSA in Canada) that specify "nominal" voltages as well as acceptable min/max values under normal and emergency operating conditions. These standards are measured at the demarcation point (typically the weatherhead on an overhead service and at the meter base on an underground service), NOT at the utilization point (i.e. EVSE in the garage).

    Bottom line is that your utility should be able to help you diagnose.
     
  10. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    This is where we get into semantics. I agree with you that under nominal conditions, as you say, the greatest number of them (in fact my observations are > 80%) will be issues beyond the service; what I said is also true, though. If your transformer is oversubscribed, or your feeder is undersized you will experience voltage drop across the entire service as the load increases -- explains lights dimming under a heavy A/C startup load, for example. In my case prior to my upgrade (prior to my Model S as well), I had 2/0 AL feeder off a 12.5 kVA transformer feeding my 200A service, and my load was measured a few times at 200% of transformer capacity. Occasionally we'd see voltage drops that were out-of-norm.

    You know as well as I that there are utility guidelines, but how many times do PoCo's actually measure voltage under real-world load scenarios? They're typically doing so only at service turn-up, not 20 years later after hot tub, swimming pool pump, heat pumps, etc., all get added. :)

    To the OP's complaint, 40A loads on #6 branch circuit conductors can be well over 250' without appreciable voltage drop.
     
  11. mitch672

    mitch672 Active Member

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    I actually spoke to a foreman, and told him I had a Tesla Model S, and it was capable of putting a 75A load on the service, for more than 3 hours (the definition of "continuous load"), I offered to meet them one morning and demonstrate the load & have them take measurements. He seemed completely disinterested in that proposition, not sure how they intend to fix a 20 Volt drop if they haven't actually measured it. I will not stop pestering them, until their is some improvement though.
     
  12. mknox

    mknox Well-Known Member

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    I've got Smart Meters deployed to 100% of customers (as do all Ontario, Canada utilities) and in addition to billing data, I get back a lot of "telemetry" including min/max/average voltages. We've built a model of all feeder-transformer-customer relationships and I have automated reports running that give me transformers with out of range voltages. We actually see as much high voltage as we do low, and then proactively change out the offending transformer. We also get "hot socket" alarms that can alert us to a bad connection in the meter or a pending "burn-off" and can get out to those quickly as well. (And don't try tampering with your meter, because I can see that too :wink:)

    I agree, however, that not all utilities may be in a position to monitor this way, but most if not all are willing to respond and investigate low voltage complaints.

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    Call your utility office directly. The foreman may have other priorities as given to him or her by the Supervisor. The office should be able to open a service order and get someone out to help.
     
  13. bluetinc

    bluetinc Member

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    Hey Mitch,

    I'll toss my experience in to this also. It took me calling 5 people and a bunch of web searching to finally locate someone at my power company (PEPCO) who understood when I asked if my local feeder and transformer are up to snuff for the additional load that I am imposing. The first 4 told me not to worry and didn't really understand the issue I was asking about. When I talked with someone in the know, they pulled the local plat and found that they have 8 200 amp services all running off of one 25kVA transformer, with each house fed with a 2/0 AL feeder. They then decided to upgrade both the feeder to my house and the transformer. Since this, I found that just speaking to the power guys about a new large continuous load of 20kVA at the house seems to get a better understanding quicker than using the words EV.

    Peter

     
  14. Al Sherman

    Al Sherman It's about THIS car.

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    Agree. As soon as I said 160amps (2 HPWC's) when the wife gets the MX, they came out and are installing a new transformer and wiring.
     

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