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You might consider charging slower

Discussion in 'Model S: Battery & Charging' started by boilerbots, Jun 22, 2013.

  1. boilerbots

    boilerbots Member

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    I see talks about people wanting to charge as fast as possible all the time just for the hell of it, or because they paid for twin chargers so they feel obligated to use them to the max.

    Well, I thought I would throw out some information as to why you might want to slow things down.

    Vehicle Losses

    I will talk about parts of the system where power is lost and talk about how this can be related to charging current.

    The chargers are lossy. They are converting the input voltage into DC and regulating the current that flows into the vehicle. Most DC/DC supplies are most efficient somewhere in the middle of their operating range, I don't think any are most efficient at the upper limit. There are several reasons why this is the case, one simple reason is resistive power loss. The charger circuitry has a fixed resistance in it's circuit that the current must pass. Using the equation to convert current and resistance to power, power=I^2xR you see that as the current increases the power is increased exponentially. Other factors that influence efficiency are losses in the magnetics used by the switching supply, the inductors and chokes. As the loads increase on the circuit, so do the losses.

    The battery cells also loose power. The battery stores and releases energy through a chemical process but the battery cell also has a certain resistance internally (also called impedance). This loss can simply be thought of as a fixed resistance and using the power equation P=I^2xR you can determine the power loss due to current and resistance. As battery cells get older their impedance typically increases, so the internal cell resistance will rise over time.

    Energy wasted is turned into heat. Maybe you need heat because you live someplace cold, maybe you don't. Let's say you don't need to warm up the battery pack, or worse yet you are parked over hot asphalt. The heat is going to need to be moved away from the critical components, the battery cells, the charger and dissipated to the air. Tesla does this first with a fluid, so a pump is involved, then a radiator and a fan. It takes power to run pumps and fans. If you charge slowly the heat may be able to naturally move through convection and dissipate without extra help. I don't know when active cooling kicks in, wish I could unlock that Tesla diagnostics screen, but you have probably heard fans and pumps running while you observe the Tesla beast sleeping. It isn't purring because it likes your garage.....

    Home Losses

    When you are charging, the car is part of a bigger system. A system that starts at some generation plant, includes power lines, transformers, your homes electrical, the car. It is big and complex and awesome. Let's focus on the part we care about, the part we directly pay for, and that is the numbers measured by our electric utility meter.

    The meter at my house is digital, it shows the voltage, power and kWhr totals on it's display. If I draw enough current to cause a voltage drop between my meter and my car, then that is a sure sign of power loss. That loss is caused again by the fixed resistance in the wire, circuit breakers, and fuses in the network. Multiply the voltage drop by the current and you get the power loss. Another problem that voltage drop can causes is higher current draw. We know the equation for power is voltage times current, we know that a twin charging vehicle is rated for up to 20kW. If the car limited it's charge by power we could have a problem since voltage sag would require an increase in current to keep the total power equal. Lucky for us the fine electrical engineers gave us control over the current limit instead. Current is what trips our breakers and melts our wires, so current is what we need to limit. By the way, a quick calculation shows that since the current limit is 80A, you would actually need 250Vrms to reach 20kW, doable in Europe perhaps. A voltage drop in our home is something we end up paying for, beyond our meter we don't BUT it does reduce the total power being delivered to the car.

    To throw out a few numbers:

    80A at 240V = 19.2kW
    80A at 220V = 17.6kW
    80A at 200V = 16.0kW

    If your car shows 240V before charging, and 220V during charging then you are loosing 1.6kW continuously due to resistive losses.

    Summary

    You loose power in your house wiring. You loose power in the chargers. You loose power in the battery cells. You consume power to cool the chargers and the batteries. All of these losses are directly related to the current. If you lower the current you lower your losses.

    I would like to see Tesla offer a software option where charging current is minimized based on a deadline. Lets say I leave for work every day at 8AM, I only care that my vehicle is fully charged the second I unplug the charging cable. I schedule a fixed starting time when my rates are lowest, lets say midnight. I set the upper current limit. The vehicle dials back the charging to the minimum required to fully charge by 8AM.

    Secondary advantage is lowering your average SOC (State of Charge). The lower the charge level kept in your battery the better for it's life time. This method would optimize the SOC.

    The information above is free. Take it, or leave it I don't care.
     
  2. yobigd20

    yobigd20 Well-Known Member

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    I was talking to a lead tech about this. It's actually harder than you think to do that. That's because the car's charging system is doing a lot more than you think. Everybody's power is "different". One person's power on a NEMA 14-50 is different than another person's power on a NEMA 14-50. (I'm no electrician so I may get some terminology wrong), but things like voltage drop (depending on length of run and many many other factors), spikes, surges, yadda, yadda.

    Consider a standard wall socket - you don't know what else is "using power" at any given time on the same circuit - lighting, fans, etc. On top of this, everyone's batteries are different too - different "health" (for lack of a better term). Basically, what may take you 10 hrs to charge may take someone else 5 hrs to charge using the same exact connector and amps. Therefore, I won't go as far to say "impossible", but the tech was more like "nearly impossible" to set a rule on the console to something like "I want 72% charge before 6am" (on the flip side obviously it's *very* easy to set "start charging at this time" but you can't regulate the exact time of completion). So I'm not sure if we're ever going to get that exact functionality even though we all want it. If your circuit is clean one moment and the car waits until a certain time to start charging, if something changes on that circuit then the car won't have enough time to make sure you have what you set by xxx time.

    I hope I explained that good enough. Personally, while an algorithmic challenge, I still think they can provide this to us with the caveat that they "can't guarantee it will be right 100% of the time".
     
  3. GSP

    GSP Member

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    The Chevy Volt has a "charge based on departure time" feature. I really like it, and it is near the top of my list for desired software upgrades for the Model S.

    GSP
     
  4. dtich

    dtich #P708

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    #4 dtich, Jun 22, 2013
    Last edited: Jun 25, 2013
    @op, good points, although I think most on this board are pretty well aware of them already.

    I myself do charge at 24A actually, for many of the reasons you state, and because I don't need to charge faster. it works out well for me. heat is the ultimate enemy of batteries and the less we can create most especially when charging, the better the efficiency and the longer the effective life of the battery.

    I think some people, like the one you were referring to, know all this but simply want the option to charge at whatever current they want, without hobbling, as they have spent money and time building open evse chargers and want to know and demonstrate that they work up to spec. makes sense to me. I'm sure ultimately they will opt to do what is best for their system.

    the option of an actual-power based charging algorithm that estimates charge finish times is doable. it may be difficult as the engineer points out, but the issue of all homes and charging systems being different doesn't really enter into it as the whole point of the algorithm would be to use actual voltage and current readings to make the schedule, so all these variables are de facto taken into account. to my mind anyway... we'll get it from tm eventually, just will take some time. I myself am fine starting charging at 130a and finishing around 6 with a full charge. works for my setup. others have tailored their own systems as well. the more tools the merrier of course.
     
  5. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    The Leaf also has a finish by X setting. An algorithm shouldn't be particularly hard.

    1. The driver sets minimum start time (for TOU), stop time, ideal SOC, and ideal amps.

    2. The car starts charging at the minimum start time and calculates when the finish will happen based on ideal amps and ideal SOC.

    3. If the calculation in #2 is prior to the stop time, ideal amps are reduced (or increased if it appears that it won't finish in time).

    4. At regular intervals the car checks the SOC and remaining time and adjusts the amps as required.

    5. Car stops charging at the stop time.

    FWIW, I've normally charged at less than the maximum--more to prevent overheating of the UMC than anything else. After the recent discussion about cell equalization, I reduced that to 16 amps. I'll see how it goes.
     
  6. mknox

    mknox Well-Known Member

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    Yes, but it doesn't throttle the charge current... it just delays the start time so that it will finish when you want it to.
     
  7. dhrivnak

    dhrivnak Active Member

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    I am not sure the Leaf and Volt comparisons are valid as they only charge at 15 amps. With a low predictable rate one can more easily calculate the finish time. But that said on my Roadster I charge at half rate (40 amps) for several reasons you may want to consider:

    A) Higher rate is not needed as I am always at a full level the next morning.
    B) Better on the utility as it is easier for them to balance smaller loads.
    C) It saves a little money as when I charge at 70 amps the A/C is frequently on but at the lower rate it rarely comes on. So for the reasons of the intial poster a higher charge rate is more wasted heat that must be gotten rid of.
    D) I suspect the batteries will last longer as in most chemical processes and mechanical machines running at half the design rate saves wear and tear. Pushing things at higher rates reduce their life. This is one reason NASCAR will wear our an engine in just 500 miles rather than the 200,000 we are hoping for.
     
  8. boilerbots

    boilerbots Member

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    I am glad to see that some are on board already with these ideas. I assure you that completing a charge at a specific time is not hard to do, a simple slow PI loop will get you there because the system can measure and adjust very quickly.

    Also some set their limits at 40, others lower like 20, remember we have virtually 3 size batteries and 2 charging options, so I am on the low end with a software limited 40 plus a 10kW charger and some are on the high end with an 85 and 20kW charging.

    Getting done as slow as you feel comfortable is a pretty good choice to make.
     
  9. RodF

    RodF Member

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    After reading this thread, I changed my charging level from 40 to 24 on the screen. But the next time I charged, it reverted to 40 amps. Do I need to change the dip switches on the HPWC to retain the setting?
     
  10. Todd Burch

    Todd Burch Electron Pilot

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    Model S remembers your charge setting based on GPS, which means you should make the change when parked at the charging location or it won't stick.
     
  11. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    It took me a couple of times to get it to stick.
     
  12. RodF

    RodF Member

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    Thanks for the info, Todd and Jerry. I'll try it again.
     
  13. Benjam

    Benjam Member

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    I've seen this many times lately - why do people use the word "loose" when it should be "lose"?
     
  14. hvb

    hvb Member

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    I'm assuming they don't know any better.
     
  15. David_Cary

    David_Cary Member

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    2013 Leaf charges at 6 kw on 240V - 25A not 15A. I have a charger that can dial back and I charge at 10A - for heat reasons and because even if I ran to zero, I would still fully charge on my off peak that starts at 9pm and be done by 6am.

    I don't know if the Tesla is the same but there is a flat overhead for cooling - about 300W. So if you are charging at 1KW (ie 120V), it winds up being a 30% overhead. Charge at 6 KW and it is only 5%. I would assume that Tesla has a variable overhead based on heat buildup rather than the Leaf's fixed overhead.
     
  16. markwj

    markwj Moderator, Asia Pacific

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  17. Vger

    Vger Active Member

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    A lot of people above, including the OP have not read Tom's or your work, clearly. The key point is that less and less current is not better, there is an optimum. It is also clear from Tom's work that the sensitivity to current is not as great as one might think. This is why Supercharging still makes sense. Yes, it is less efficient and harder on the batteries, but doing it a few percent of the charge cycles of the car's life is really not that significant.
     
  18. boilerbots

    boilerbots Member

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    The roadster and the Model S do not behave the same.

    Also, you can not lump 120V charging in with 240V charging because that is going take a huge hit since the incoming voltage is too low and most likely gets doubled during rectification to DC which is not efficient. I am speculating but this is generally the easy way out since 120V is kind of a desperate situation in the first place.

    The data in the thread posted by Markwj says to me, 70A charging 66.7% efficient (80.78/121.2=66.7), 32A is 64.3% (43.01/66.9=64.3%) because you should care about the watts in vs. the watts out of the battery. I don't care how this translates to Wh/km because that is influenced by driving and environment.

    The data from tomsax looks a little fishy too. I have never measured my voltage to be exactly 240V when I am drawing 70A out of a receptacle. So how can I believe the numbers posted when it appears the the voltage is always assumed to either be exactly 240V or 120V. A few volts dropped at 70A is a lot of wasted power.

    For an accurate and verifiable measurement one would need to measure the power entering their home at the point of their utility meter. Measure the power being drawn out from the battery during non-charging time and calculate the total efficiency under different charging scenarios. All while the vehicle and battery are kept in the exact same environment and driven exactly the same.

    If someone did this and I missed it please let me know.
     
  19. markwj

    markwj Moderator, Asia Pacific

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    I agree the roadster and Model S will most likely be different. However, absent any Model S data, the roadster studies are the best we have. It is, at least, real data rather than speculation.

    My measurements were taken (a) at the wall near the utility via a permanently installed meter, (b) in the car displays, and (c) verified by car logs. If you check the thread, you will see I reference kWh and Amps, not volts.

    My house gets 200Ax3 from the utility, and that circuit is split via bus bar to a MCB 60Ax3 to the house and another MCB 90Ax3 to the EV charger, with nothing else on the 90A side and a very short cable run. Voltage in Hong Kong is stable around 220V, and I see little difference in voltage when charging at 13A vs 70A, in my environment. I did no testing at 120V.

    The efficiency figure I was looking for was charging losses only at 13A, 32A and 70A, in high temperature environments. The result was clearly that little was to be gained beyond 32A.
     
  20. markwj

    markwj Moderator, Asia Pacific

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    Just noticed this comment. Can you explain how my data looks fishy?

    As for Tom - he knows more about Roadster than most anyone here. I think fishy is a strong word to use.

    It is real data, with logs to back it up. Not just idle speculation.
     

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