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Roadster charging puzzle

Discussion in 'Roadster: Technical' started by gregd, Jun 28, 2017.

  1. gregd

    gregd Active Member

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    Hi folks,

    So I'm trying to understand why it seems to take 30-ish % more time to charge my car than what I calculate it to need.

    Scenario: charged from 69% to 85% SoC (per the logs) in 1hr 32 minutes. Charging was at 30 amps / 240 volts (7.2kw), in Range mode. This was after a 70 mile freeway drive that was mostly down-hill, so the car wasn't panting.

    So, by the math, I should have added 16% of the battery's capacity, or 8.5 kwh (0.16*53kwh). At a 7.2kw rate, I should have needed just 1hr 11 minutes to accomplish the charge, plus a little for inefficiencies. The logs for the event shows that I actually consumed 11.2kwh. Now, 7.2kw for 1.5hrs is 10.8kwh. So, the difference between 10.8 and 11.2 is likely that charging inefficiency (4%). But that 10.8kwh should match the 16% of 53kwh, and it's not even close. Why the difference between 8.5 and 10.8?

    I read Tom Saxton's very informative article (Tesla Roadster Charging Rates and Efficiency - Tom Saxton's Blog), but it predicts the lower number too. 16% charge * 530 wh/% is 8.5 kwh needed.

    Is this a matter of having an older battery (CaC of 140 vs 160 new), or the A/C coming on (I don't recall it doing that, but perhaps it did a little), or something else? Any way to better predict this for Roadsters in general? What information is needed?

    Thanks,

    Greg
     
  2. hcsharp

    hcsharp Active Member

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    I haven't read Tom Saxton's article but if I were to guess I'd question your charging efficiency. Below 30A the efficiency drops off considerably and if you were charging to 85% then a lot of your time was spent at much lower than 30A due to tapering down. If you had started with an empty battery and added 16% you would have probably been closer to 9 or 9.5kWh. That's my guess for what it's worth...
     
  3. Stefan T

    Stefan T Member

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    The charge speed is 42 km/h(26 mph) with 32A 230V and that must be sinular to 30A 240V befor it tap down
    it's approx 180 wh/km but idial is approx 143 wh/km so the loss in this charge senarie is approx 20%
     
  4. snd92

    snd92 Member

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    #4 snd92, Jun 29, 2017
    Last edited: Jun 29, 2017
    You can compare with my setup, but keep in mind that you should have better efficiency due to higher amperage. Also, at higher SOC the tapering is a factor.

    13 A at 230-235 V gives:
    15 km/h or 2.3 kW with 75 % efficiency

    I did attach the claimed charging times in respect of amps/voltage
     

    Attached Files:

  5. dhrivnak

    dhrivnak Active Member

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    I also believe the charging losses are closer to 15% as there is heat and inefficiencies keeping the batteries cool and in the AC to D.C. Conversion.
     
  6. hcsharp

    hcsharp Active Member

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    Charging losses can reach 50% or more at low amps and/or low voltage.
     
  7. gregd

    gregd Active Member

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    Ok, so the relevant line from the log file is this:

    06/23/2017 11:43:14 - 06/23/2017 13:15:55 (01:32:41) Charge 69% -> 85% 242V 30A of 30A 11.2 kWh 0.0 Ah 0.0 Ah

    I check the current on the VDS during the charge, and it stayed constant at 30A (no taper), and the voltage bounced around the 240 to 242 range. Good, stiff power source (and solar powered!). This is at the Vacaville, CA Bella Vista Park & Ride (Davis st. exit off I-80), by the way - Recommended.

    So, when I got home and charged there, I see an even worse situation in terms of apparent overhead. Note this is at 40A / 240V, so the "low voltage / low current" overhead shouldn't be significant (right?)

    06/25/2017 17:18:06 - 06/25/2017 21:07:52 (03:49:46) Charge 34% -> 81% 238V 41A of 40A 34.9 kWh

    238*40*1.8hrs = 36.3kwh drawn from the wall, 34.9kwh reported by the car (same 4% overhead as before), but this to replace (81% - 34%) * 53kwh = 24.9kwh in the battery (46% over!). Really? Actually, it's worse than that, since my ESS isn't the full 53kwh anymore; more like 46kwh now. So, I really only replaced about 22kwh in the ESS, or 65% over. Yes, the A/C kicked in to cool the battery, as I had just climbed 1,200ft at freeway speed with a 97F ambient temp. But I believe the A/C only draws a kwh or so (3A @ 375V, watching the amp display), and it was not running the entire time, so it alone shouldn't have caused the significant lengthening of charge time. What did? Any way to predict?

    How does one draw an extra 14kwh in under 4 hours, and not put any of it in the ESS? Is there an electron bucket under the car that needs to be emptied before it makes a mess on the garage floor?
     
  8. MileHighMotoring

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    It's personal preference, whether you mind the staining or not.
     
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