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Charging rates, current draw, and efficiency data for 120V charging

Discussion in 'Model S: Battery & Charging' started by EarlyAdopter, Feb 21, 2013.

  1. EarlyAdopter

    EarlyAdopter Active Member

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    #1 EarlyAdopter, Feb 21, 2013
    Last edited: Feb 23, 2013
    I've had my Model S for a month now and been doing just fine charging on a standard 120V outlet in the garage, given that I drive 30 - 40 miles a day. Still, I've been curious how the efficiency compares to 240V charging. I hooked up a Kill-a-Watt meter the other day and watched it like a hawk (worried about smoking it out), but also got some good data out of it.

    Quick caution: I don't recommend charging with any sort of device inline due to risk of overheating and fire. While the Kill-a-Watt is rated for 1875w (125v @ 15A), I don't know that it can safely handle that rate for a sustained period of time. Also, it's cheap and made in China. Please don't do this overnight and risk catching your house on fire.

    Ok, with that out of the way, here's the data:

    • Battery was down 5.9kwh from driving 15 miles before coming home to charge.
    • Garage temperature ranged between 50 - 55F.
    • Charged for 5hr 45min to full standard charge.
    • Car showed 2kw/2miles per hour rate of charge.
    • Car showed 6kwh recovered at the end of charge (more like 6.25kwh based on when it flipped to 6).
    • Car showed 15 miles recovered at end of charge.
    • At the wall 8.54kwh were used.
    • Current draw reported by the car (116v-117v @ 12A)
    • Current draw at the wall was 1490w (116v-117v @ 12.82A)
    • Current draw at the wall after charging complete was 1w
    Some conclusions:
    Charging efficiency at 120V: 73%
    [edit: shokunin points out I forgot to factor in idle load draw. With that, charging efficiency is 85%]
    Real rate of rated miles recovered: 2.6 miles per hour of charging
    Real rate of watts recovered: 1.09kw/hr

    Some open questions:
    I heard a circulation pump running the whole time during charging. It stopped when charging completed, so I assume that was for thermal management of the batteries. It could be that some of the charging losses were from heating the batteries, in which case the charging efficiency number would be higher. I'd need to repeat this experiment on a colder and warmer day to say for sure.

    One interesting thing I noticed:
    With the car fully charged and still plugged in, opening the door to start the cabin air heater drew no current from the wall (meter still read 1w). This means opening up the door in the morning to preheat the cabin will draw from batteries, not the wall. But, turning the heater on with the phone app without opening the door does appear to draw from the wall, at 250w. So, lesson learned - preheat from the app, not at the car, to draw from wall power. Cranking the heat up to max only increased this number by a few watts, presumably extra current from running the fan blower faster.
     
  2. Robert.Boston

    Robert.Boston Model S VIN P01536

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    Interesting data overall. This last note is a gem; thanks.
     
  3. Todd Burch

    Todd Burch Electron Pilot

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    Or, if you're like me and you have no 3G in your garage, get in the car, start the cabin heater, switch to range, and then back to standard. C'mon, time for Wi-Fi!

    Thanks for the info EarlyAdopter. I suspect from this information that the Model S, like the Roadster, charges most efficiently at closer to 10 kW.
     
  4. shokunin

    shokunin P85 & S40

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    The efficiency at 120v seems pretty bad. I'm guessing that 240v would be better since the percentage of 'overhead' amperage being used for heating/cooling/battery management would be less.

    Wait, actually 6 hours of charging and average 4KW daily vampire = 1 KW (6 hours). So looks like the 20% efficiency is still there at 120v, but you're also using 1KW for vampire load.
     
  5. jerry33

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

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    But the vampire load is only temporary and will go away once the get the bugs out. The 120V inefficiencies are forever.
     
  6. EarlyAdopter

    EarlyAdopter Active Member

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    #6 EarlyAdopter, Feb 23, 2013
    Last edited: Feb 23, 2013
    This is a good point. My car is on v4.1 with sleep enabled, but it didn't go to sleep the whole 6 hours of this test. Probably because I was around the car with the key the whole time. Normally it goes to sleep after 2 or 3 hours of no activity.

    I know that I lose 4kWh/day without sleep in my garage at 50F (and only 1kWh/day with sleep, btw). 4000W/24hours = 166W/hr. Let's take that as the idle load of all the processors on the car.
    • 8.54kWh pulled at the wall
    • 6.24kWh went into the battery
    • 1.00kWh went to the idle processors
    • (?.??kWh went to heating)
    So, the charging efficiency at 120V factoring in idle processor load but not heating is 85%.

    This is inline with what I've seen for 240V charging efficiency. But we need to factor in idle load there, too.
    Taking a 1 hour charge at [email protected]:
    • 9.60kWh pulled at the wall
    • 8.16kWh into the battery (85% efficiency)
    • 0.16kWh to the idle processors
    • (?.??kWh to heating)
    The charging efficiency for 240V increased from 85% to 87%.

    I suspect once we factor in losses from the battery heater we'll see the charging efficiency numbers between 120V and 240V converge even more. That said, the loss to battery thermal management should arguably be kept in place, since the longer charge time of 120V requires more time and thus more energy to go into heating. Thus, the true overall efficiency between 120V and 240V depends on temperature. I'm good with calling these numbers final.

    Overall charging efficiency, including thermal management, at 50F:
    120V: 85%
    240V: 87%
     
  7. Trnsl8r

    Trnsl8r Blue 85kwh since 12/8/12

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    That... or can it actually go to sleep while charging (on 4.1)?
     
  8. EarlyAdopter

    EarlyAdopter Active Member

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    I've been testing your hypothesis and it appears to be correct: the car does not enter sleep mode until after charging is complete.

    So, I'm willing to conclude that the energy efficiency of charging at 120V is the same as 240V, but overall 240V will waste less electricity as the amount of time the battery thermal management will need to run and the amount of time the car will need to spend out of sleep mode will be considerably less.
     
  9. mknox

    mknox Well-Known Member

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    I used a UPM 120v power monitor on a Chevy Volt for a week and it was fine. Didn't even get warm. I was worried at first too, and went out to the garage to check it every hour or so for the first day.
     
  10. Puyallup Bill

    Puyallup Bill Member

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    Let me strongly second that recommendation. I initially used a Kill-a-Watt 24/7 while charging a LEAF at 12 Amps. The male prongs are soldered to the internal circuit board, and one got hot enough to melt the solder connection.

    I repaired it, and still use it for a quick check with a load, but I will never leave it in-line unattended for anything.
     
  11. breser

    breser AutoPilot Nostradamus

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    EarlyAdopter, Can you redo your tests with your P85D now? There's some speculation that newer vehicles have improved charging efficiency.
     
  12. Max*

    Max* Autopilot != Autonomous

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    [yes I know this an old thread] I charge my 70D on a 120V outlet also (my roundtrip commute is about 20 miles), but I get 4 miles of charge per hour from the standard outlet.
     
  13. TexasEV

    TexasEV Active Member

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    This is going to depend on how far the outlet is from the breaker. I've charged my 60 at 4 miles/hr from a garage outlet with a short run from the breaker, but with other outlets I get 3 miles/hour when there is more of a voltage drop.
     
  14. Max*

    Max* Autopilot != Autonomous

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    Interesting, thanks.

    I think my outlet is... at least 30 physical feet from the breaker. I get 114-115V/12A.
     
  15. FlasherZ

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

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    I have a Signature car, and I charged at a condo while staying in Florida. App reported 4 mi / hr of charge @ 119V, 12A on my older car or at my home when not in the cold temps of winter (My home runs around 124V-ish under normal conditions).

    I've found that @ 116-117v, it will teeter between 3 and 4 mi/hr as long as it's above 75 deg F.
     
  16. EarlyAdopter

    EarlyAdopter Active Member

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    The biggest factor I've found that affects reported charge rate on 120V is ambient temperature. At 40F-70F I see 3 mi/hr. At 80F+ I see 4 mi/hr. Never charged <40F, but reports here show a continued drop off.

    I did notice that the onboard charger for the P85D are listed as a different part number. Perhaps there is something to the improved efficiency claim. I'll try to repeat the tests when I get chance.
     

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