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Backing down the amperage - does it matter ?

Discussion in 'Model S: Battery & Charging' started by slipdrive, Sep 4, 2013.

  1. slipdrive

    slipdrive Member

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    When daily average is 50-80 miles, the full 40 amp charges in two hours. The mobile connecter and cable gets pretty warm. Does taking the amperage down to say 24amp save component life. Good idea to adjust amps to meet charging time available ?
     
  2. FlasherZ

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

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    #2 FlasherZ, Sep 4, 2013
    Last edited: Sep 4, 2013
    There is another thread on this somewhere. Bottom line is that probably, in the very very very very long run, it will have some effect, but likely very minimal. There is going to be a trade-off curve with energy consumption if the car staying awake to manage the charge vs. going to sleep with a full charge more quickly.

    Warm isn't a problem -- hot to the touch is. The UMC and HPWC will each dissipate up to 150W or so across the charge cable at full current.

    It would be a convenience problem for me to move the charge target all the time depending on how much I've driven the car, so I leave it at 80A on the HPWC. If that's not an issue, you may want to lower it to a number that guarantees a charge overnight for you.
     
  3. mknox

    mknox Well-Known Member

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    A number of folks have reported melted UMC connectors and "excessively" hot parts, so lowering the amperage is probably not a bad idea. To my eye, the UMC seems to be designed with not a lot (if any) of over-capacity.

    For reasons unrelated to the UMC, I have been charging at 16 amps / 240 volts, and even at this level, the UMC box gets warm. The connectors (both ends) stay close to ambient, however.
     
  4. jerry33

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

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    Lower amps are easier on the electrical service. I generally charge at 32 amps.
     
  5. rlang59

    rlang59 Member

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    Agree, that's why I charge at 25A. Only go higher if I need it charged faster, but most nights 25A is plenty fast enough for my next days commute.
     
  6. AC1K

    AC1K Member

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    the lower the amps, the less heat, the less stress on your components, the less load on the utility, the longer everything lasts.

    i backed mine down to 20A and my voltage never goes below ~243v, this tells me i am not stressing the utility or transformer. the car will always recharge to my 60% cut off in the morning (battery is never below 30%)
     
  7. neroden

    neroden Happy Model S Owner

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    I believe Tom Saxton did some experiments on the Roadster and concluded that between 32-40 amps was the optimal rate for minimizing electrical transmission and conversion losses. I'm not sure if the same is true on the Model S, but I've been assuming it.
     
  8. caddieo

    caddieo Member

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    I use programmed charging starting at midnight. Depending on how many miles I need to recover by 6 AM, I vary the amps between 12 and 30. I have not noted any excessive warmth or heat at those levels. I usually charge to 70% of full except when anticipating a longer than usual drive next day in which case I will also up the SOC to 80-85% of full. I am now away on a 3+ week trip and I left it plugged in with the same midnight start at 12 amps. I've checked on the car remotely a few times and it is holding steady at 170-175 miles every morning.
     
  9. AmpedRealtor

    AmpedRealtor Active Member

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    Not necessarily true. I read in another thread (here or at Tesla Motors, don't remember) that showed efficiency increasing with amperage. Whoever put that information together used a meter on the power connection and did a pretty thorough evaluation. The car draws a minimum amount of current to maintain the battery and other systems while plugged in. At lower current, more of the electricity that you feed into the car is used to feed the baseline usage as described above. Only excess beyond that will charge the battery. This results in a large loss of efficiency as very little of the current you are drawing is making it into the battery - the rest is feeding battery and other system maintenance. I don't remember the exact numbers from that thread, but 110v/15A was around 65% efficient and 240v/40A (or 60A, don't remember) was over 85% efficient. I apologize for not linking the thread, as I'm unable to find it and teslamotors.com forums are down right now.

    Model S is designed to use Tesla's 120 kW chargers. That's 120,000 watts getting dumped into the battery. Compare that to an HPWC running at its maximum 240v/80A level at just 19,200 watts. That is 16% of the power provided by a supercharger. Granted, the supercharger is DC and the HPWC is AC converted to DC, so there is some conversion loss and inefficiency there. However, it's still just a fraction of what the superchargers do to the battery. I wouldn't worry about thermal stress at 80A levels.
     
  10. spaghetti

    spaghetti Member

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    If I remember my Physics correctly, Joule heating (which is the dissipation of electrical energy via heat when current flows through a conductor) is proportional to the square of the current. Thus, very roughly speaking, charging at 30A will result in approximately half the heat when compared to 40A. And 20A will result in a quarter of the heat when compared to 40A. I charge typically between 20-30A. Like others have said, at 40A everything gets very warm. For my daily commute levels, I just don't see the need to charge at 40A.
     
  11. FlasherZ

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

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    #11 FlasherZ, Sep 6, 2013
    Last edited: Sep 6, 2013
    Yes, P=(I^2)R. 14 AWG wire is ~2.5 mOhm/ft. The UMC uses 2x 14AWG for each of its conductors, so resistance would be halved with identical parallel conductors; yet since there are two conductors (L1/L2), we multiply x2 again, so total 2.5 mOhm/ft on the cable. 18 ft cable on the UMC, then, should have a total resistance of 45 mOhm. So, the power dissipated across this is (40^2)*0.045, or 72W at 40A. It's 40.5W at 30A, and 18W at 20A.

    Now that's just the power dissipated across the UMC cable, nothing else. There's the question of whether cooling pumps have to run and what they draw, the vampire load of having to keep the car awake while charging (charge faster, car goes to sleep faster, for example). We could probably create a huge multi-factor spreadsheet to model it, but why? Elon tells us not to worry. Charge at whatever's comfortable for you.

    EDIT: For those wanting HPWC figures, the HPWC uses 1x 6AWG for each conductor. 6 AWG wire is 0.395 mOhm/ft. Multiply 2x for 2 conductors, and 25 ft of cable, so 0.395 * 2 * 25 = 19.75 mOhm. At 80 amps, that's 126W dissipated across the cable. 60A is 71W. 40A is 32W. Note this is for the HPWC, not the UMC.

    Me? I don't want to have to adjust the car all the time. I leave it at 80A.
     
  12. brianman

    brianman Burrito Founder

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    You blessed few with 5.0. Rabble rabble rabble.
     
  13. spaghetti

    spaghetti Member

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    Thanks, FlasherZ, for this information. It's great to understand the level of heat generated given the various resistances and currents. For my situation, it is the breaker that bothers me more than anything else, and perhaps you can help me figure out if I need to get the breaker checked out. The breaker is a quad 40/50/40 type (Cutler Hammer BQ240250 Quad Circuit Breaker 40 50 40 Amp 120 240 Vac New | eBay), with the 50A circuit feeding the NEMA14-50 outlet, and the 40A circuit for our dual oven. For my daily charge to recover around 60 range-miles, at 20A, I don't feel any heat on the surface of the breaker. And just slightly warm at 30A. But at 40A, it gets very warm. Now clearly my perception of warm breakers is very subjective, but still I can tell easily tell the difference between heat levels at 30A vs 40A. None of my other breakers get this warm. Any suggestions on what could be going on, what I should do, etc?
     
  14. FlasherZ

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

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    If a 40 amp current makes a breaker more than just lukewarm, I'd first suggest checking the torque of the bolts holding the wires to the breaker. Ensure they're tight, a loose connection will generate a lot of heat. If that doesn't cure the heat, you'll want to pull the breaker and check its connection to the buss bar - look for corrosion, pitting, loose clips on the breaker.

    Warm @ 80 degrees F with normal inside ambient temps isn't too bad, anything warmer you should worry a bit.
     
  15. slorojo

    slorojo New Member

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    Just for more data points, with my IR temp gun in a 95F garage and the car at a solid 40A for more than an hour, I measured 130F on the head unit of my UMC (to the touch it is very warm, but not so hot I can't hold it). The cable cools down to about 110F as it leaves the head unit, and the connector at the car is practically ambient. The 6-3 NM-B cable feeding the 14-50 is about 110F.
     
  16. spaghetti

    spaghetti Member

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    Thanks for the suggestions. I will check this out.
     
  17. Klaus

    Klaus Member

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    The UMC uses 2 pairs of 2.5 mm^2 wires according to Ingineer: Tesla Model S UMC cut open and modified to J1772 http://www.TeslaMotorsClub.com/showpost.php?p=341617

    Won't affect your calculation much, just to be accurate.

    Excellent information though!
     
  18. TommyBoy

    TommyBoy Member

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    This kind of information is why the forums are so awesome! I never thought about dialing back the amperage.

    I'm one of those 40-50 miles a day maximum people. I was concerned with the charging cable getting hot and had read, with great concern, some of the stories about connectors melting and plugs melting down. I sure don't want a fire.

    I can easily dial down the amperage to 20 amps and charge at maybe 15mph for two to three hours a night. That should keep the heat down to a quarter of what it would have been at 40A.

    Thanks for the advice!
     

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