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HPWC Reliability: an Overnight Charging Strategy

Discussion in 'Model S: Battery & Charging' started by m6bigdog, May 31, 2015.

  1. m6bigdog

    m6bigdog Member

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    While I haven't had my MS charging on the HPWC for very long, I am certainly concerned about its reliability given the conversations on this forum and I have a strategy that may help prolong the life expectancy of the HPWC. This strategy also applies to the UMC although the max 40 amp capacity to nominal daily charging rates are not as great.

    My strategy while not proven is based on the reality that high amperage is the enemy (the big stressor) of any electrical system; its individual components, contacts and wiring terminations.

    From the electrical engineering perspective when you want the HPWC to last, decreasing the HPWC’s normal daily current draw from 80 amps to 40 or below should greatly improve reliability and life expectancy. This doesn't mean you never use the 80 amps capacity, it means you extend the life expectancy by only using the HPWC at 80 amps if and when you actually need that level of charge rate to save charging time. The 80 amp charge rate is a fantastic rate for a quick mileage boost when you need the extra distance before you leave home but apparently not for the daily overnight charging. Hence the Dual Charger and HPWC with 80 amp is a worth while home charging convenience when you need it.
    I anticipate the main contributors to the HPWC reliability is the plastic enclosure that is covered in rubber with no ventilation so the internal heat generated is trapped inside the HPWC and cannot escape.

    The strategy is simple, just dial down the HPWC charge rate on the MS charging console to as low as practical when you don't need a full 80 or 40 amp charge rate. Therefore, only use the 80 amp charging capacity for short periods when necessary.
    If you are at a destination charger I would always go for the max current available and start charging as soon as you arrive/plugin; so you can disconnect from the charge station ASAP (charging etiquette). However at home I would dial it down.

    Strategy Reasoning:
    The HPWC's internal power dissipation due to internal resistance (fuses, relay contacts and wiring terminals) at 40 amps is 25% the value it is at 80 amps. The heat generating power from the current flowing through the HPWC’s internal resistance is the square of the current flow, so if you reduce the amps the power and heating is significantly reduced. Therefore, if you can feel the HPWC get warm when it is operating at 80 amps it will operate significantly cooler at 40 amps and below. Note: If your HPWC is getting uncomfortably warm to the touch you will want to have the HPWC diagnosed for loose internal wiring terminals (including fuse and relay terminals) before it completely fails.
    Also, overnight home charging with the HPWC's 80 amp capacity is excessive since a seriously depleted 85kWh battery can pretty much be charged in 6 hours with 56 amps or 41 mi/hr. charge rate.

    How would this work?
    I personally chose 6 hrs. for my overnight charge period because my utility company’s lowest EV electrical rates are available from 11PM until 7AM and the 6 hour normal nightly charge period provides some tolerance for variation in the rate and time required to reach the charge goal without expanding the charging time into the partial-peak period where the kWh cost doubles. If you can set a longer charge time, all the better especially for more miles/day driving range.
    I limit my MS-85kWh to a daily 150-175 mile range and it is typically driven 50 +/- miles/day. That leaves plenty of range for unexpected local trips and I can reach a Supercharger in any direction if my day starts off with an unexpected long range trip or I can boost the range up to 200+ miles by increasing the battery range and turning up the charge rate first thing the morning. Recharging the daily 50 miles only require a 12 amp charge rate during a 6 hour overnight period.

    Strategy Details:
    The HPWC summary chart uses a default 345 Watts per mile driven baseline (your actual Watt mileage my very). Charging at 40 amps is 10kW @ 250Vac and that provides 29 mi./hr. charge rate. 10,000Wh/345W = 29 mi. and 6 x 29 = 174 mile charge.

    Therefore: 50 * 345 = 17.25kWh and with 6 hours to recharge then: 17250/6 = 2875Wh. With 250 Volt supply the 2875/250 = 12 amps charging current.
    However, 12 amps is only 15% of the HPWC full charging capacity and 2% of the power of an 80 amps charge rate and that is far below the point where I anticipate the charging current would stress the HPWC’s internal components.

    IMHO, 24 - 32 amps (30 - 40% of the HPWC 80 amp capacity) is a good balance between useful miles/hour charge rate and 6 hr. recharge period, as 24 amps = 17 mi./hr. charge and 30 amps = 23 mi./hr. charge rate. And, at the 32 amp charge rate the HPWC will produce only 16% of the HPWC’s internal power dissipation of the 80 amp charge rate.
    Also, in 6 hrs. at 24 amps = 102 miles range and 32 amps = 138 miles range; that provide for plenty of daily mileage flexibility without the need to adjust the charging rate for the mileage driven each day while only using a fraction of the HPWC's capacity for daily charging .

    If you need to reach 100% SOC/range, I would only dial in enough current to reach the 100% charge after 6 hour.
    Example: 48 amps should provide nearly 200 miles of range in 6 hours. That is 36% of the 80 amp HPWC power dissipation and that certainly will be easier on the HPWC than allowing an 80 amp charge rate or 58 mi./hr. for 3.5 hrs. to add 200 miles of range. In the final analysis the difference with respect to total power dissipated by the HPWC’s internal components at 58 amps is reduced by 50% when charging the 200 miles slower at 48 amps rather than 200 miles faster at 80 amps, while the continuous power as stated above is reduced to 36% of the 80 amp charging rate.

    Save your HPWC and UMC by turning the charge rate down.
     
  2. chriSharek

    chriSharek Member

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    That's a long post. So, since i install the wiring for my HPWC for the 40 amp service (50 amp breaker w/ #4 wires) and not getting dual chargers on my car, I'm aligned with your protocol above, right?
     
  3. breser

    breser AutoPilot Nostradamus

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    I'm firmly of the opinion that the time and energy to manage this isn't worth whatever you're saving. There's no guarantee that your HPWC will fail if you use it at 80A all the time. So the way I see it you're just prematurely spending time and energy figure out this when you may never have any issues with the HPWC. But each their own.
     
  4. S4WRXTTCS

    S4WRXTTCS Active Member

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    My solution would be to buy the HPWC with the car so that it has the warranty of the car.

    Then don't worry about it. if it fails it will likely fail under warranty.
     
  5. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Reducing current will reduce power in the connectors, the wires, and the chargers. OTOH, all of this is designed to take 80 Amps. I am set up for 80 Amps, but normally charge at 56 Amps. That cuts the power down 50% in all the above parts, but charges at 70% of max rate. When I want to charge faster, I have no qualms about charging at 80 Amps as many times as I want, but if I don't need fast charging, why add stress to the components, even if that is a slight stress.

    If you want to go all the way down to the 40 Amp region, I would suggest using something like 44 Amps. At 40 Amps and below, the car will only use one of the charging modules. At higher than 40 Amps, it will split the current across the two charging modules. Splitting the current across the two charging modules will both reduce the power dissipated in each module as well as test both modules each time you charge.

    Enjoy your Tesla, and may you enjoy the usefulness of your dual chargers many, many times!!
     
  6. FlasherZ

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

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    I've had the HPWC since they shipped it and have run at 80A regardless of circumstance. The only issue that I've had is dirty contacts at the car coupling that results in higher temperatures there (and Tesla has replaced the cable for me in that case). I don't see that many HPWC failures here, other than the cord sets getting extremely hot because of dirty contacts. UMC's, on the other hand...

    I'm sure that very, very, very, very long term testing might indeed prove you right on the reliability, but I'm not convinced it's worth the hassle to deal with it. I'd suggest that if you're going to set it back, you consider 56-60A which would be half the power dissipation across components; I don't know that you need to go all the way back to 25% of the rated capacity.

    To each his own... but I'll stick with 80A and not worry about it. :)

    - - - Updated - - -

    Yup. Nothing to worry about.
     
  7. Kalud

    Kalud Member

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    I haven't investigated much to give a clear threshold but sometimes I charge at less than 80A on my HPWC and recently I observed that the split of current is sometimes differently than just 0-40 on the master charger and the balance on the slave. For example I charged at 60A and it did a ramp up to 30 then paused (like it normally do at 40A) then you can hear the second charger engage then slowly ramp up to 60. This needs more tests but I guess one of the latest firmware update included some tweaking in the master/slave charger load balancing for wear.

    Back on the original topic, I do normally always charge at 80A but share the idea that charging at lower current when not needing the faster charge is a good idea (less thermal stress in the long run).
     
  8. FlasherZ

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

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    I don't think we have any positive confirmation as to how it works. We do hear a contactor clicking on just after 40A, but I believe it's unknown whether the load shifts to balance across them at that point or whether the slave is only handling the excess over 40A. I would assume they'd shift to balance, but I don't believe it's confirmed.

    It might be easy enough to determine, if someone had the back seat out, a clamp ammeter would tell you... perhaps someone has done this and I haven't seen the thread?
     
  9. m6bigdog

    m6bigdog Member

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    Not exactly, as the dual charger is a bonus for destination charging where the 80 amp HPWC or J1772 charge station is available.
    However, at your home if your service is limited to an EVSE load of 40 amps you are not going to miss out on much.
     
  10. m6bigdog

    m6bigdog Member

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    Yeah, your probably right.
    For those that understand the post are most likely already doing some form of the strategy and for those that don't understand it is just so much gibberish and a waist of forum bandwidth.
    I appreciate your feedback.

    But for those that want to know... here is some really good stuff I just found from wk057; as it is more of the technical junk on the HPWC at 80 amp usage from a different perspective,
    http://www.teslamotorsclub.com/showthread.php/32704-HPWC-IR-Pics-(more-fun!)/page2
    Thanks wk057 for your efforts, IMHO, time well invested.
     

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