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Charging the Roadster

Discussion in 'Roadster 2008-2012' started by malcolm, Sep 14, 2008.

  1. smorgasbord

    smorgasbord Active Member

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    #841 smorgasbord, Feb 15, 2021
    Last edited: Feb 15, 2021
    I finally got around to installing the Gen-2 HPWC's that I accumulated over the years (one as a referral bonus, one from a Canadian), so I can now charge at 70amps. Previously, my Roadster UMC would crap out over 30 amps, so I had it limited to that instead of the 40 of which it's capable.

    So, tell me why I shouldn't just set it to charge as fast as it wants now? Sorry if I've missed some pinned post.
     
  2. X.l.r.8

    X.l.r.8 Supporting Member

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    The max rate is way under the minimum C recharge rating so personally as long as it was not getting the PEM hot I would/do charge at max rate.
     
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  3. gregd

    gregd Active Member

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    Unless you need to have the charge finish more quickly, I've read that the "sweet spot" for charging is 240v at 24 to 32 amps. Higher wastes power with I2R heating, lower accumulates more of the base "overhead" load because of the longer charging time. 24 amps is plenty quick for fully recovering a day's use overnight.

    I was initially limited to 24 amps for charging my car because I was using a clothes dryer 10-30 outlet. When I upgraded my panel to install a full 14-50 outlet (so, 40 amp capable), I kept the lower charge rate as an efficiency measure. The lower current is also kinder to the contactor in the charging station / UMC. The original Roadster UMC 240 is notorious for failures, and until I installed an OpenEVSE, I wanted to go easy on it.
     
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  4. smorgasbord

    smorgasbord Active Member

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    It used to be part of the Annual Service that they'd try charging the car at 70 amps. Several years ago, the local Service Center put a note in the report that since they no longer had a Roadster HPWC they couldn't test that out.
     
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  5. X.l.r.8

    X.l.r.8 Supporting Member

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    My PEM was getting warm because of a tired fan so I can see why they would check, it was fine in lower amps, with the new fan it’s crazy cool all the time now. I think high amp charging has a worse effect on the PEM than the battery. We discharge at much higher rates.
     
  6. hcsharp

    hcsharp Active Member

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    There are lots of opinions on this subject, with some of the experts contradicting each other. I generally agree with the theory (suggested above by X.l.r.8, gregd and others) that it's best to strike a balance between efficiency and keeping heat generated to a minimum. Testing has revealed that the best efficiency is achieved between 24 and 48 amps depending on other conditions. Below 24A and the system overhead losses become a significant factor. Above 48A and thermal management starts reducing your efficiency. All of this depends on other factors. Is the car already hot from driving or high ambient temps? Is it so cold that the cooling efforts will be minimal and you might even save some energy from not having to heat the battery as much after charging? Apparently this can even apply to people in Texas this week!:(

    Your question was probably not limited to matters of efficiency. As for what's best for cell life and PEM longevity, less heat is always better. That's an argument for lower amps. Maybe even lower than 24A. It depends on other environmental factors. Are you only adding 40 miles, it's a cool 50 - 60F in your garage, and the pack is not already hot from driving? In that case you're not going to hurt anything by charging at 70A. In fact charging at 70A every day would probably have very minimal impact in my opinion. You'll find lots of opinions on this. ymmv.

    Yeah, it's disappointing they don't test that anymore. A few service centers purchased CAN SRs from me so they could continue this test using newer HPWCs after they lost their Roadster chargers. But now most of the 80A wall chargers are disappearing.

    If the charger is working properly the contactor only opens and closes when there's no current flowing. Having said that, you're probably still correct that it's kinder to the contacts to have less current flowing through them when they're closed.
     
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