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Cold Weather Regen Limiting and Accessory Draw

Discussion in 'Model S: Battery & Charging' started by scaesare, Feb 16, 2015.

  1. scaesare

    scaesare Active Member

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    I'm starting this thread new rather than post in the existing "Cold Weather Regen" thread, because I'm explicitly discussing the accessory draw here.

    We all know that the pack can't accept significant charge if it's too cold. I often will have regen limited in the cold, and occasionally it will be disabled altogether. Yesterday was such a day for me (along with the first time I've ever seen the dash "range snowflake") where, after sitting out all night in nearly 0 degree (F) weather, the battery was completely cold-soaked and regen was disabled.

    As I began driving, my accessory power draw was somewhere on the order of 10kW. This was cabin heater on, both seat warmers on full, and the pack heating obviously engaged. I determined the draw based on the the dash gauge when the car was stopped at an intersection.

    So, we know the battery pack can't accept charge when it's too cold. My question is: Why can't the car offset the current draw for accessories with regen power, even if it doesn't provide current to the pack?

    Even 10kW of regen helps slow the car when you are approaching an intersection. And when the pack heaters are running full-bore it would be nice to recover as much energy as possible.

    My guess is that the accessory draw can be instantly variable: The heaters could go off at any time, thus forcing regen to instantly disable during deceleration, thus causing a safety concern. Of course, regen can instantly disable when traction control detects a slip too, causing the same safety concern... but in that circumstance there may be little other choice, and halting regen may be the lesser of two evils.

    Thoughts?
     
  2. FlasherZ

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

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    This is a great point. My guess is that the drivetrain doesn't have the instantaneous power draw information from the pack to make the right decision on regen. Even when plugged into the mains, it seems the car reactively adjusts amperage to balance out an average draw from cabin heating, etc. If you sit in the car while it is using shore power to heat, and you turn up the heat, it doesn't suddenly jump up, it ramps up to match the average over a few seconds.

    That would be my guess, uneducated WAG though. Perhaps they could instrument it somehow and get the messages to the drivetrain.
     
  3. scaesare

    scaesare Active Member

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    Hmmm, the pack teardown threads have revealed current shunts in the pack allowing for current draw measurement... and the dash obviously displays power draw with a sub-second resolution... so the car knows what the pack is putting out....

    But I wonder if the accessory draw is essentially a parallel load behind the DC-DC converter, thus making it impossible to avoid supplying HV from the drivetrain during regen to the accessories without also applying that voltage across the pack terminals. Your point about the accessory behavior on shore power might support that...
     
  4. FlasherZ

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

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    It's not that the loads aren't known, it's that they're not communicated in real-time between systems. It places a dependency on two components that don't have them and keeps things similar. That's just a crazy guess on my part.
     
  5. ArtInCT

    ArtInCT Always Learning

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    #5 ArtInCT, Feb 17, 2015
    Last edited: Feb 17, 2015
    All of the accessories which you mentioned are traditional 12 vold DC accessories.
    They get their power from the 12 volt DC battery as I understand it.
    So what you are asking for then, if I understand you correctly, is for the ReGen to realize that the 12 volt DC battery is providing load and that the ReGen should assist the 12 volt battery IF and WHEN the large battery pack is compromised with the inability to accept current from the ReGen?

    In other words, if the output of ReGen is 12 Volts DC, ReGen could appear to be a second 12 V DC battery wired in parallel with the actual 12 V DC battery.

    My tuna boat has quite a few deep cycle 12 volt batteries which are used for bait wells, washdown pumps, deck and tower lights and electronics. When the generator or engines are running, they essentially recharge the 12 volt batteries via their charging system. ReGen would or could be such a power source I recon. There must be a reason why the S does not do this. I suspect my ignorance of the electrical systems in the S is making a difficult thing seem a bit simpllistic.
     

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