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Idea: Vampire drain is partly battery pack balancing?

Discussion in 'Model S: Battery & Charging' started by rebra, Aug 9, 2015.

  1. rebra

    rebra Member

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    I got thinking about vampire drain, and observed my car getting less and less drain after sitting for a while with a quite low SOC.

    Earlier posts have mentioned no active balancing hardware, but the possibility of "bleeding off" the higher voltage packs through resistors. So maybe the higher vampire drain at high SOC really is some balancing going on? Could explain why different cars have different vampire drain. And why (it seems to me) the car consumes less energy sitting still after a few days and maybe on a lower SOC (although it does look like the energy saving mode is more aggressive if left alone a few days).

    (oh, and first post :))

    rebra
     
  2. djp

    djp Roadster 2.0 VIN939

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    You're right, pack balancing could account for some of the loss in range while the car is sitting at high SOC. Li Ion batteries also self-discharge at a faster rate at high SOC, which also contributes to the range loss. The vampire drain isn't just from the electronics.

    Welcome to TMC!
     
  3. apacheguy

    apacheguy Sig 255, VIN 320

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    Good idea. I had suggested the 16 BMBs contained within the pack (which control balancing) could explain part of the loss. Wk057 though said these only draw at the milliamp level and are unlikely to be significant contributors.
     
  4. jbcarioca

    jbcarioca Active Member

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    From piecing together many disparate comments and disclosures I think the vampire drain certainly has several components.

    That said, the largest single one of those seems likely to be battery temperature control. My own observations suggest that the drain is higher at low and high temperatures than it is in moderate ones. I have no personal observations from cold weather but i do with hot and moderate ones. My car loses about 1.1% per day when temperatures are above 30C, around .8% per day when between 20-25C. Both of those i have had experience with while my car sat without charging for a week or more beginning with an 80% SOC. The location was identical, the only variable altered was ambient temperature.

    Without a doubt keeping the car awake speeds the drain, and asking remote S or another app for SOC does wake the car up. I have not enough experience to judge the impact of this but it must be easily measurable.

    As several people have mentioned in various threads, equalisation is a pretty minimal draw so is unlikely to have any significant deleterious consequence.

    I am not an expert, but some people posting on TMC are qualified electrical engineers who have specific li-ion battery experience. if I ever have one of my battery-related posts disputed by one of those people you'll see me make a lightening-fast retraction. I think my observations are correct, though.
     
  5. AmpedRealtor

    AmpedRealtor Active Member

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    Or simply that hotter weather causes the 12v to lose charge more quickly, requiring more frequent top-off, and hence the higher vampire drain in hotter temps?
     
  6. stopcrazypp

    stopcrazypp Well-Known Member

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    The Model S supposedly has a "deep sleep" mode when it reaches every low SOC. That may be what you are seeing instead.
    "For example, a Model S battery parked with 50 percent charge would approach full discharge only after about 12 months. Model S batteries also have the ability to protect themselves as they approach very low charge levels by going into a “deep sleep” mode that lowers the loss even further. A Model S will not allow its battery to fall below about 5 percent charge."
    http://www.teslamotors.com/blog/plug-it
     
  7. rebra

    rebra Member

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    That may be the case. SOC was around 35% when I parked.
     
  8. djp

    djp Roadster 2.0 VIN939

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    I think "deep sleep" refers to the anti-bricking protection that disconnects all systems to prevent the pack from being fully discharged. The deep sleep only kicks in at extremely low SOC, and has a side effect of sacrificing the 12V battery, so needs a service tech to wake the car back up.
     

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