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Battery discharging even when plugged in?

Discussion in 'Model S: Battery & Charging' started by znib, Oct 15, 2015.

  1. znib

    znib Member

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    I have my P85D set to charge to 70% at 01:00am. Usually this gives me about 275 km (171 miles) in the morning.
    After not using the car for a day it was at only 270 km (167 miles). The next day at only 266 km (164 miles). The car was plugged in all the time.

    Why does the battery go down if the car is plugged in? Shouldn't it use the power from the outlet?
    Why does it not charge the battery back to 70% at 01:00 when it drops below during the day?
     
  2. Electric700

    Electric700 Member

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    I'm not sure why, but that's under a 10 km drop and there may be a trigger to charge it back up once it goes lower. You may want to contact Tesla about it as I'm curious to know too.

    The car does use power to maintain the battery temperature over time.
     
  3. Father Bill

    Father Bill Member

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    My experience is that it has to drop 5% before my HPWC charges the car again. I recently was out of the car for 2 weeks and kept track of the level of charge via the iOS app while I couldn't drive. I know that I lose about 1-2% overnight even when plugged in. Just my experience but I think this is the normal "vampire" draw of the system discussed in many threads here.

    Peace,

    Father Bill
     
  4. znib

    znib Member

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    Good to hear it's normal, but I didn't expect vampire drain while the car is plugged in, since it could simply draw current from the outlet. Wouldn't this make more sense, also in terms of battery life?
     
  5. znib

    znib Member

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    This morning, the battery was at 275km again, so it does indeed re-charge once it drops below a certain level.
    And now it's about time to drive it again. :-D
     
  6. Electric700

    Electric700 Member

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    Glad to see that!
     
  7. nwdiver

    nwdiver Active Member

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    Not sure if it's a physical characteristic of Lithium batteries or simply how the car was designed but the battery isn't kept on a float charge like we're familiar with for Lead Acid... the battery gets charged then the car disconnects itself from the grid which is opposite to the way most chargers I'm familiar with operate... maintaining a pre-determined voltage on the battery.

    I've definitely seen this too; I'll set the car to charge to a SOC of 60% then it's sometimes as low as ~57% in the morning. I think battery balancing could also cause the SOC to drop slightly. I know wk057 discovered that the battery modules operate autonomously even when separated from the pack to maintain the individual cell voltages.
     

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