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Traction Battery, 12 volt battery and Vampire Load... How does this work??

Discussion in 'Model S: Battery & Charging' started by lolachampcar, Jan 7, 2013.

  1. lolachampcar

    lolachampcar Active Member

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    The nerd in me is getting curious about how the traction battery, 12V battery and vampire load are interrelated.

    Assumptions based on other posts
    There is a 12V battery that powers the car when the traction battery is not engaged (when your butt is not on the driver's seat).
    This battery is a "normal" lead acid type.
    The traction battery looses x miles per day above simple self discharge in what has been described as Vampire Load.
    The 12V battery is charged from the traction battery at 15V when the traction battery is engaged (car on).

    Questions
    15 volts seems mighty high to be charging a lead acid battery. Is the 12 volt battery a (sealed) lead acid type?
    A vampire load on the traction battery while parked would indicate that the traction battery is being engaged for a period of time on a regular basis to replenish the 12 volt battery system. Has anyone verified this?
    What is the current draw profile on the 12 volt battery? It would be interesting to log this with a current probe to see what the static draw is, how often the traction battery is engaged to replenish the 12 volt battery and exactly what charge rate is used to replenish the 12 volt battery.

    I am sure there are lots more questions to be asked but I figured this would be a good place to start.
     
  2. Doug_G

    Doug_G Lead Moderator

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    Given that the open circuit voltage for a fully charged "12V" battery is 13.8V, it is not surprising that the charge voltage is higher. You do have to charge at a higher voltage than the battery; otherwise you can't force electrons into it. There will be some form of current regulation.
     
  3. wycolo

    wycolo Active Member

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    Please, too many threads. TMC expanding exponentially. Mods unable to contain all this. Only solution is to require Mod approval for ANY new thread.

    I know, I'm way outta line here, but having upped a detailed post I have to then post references to that post on all sister (parallel) threads- this should not be necessary in a perfect, tidy TMC.:smile:
    --
     
  4. lolachampcar

    lolachampcar Active Member

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    My last interaction with alternator charge set point was 13.8 volts (+/- .2 volts) if I remember correctly. I'll check the at rest voltage of a few SLAs I have around the shop to see where a fully charged battery normally rests.
     
  5. FlasherZ

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

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    The open voltage of a fully-charged lead-acid battery is 12.6v (2.1v per cell). 13.8v comes from alternator charging circuits, and represents a float/maintenance charge value for flooded lead-acid cell batteries. You need a lower voltage for gel-cell (13.4v) and/or AGM batteries (13.5v).

    Charging voltages are at 20 deg C, and you have to compensate accordingly at other temps. Active charging of a lead-acid battery from fully discharged state should be done between 14.2 and 14.4v, up to high SOC. Equalization takes place at 15v for no more than 2 hours and battery temp must be monitored.

    Undervoltage causes sulfation, while excessive voltage causes corrosion and electrolyte loss -- both very bad.

    So if the car is charging at 15V for as long as the traction contactor is closed, short of a special battery chemistry and/or materials, it sounds like it's being overcharged.
     
  6. lolachampcar

    lolachampcar Active Member

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    FlasherZ,
    I'm an ME with enough EE knowledge to be dangerous. Thanks for contributing.

    Any interest in putting a current probe on the hot lead and logging the maintenance profile? I'd like to see just how hard the 12V maintenance is hitting the traction battery. I'd also like to see what the steady state drain is on the 12V system with the car off / displays on standby and car off / displays powered down.

    Your comments on 15 volt charging are particularly interesting. I wonder if they are using something other than SLA (sealed lead acid) for the 12V battery. This might explain the 15 volts (BTW I've not verified that post).

    I would also like to understand the increase in vampire load at low temperatures. I assume the traction battery powers the 12 volt system which then heats the traction battery although that would be horribly inefficient (buck to 12V then into a resistive element).
     
  7. FlasherZ

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

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    I'd like to but I haven't seen the procedure to open the front to the battery yet, which is where I'd have to put the current probe. Then, I have to confess that I don't have a particularly good analyzer so I'd be sampling manual readings. :)

    I haven't, either. But if it's 15V for a significant length of time, and it is lead-acid, there's no doubt in my mind why the 12v batteries are failing. My car is headed to Chicago tomorrow for a service visit, so I doubt I will have time to perform my own experiments.

    Reportedly the battery heating circuit is part of the overall HVAC system, so it would likely be on the 12V side -- of course, a guess on my part.
     
  8. lolachampcar

    lolachampcar Active Member

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    I've written parsers to put raw data into MegaLogViewer (a free log viewer software) format. It's not fantastic but it does the job. If you have a raw data logger and current probe that will do something like 1 hz then we should be able to get a good profile. Are you in cold weather?
     
  9. brianman

    brianman Burrito Founder

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    Huh ?
     
  10. lolachampcar

    lolachampcar Active Member

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    I hesitate to post my personal experience unless it is backed up with documentation. I've been known to be wrong before and I do not want to mislead anyone :)

    A reader was kind enough to send me the following link on Sealed Lead Acid (SLA) battery charging and failure modes-
    Reasons for Battery Failure: Battery case buging /distortion, Acid leakage
    It confirms my experience with automotive an aircraft 12 volt systems. Normally they are charged with a current limited source until a float voltage is reached (normally 13.8 volts) then held at that float voltage by decreasing the charge current to near zero if needed.

    One post said Tesla told the writer that the 12 volt system is charged at 15 volts.
    Another post indicated a failed battery was deformed.

    We can check this on Model S by attaching a volt meter to the charge posts behind the bumper. The voltage can be checked shortly after locking and leaving the car, several hours later when the displays have powered down (power down on, of course) and then shortly after opening the door and engaging the traction battery.

    Most step down DC to DC converters (Buck Switcher) use a resistive divider to set the output voltage. Some are programmable like the ones used to source one or more of the multi rail processors from Intel and others. It would be worth noting the Model S firmware version when doing the above test to see if Tesla can indeed set the 12 volt system charging voltage via firmware.
     
  11. FlasherZ

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

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    So I did a set of measurements today...

    First, the car was plugged in and had both screens lit, reading "charging complete". I measured the terminal voltage at 13.84v.
    Next, I removed the UMC, opened the door, and stepped on the brake to turn the car "on". I measured the terminal voltage at 13.75v.
    I allowed the car to power down and waited a few minutes. With the car powered down, doors locked, and the screens off, I measured the voltage at 14.53v.
    I then plugged in my charge connector and it began charging from 231 miles... Terminal voltage was 14.54v.

    I'll try to catch some more scenarios -- right now I'm waiting for the car to go to deep, power-down sleep mode.
     
  12. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Could it simply be faulty software (firmware) here trying to charge an already charged 12V battery, thereby damaging it. From FlasherZ's above post it would seem the battery was at a perfectly fine voltage of about 13.8 with the car on, still after powering down and during charing (of the big battery) the car increases voltage on the 12V to 14,5V. Overcharging damage?
     
  13. jomo25

    jomo25 P4398

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    Out of curiosity. Is the 12V battery a special battery or is it a run of the mill car battery?
     
  14. lolachampcar

    lolachampcar Active Member

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    Alternatively, Tesla could be using two different systems to deal with the 12 volt side. One would be while the car is "On" which is 13.8V (a perfectly normal number) and the second system deals with the 12 volt battery when the car is "Off". In my opinion, the 14.5 volt charge point is too high. FlasherZ, what firmware were you running in case someone else checks their car so we can see if the "Off" mode voltage is firmware selectable?

    I appologize for not pitching in. I will set up a current probe and logger when I get my car.
     
  15. FlasherZ

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

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    #15 FlasherZ, Jan 8, 2013
    Last edited: Jan 8, 2013
    It's too early to draw any conclusions based on anything I've posted. I just checked again, after charging had been completed, and it still showed 14.5v. I didn't get the car to sleep because I (stupidly) left it plugged in. I'm waiting for the car to go to sleep now so I can measure it.

    EDIT: After the car entered "deep sleep" mode, displays powered down, the terminal voltage went to 12.8v. Based on this, I'm guessing the battery voltage itself might be 12.8v... the DC-to-DC converter runs and supplies 13.8v when the car is doing more than sleeping as a float/maintenance charge, and when the car thinks the 12v battery needs an active charge, it uses 14.5v... That seems like the most plausible scenario but I could be wrong.

    Keep in mind this is all with v1.19.31.

    Given the physical description of the faulty batteries that have been removed (bulging, etc.), I can only guess they were somehow overcharged. 14.5v seems pretty high for an active charge, wonder if that's causing it?

    I keep hearing that 1.19.31 has definitely fixed whatever problem was there with the low-voltage battery. Let's hope so.
     
  16. jomo25

    jomo25 P4398

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    Well, according to Lontok in the other thread, the rep told him the next release would fix this (after replacing his battery).

    Edit: re-read lontok's post. I guess he could have meant .31 fixed it. Or the next one.
     
  17. tezco

    tezco Sig P85

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    I wonder if the temperature compensating circuit is measuring temperature at some place other than at the 12V battery itself, resulting in a higher than needed float level. Also, from what I have read, it sounds as if the 12V is snuggled into tight quarters deep in the frunk, but I haven't tried to get a look at it myself. It may be that the battery is overheating if there is little air circulation, particularly with the high "vampire" loads that exist when the display remains powered 24/7.
     
  18. widodh

    widodh Model S R231 EU

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  19. lolachampcar

    lolachampcar Active Member

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    widodh,
    I specifically started a new thread as there would be fewer forum members interested in the guts of the three topics then those posting having 12V battery issues. My goal here is a comprehensive look at the system so that those technically minded can understand the system and form conclusions about how it can/will be fixed by Tesla and perhaps how to manage their car until it is fixed. These posts would get lost in the larger thread.
     
  20. widodh

    widodh Model S R231 EU

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    I get that idea :)

    We should just prevent having two topics going the same direction. If this one stays very technical about how the system works, then you're probably right that it needs it's own thread.
     

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