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Any reason not to hook up a battery tender to the 12 volt battery ?

Discussion in 'Model S: Battery & Charging' started by edgrzy, Aug 15, 2015.

  1. edgrzy

    edgrzy New Member

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    My neighbor who does not work on cars just had his 12 volt battery replaced. He does not look forward to doing this every 12-18 months. He asked me if I could hook up a Battery Tender to his 12 volt battery. It looks pretty simple to me to do this. He would like to top off his 12 volt battery for 8 hours once a month. He intends to do this while not charging the main battery. Any one have any concerns on why he should not do this ?
     
  2. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Not a bad idea, but you will need more than a little battery tender for 8 hours a month.

    Do some math:

    1. The thirsty Vampire drinks about 1 kWh per day.
    2. That is an average of about 42 Watts. 1,000 Wh/24hours
    3. At 12 Volts, that is about 3.5 Amps. 42 Watts/12 Volts

    To keep the Vampire from sucking down the 12V battery, you should get an 8 Amp charger like the BatteryMINDer 12 Volt 2/4/8 Amp Wet/Gel/AGM Battery Charger. Of course, to avoid having to reprogram it to 8 Amps and AGM on every plug in, you will want to leave it in the Frunk, connected to the 12V battery, and just plug in its AC plug every time you get home.

    The Vampire is very thirsty and it takes way more than a little wall wart charger plugged in occasionally to feed it...
     
    • Informative x 1
  3. wk057

    wk057 Senior Tinkerer

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    Makes little sense when Tesla already has charging for the 12V built in. Just plug the car into 120V instead.
     
  4. CHG-ON

    CHG-ON Still in love after all these miles

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    I don't get this at all. The main pack charges the 12V whenever needed. I am not sure that a battery tender would solve the deep discharge issue that is inherent in the MS design.

    I would love to learn more about this.
     
  5. SteveS0353

    SteveS0353 Member

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    The vampire drain is such that the 12V battery is cycling (charge - discharge cycles). That's what kills batteries. All rechargeable batteries have a wear-out mechanism related to the number of charge - discharge cycles. A battery tender (a decently powerful one, as Cottonwood mentions), connected whenever the car is parked (when practical) will supply the vampire load and avoid cycling the 12V battery. The significant deduction in cycles over time will mean the 12V battery will have a longer and happier life.
     
  6. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    The only reason to do this is that something like the 8 Amp BatteryMinder that I mention above would keep the battery always charged and avoid the "Death by a 1,000 Cycles" that the Tesla greedy Vampire creates. It is the constant discharge/charge cycling that typically kills the 12V battery in a Model S every 12-18 months.

    Personally, I consider this to be Tesla's design problem and just let them replace my 12V batteries under warranty.
     
  7. S85D

    S85D Member

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    How do you get to the 12v battery to hook up the battery charger? What do you have to take off, etc?
     
  8. smorgasbord

    smorgasbord Active Member

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    I think the profile for lead-acid battery cycle life is quite different than lithium-ion. What kills lead-acid batteries is deep cycling - taking the charge down too far. Heat is also a factor. Assuming Tesla's 12V charging algorithm is proper, I have a hard time understanding how a battery tender can significantly help. But, I'd be interested in hearing the mechanism(s) involved if you know.
     
  9. SteveS0353

    SteveS0353 Member

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    Indeed, the wear out cycle count is not a linear function of discharge depth. Cottonwood and I have contributed to a detailed engineering discussion on this in this thread -> Near annual replacement of 12V battery is typical according to Tesla Service Tech. specifically entry #32 in that thread. Bottom line, the cycling of the 12V battery to support the 1.2kWh per day of vampire load will lead to that battery wear-out in 12-18 months. Tesla's battery charging algorithm may be proper, but the real issue is the level of vampire load requiring frequent recharging of the 12V battery. The reason a battery tender would help is that it would supply the vampire load from the shore power, removing the need to cycle the 12V battery so often, or at all.
     
  10. jbcarioca

    jbcarioca Active Member

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    I agree. especially since i have just received a new 12v from Tesla today. If I did not think that I'd put an 8amp 12v charger in the franc, connected to the battery and connect it immediately when i connect the main charger cord.

    The problem really is that the 12v continues to work at all times handling the charge port door, the communication to Model S and tesla reporting, charger communications, essentially everything that goes through the CANbus. That means the 12v discharges and is recharged as many as 6-8 times per day when the car is parked. As everyone agrees Tesla need a better solution for these requirements. By 'everyone' I mean Tesla service people also, This may be the largest single warranty budget item because so many cars must be retrieved and carried to the SC by tow truck. The cost of that is ridiculous. I predict a better solution is almost ready to emerge because they cannot survive the cost and inconvenience of this situation as production ramps.

    It looks easy to solve, but I suspect it will take some careful engineering to do. After all, nearly all the vehicle systems were designed for 12v systems and are parts used in order high end cars. It ought to be easy to solve, but it is not, so I am told. Some people here are more competent than am I on this subject so maybe somebody knows how to solve these issues.
     
  11. st50maint

    st50maint Member

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    Why doesn't the car charge the 12 volt battery more often?
    The 12 volt battery would not discharge as deeply and I assume the amount of energy required from the main pack would be similar on average?
     
  12. scottm

    scottm Active Member

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    Or why not make all the accessories run off 400V and just leave the contactors closed all the time? :)
     
  13. blanche

    blanche Member

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    Because most accessories (lights, wipers, etc.) are standard parts sourced from the auto industry supply chain. The standard in that industry is 12 volt power. So running everything off 400V would require custom building every single accessory, which is neither financially nor logistically a possibility for Tesla. Plus, I'd be a bit worried about 400V running through every single accessory in the car. That sounds like a real bad accident waiting to happen.
     
  14. FlatSix911

    FlatSix911 918 Hybrid

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  15. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    That unit puts out 4.3 Amps, so it might be just enough for the incredibly thirsty Vampire in the MS if it is connected continuously, but there is also the problem that the MS disconnects the 12V CLR from the 12V battery after 10-20 minutes and then there is no way for this unit to charge though the CLR.
     
  16. stevezzzz

    stevezzzz R;SigS;P85D;SigX

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  17. snort

    snort Member

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    a switching power supply that converts 400VDC to 12VDC is a trivial thing--there's one in the car already to charge the12VDC battery. Functionally the same thing as the external trickle chargers you're suggesting. Why doesn't tesla run the 12VDC components off of that and dispense with the 12V battery?

    No engineer capable of doing any part of this would overlook that possibility unless there was a darned good reason. My guess is that the 12VDC loads fluctuate wildly and the worst case is several tens of amps. They chose to go with a small trickle charger and a bigger lead battery. That's probably a bunch cheaper than having a 12VDC power supply big enough to supply the worst case 12V load and is available off the shelf.

    my guess is that the external trickle chargers won't help the problem at all....if they do, simply building a slightly bigger trickle charger into the car than the one that's already there would solve the problem trivially and telsla would have already done it. my guess is that they're working on whatever the loads are that give the 12V system a hard time but that they're proving challenging. do any of the folks that have put current monitors onto the 12VDC system have any thought what they may be?

    --Snortybartfast
     
  18. Larry Hutchinson

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    My guess ts that they don't want the 400V connected to anything unless the 12v system validates that it is safe to do so.

    So a continuous 12V charger is out.
     
  19. SteveS0353

    SteveS0353 Member

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    The architecture of the Model S has a safety feature involving the 12V battery. There are contactors inside the LiIon battery pack, which are normally open, to isolate the high voltage inside the pack. The contactors and closed by the 12V system when the car is operating. In the event first responders need to intervene to extract someone from the car, all they have to do is sever the 12V loop which is in the frunk to ensure those HV contactor stay open. That isolates the HV to inside the pack, making it safe for first responders to work around the car as long as they don't puncture the HV battery.

    To feed the 12V from the DC-DC all the time would require the contactors stay closed; not a safe situation. If the contactors are allowed to open, then the architecture requires an external power source (i.e., the 12V battery) to close the contactors to get power to the drive drain when you want to drive the car.

    The vampire drain is typically 1.2kWh per day. That's 1200 / 24 = 50W running continuously on average when the car is off. We don't know for sure what makes up that level of power, but here are some potential contributors...

    3G/LTE radio (needed all the time for access to the mothership, unless on WiFi)
    WiFi Radio (needed 100% of the time when in a hot spot)
    Bluetooth radio (might be powered off when the car is off)
    Key fob radio (needed 100% of the time when the car is off to detect approaching key fobs)
    Center console CPU and memory subsystem (could do done much more economically by reducing the CPU frequency when the cars off)
    Battery Management System (BMS). This is probably a big contributor to the vampire load.

    In any event, 50W continuous on average is a HUGE vampire drain. A 12V charger from the shore power capable of supplying 50W (4.5A, say) would eliminate the deadly cycling of the 12V battery. That's a band-aid. The solution to the 12V battery longevity is to reduce the vampire drain by an order of magnitude at least.
     
  20. tga

    tga Active Member

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    Safety. You don't want the HVDC system energized unless necessary. Plus it gives a little bit of run time to put the car in jack mode and run the flashers after your main contactors go boom. :wink:

    I have a 36A 12V switching supply that I use to power ham gear. I bought it from these guys: http://www.12voltpowersupplies.us/ (it's the S-400-12 model)

    It would work as a "battery tender on steroids", but, as Cottonwood points out, it's seems like a lot of hassle while the car is still under warranty.
     

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