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Near annual replacement of 12V battery is typical according to Tesla Service Tech

Discussion in 'Model S: Battery & Charging' started by ecarfan, Jan 15, 2015.

  1. ecarfan

    ecarfan Well-Known Member

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    #1 ecarfan, Jan 15, 2015
    Last edited: Jan 15, 2015
    Yesterday my S showed a message saying something to the effect of "12V battery needs service. Contact Tesla Service." I called my local service center (Burlingame, CA) and they said they would come out and replace the 12V battery, which they did. No charge of course, my car is just over a year old, has 20K miles on it.

    The service guy was very nice and took about 20 minutes to change the battery. As has been posted before, it is located below the cabin air filter at the rear of the frunk on the passenger side, actually just below the most forward portion of the dash. It is not easy to get to.

    The service guy said that the 12V battery is typically replaced every 12 to 16 months. It runs all the microprocessors (as well as 12V motors for the windows, wipers, etc.) and has to deliver a very precise and stable voltage for the computers to run properly. He also said that when the car displays the message about the 12V battery needing service, it is still safe to drive the car for a few days on short trips, but to immediately contact Tesla Service and set up an appointment to have it replaced

    The battery model that came out of my car, and the new battery, were both C&D Technologies DCS-33IT "Deep Cycle Series" C&D Deep Cycle Battery

    I searched the web for them to see how much they cost and could not find them listed for sale with a price, but found companies selling what they say is an "equivalent replacement" for about US$53.00, likely less than the DCS-33IT would sell for.

    I did not realize that the Model S 12V battery would need to be replaced every 12 to 16 months. When the car is out of warranty, that is a higher maintenance expense for 12V battery replacment than any ICE I have ever owned, as ICE 12V batteries typically last 5 years or more if you buy a good one. And the Model S 12V is not a battery I would try to replace myself, it's rather tricky to reach and you have to know exactly how to do it.

    I am posting this because I do not recall reading before on TMC about the need for the 12V battery to be replaced every 12 to 16 months.

    Model-S-battery.jpg
     
  2. Cattledog

    Cattledog Active Member

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    One of the EEs on the forum can answer this, but I assume the 12v battery does not get recharged by the main battery like the battery in an ICE does by the alternator. It might be that it can last longer than 12-16 months but replacing at annual servicing is good practice, or as in your case when you get a message.
     
  3. wycolo

    wycolo Active Member

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    Bears repeating, early and often. Yes this guy gets trashed on a regular basis by an over-reliant system so annual replacement is prudent as well as convenient for user and TM. I'd peg the quality level @$100-US. You don't want to use a run of the mill garden tractor battery in this application!
    --
     
  4. TexasEV

    TexasEV Active Member

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    Yes lots of people on this forum have reported 12V replacements, many proactively, at a year or a little more. I got a call from Tesla service that mine needed to be replaced about a month after my 1 year service. I don't think Tesla expected the 12V to need to be replaced this often, but that's what experience has shown.
     
  5. ckessel

    ckessel Active Member

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    I'm curious how much Tesla will charge for this out of warranty. At $100 a pop just for the battery alone each year, that's more than I paid for oil changes/filters for my ICE. Then add in the labor to have it replaced, that's probably another $75 for a 1/2 hour labor.

    I keep hoping the claim that EVs are less expensive to maintain will pan out, but the data points so far don't seem to be supporting it.

    I suspect Tesla's warranty costs so far have been within the set aside amounts (according to ER calls) not because there isn't as much work as an ICE, but primarily because they do the work in house by their own salaried employees rather than pay dealers exorbitant rates for the repairs.
     
  6. rlang59

    rlang59 Member

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    The 12V battery does get recharged by the main pack. There is a DC-DC (high voltage DC to low voltage DC) converter that charges the 12V battery from the main high voltage traction battery.
     
  7. mitch672

    mitch672 Active Member

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    Just a guess, but it seems they need a better "float charging" circuit from the DC-DC inverter, so the battery isn't over/under charged, it should last far longer than 12-18 months with a properly designed charging circuit.
     
  8. Vger

    Vger Active Member

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    #8 Vger, Jan 15, 2015
    Last edited: Jan 15, 2015
    Same experience here. I am now on my third 12V battery after 25 months. The first replacement was due to the early batteries being of lower quality than Tesla expected, but the replacement last week was unexpected (by me) and proactive by Tesla.

    It seems to me that this is an issue of the car allowing the battery to "deep cycle" too deeply on a routine basis. Lead-acid batteries are typically only supposed to routinely use the top 50% of their SOC. Anything below that, on a routine basis, is considered "battery abuse." The Model S DOES recharge the 12V from the main battery, but apparently not proactively enough to keep the SOC of the 12V battery above 50% the great majority of the time.

    If there is a good reason why they need a deeper discharge cycle on this battery, I don't know why they do not switch to a Lithium drop-in replacement. There are now such 12V batteries (with integral BMS) available for the automotive and marine markets. See for example here. Of course they are more expensive, but if they can indeed last for 5 years or more, that is a lot cheaper in the long run, especially considering the labour for replacement under warranty!
     
  9. techmaven

    techmaven Active Member

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    Tesla originally shipped the car without deep cycle 12v lead acid batteries. I think all of those will have failed between 6-18 months. I don't think a proper deep cycle battery will fail nearly as often... The duty cycle isn't any worse than off grid solar for instance. Now, Tesla did go through some different batteries so some early adopters got basically the same non-deep cycle batteries as their first replacement and those will have to be replaced. Some early cars had problems with the quality of the non-deep cycle batteries and some cars have problems with the DC-to-DC converter that charges the 12v from the main pack. At this point, a proper deep cycle 12v battery that would be suitable for solar use for example should last quite a while unless the DC-to-DC converter has failed.

    BTW, this is one of the reasons why I want to be able to top off my 12v with a solar panel to reduce stress on it.
     
  10. caddieo

    caddieo Member

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    I just had mine replaced yesterday @ <17K miles/18 months old. I got my warning dashboard message mid-December but the Longwood SC did not have any 12 V batteries available and the next supply was not due to arrive until January. The service guy assured me I could continue driving without worrying about the car suddenly dying since the message is apparently a pro-active warning rather than a dire prediction. And so I did for almost 4 weeks - including a few +/-200 mile trips above the speed limit involving airport runs for holiday visitors. The service guy was somewhat vague about what the battery state is that triggers the warning but I gathered that it had something to do with ability to recharge. Feel free to correct me if I'm wrong.
     
  11. Jaff

    Jaff Active Member

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    Vger, do you replace your 12v at your annual service date on your Roadster?...I always followed that practice as preventative maintenance...the 12v Lithium battery that I looked into may not have had enough "operational temperature range" (low end only -4 F) for my area...might work well for milder climes like yours though... Smart Battery® | 12V Lithium Ion Batteries for RV Marine and Automotive

    **edit**...sorry, didn't see your link...

     
  12. Vger

    Vger Active Member

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    So far, I have still been having Tesla do the annual service on our Roadster, and I know they replaced it at least the last time (just don't recall previously). As for the temperature range, that could be an issue I suppose.
     
  13. Jaff

    Jaff Active Member

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    Yeah, but I'd bet the temperature thing would not be an issue for many in the South, or you on the coast...I chickened out of replacing my 12v lead acid battery with a lithium one due to this (and the fact that I don't know if the trickle charger from the main pack to the 12v is meant to handle a lithium battery) ...I'm not going to do anything that might damage the main pack...
     
  14. SteveS0353

    SteveS0353 Member

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    The 12V lead-acid battery does get recharged by a DC-DC converter from the high-voltage main battery. The issue with the 12V battery longevity is related to the so-called "vampire load", power to run the on-board electronics. Typically, the vampire load is 1.2kWh per day. There are various sources on the forum for this number, and represents ~4miles of range loss per day from the main HV pack.

    All rechargeable batteries have a cycle limit. In other words, you can only discharge and recharge batteries a specified number of times before they "wear out" and can't support the load. The above battery had a capacity of 33 ampere-hours (33AH on the label). At 12V this is 0.396kWh of capacity. The 1.2kWh vampire load represents 1.2 / 0.396 = 3 charge / discharge cycles per day. The data sheet for the Tesla battery (thanks, by the way to the OP for providing the picture) has a capacity limit graph. Assuming Tesla allow the 12V battery to discharge 50% before recharging it from the DC-DC converter, the battery has a cycle limit of 1,800 before wear out. At 3 cycles per day, the 12V battery in the Model S would have a typical lifetime of 600 days, or ~18 months. Some batteries will last longer, some will fail sooner, but if one plots the bell curve, I would expect the mean will be about 18 months to 12V battery replacement.
     
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  15. caddieo

    caddieo Member

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    I think that's what the service guy was trying to explain to me. As per above calculations, it looks like I was smack in the middle of the bell curve. (Post #10) :tongue:
     
  16. Johan

    Johan Took a TSLA bear test. Came back negative.

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    I've had one replacement so far, got the warning after about 14 months of ownership. I have a voltage monitor plugged in from time to time in the 12V socket in the center console. It typically shows 14.3V while the car is on which seems high to me? When I got the warning I measured the voltage on the battery terminals with a volt meter, it was something like 12.3V with the car off and the key not present. So completely normal. Spoke to service and they say the warning comes on whenever there is low voltage, if even for a very short period of time, and won't go away until the battery is replaced (I asked for them to clear the warning remotely, as I had an appointment to replace it, but was told this is not possible).

    My guess is they voltage delta during 12V charging is too high and this caused premature wear on the 12V battery?
     
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  17. SteveS0353

    SteveS0353 Member

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    Right on average, I would say. Rechargeable batteries don't typically stop working suddenly. Some are catastrophic failures, but more often than not, they simple wear out and degrade over time. The warning will come when the battery is no longer able to hold a specified threshold capacity. It's not the end yet -- just a warning that the battery is nearing the specified wear out limit. That's why the SC tells you it's OK to drive for a while -- they have some headroom built in to the algorithm so that owners won't typically get stranded. Its not a warning to be ignored though as the 12V battery is required to close the HV contactor in the main traction drive battery to get power to the motor.

    - - - Updated - - -

    No, it's all about depth and number of charge / recharge cycles. The vampire load is large -- remember, a 40W light bulb running 100% of the time. To recharge a battery, the voltage across the terminals needs to be higher than the open circuit voltage to drive some charging current. It's normal for a 12V battery to see about 14V or so when charging -- about 300mV delta per cell. The capacity is computed by measuring the terminal voltage under a defined load. Too much voltage drop after a charge cycle, and the capacity is deemed to be low.
     
  18. Johan

    Johan Took a TSLA bear test. Came back negative.

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    OK you're probably right. The question then is why they didn't go with something bigger and better than a standard 12V automotive battery? I know there are other types, for boats, for people with very heavy duty sound systems in their cars etc.
     
  19. roblab

    roblab Active Member

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    Size, weight, amp hours are satisfactory. Most of the thousands out there cause no trouble. I've had mine nearly 2 1/2 years, 62000 miles, never a whimper.

    Cost, small batteries (if high volume stuff) are cheap. It's probably a standard Toyota or Honda size! Who knows. They may switch to some LiIon stuff when they gey the Gfactory going. Lead acid is a poor choice.

    they have the warning system in place, it's not deep discharge. The car goes to sleep so the vampire load is cut down on long off times.
    They are going to be servicing it every now and then, so they check it when you're in, replace it, probably don't even tell you, big deal. I thought it was a pretty small "automotive" battery: I was thinking more "motorcycle" type. I've never seen it. I asked yesterday, and they had not changed it; it didn't need it.
     
  20. SteveS0353

    SteveS0353 Member

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    What's important in the Model S application is cycle count -- the capacity, size and weight are also factors, but the application is demanding on cycles. The C&D is a deep-cycle battery, and suitable for the application. The issue is really the vampire load -- it's way too high IMHO, and is stressing the 12V battery, even a very good battery like the C&D.
     

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