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12 Volt Battery in Model 3, lessons learned from Model S and X

Discussion in 'Tech Talk' started by Mike, Nov 20, 2016.

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  1. Mike

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    Read a rather interesting blog on the techniques used by the Tesla software to tender the on board 12 volt battery.

    Briefly, there is some history of the onboard 12 volt (lead acid) battery on each Tesla (S and X) wearing out much quicker than in a modern ICE vehicle.

    The blog I read (sorry, no link) talked about how the onboard software was essentially never letting the 12 volt battery fully recover with a float charge.

    The blogger recommended a larger capacity 12 volt battery, have the software rewritten to allow the 12 volt battery to get a float charge as soon as being plugged into shore power (even to the detriment of recharging the main traction battery as fast as possible) and have the 12 volt battery kept at float charge off of the main traction battery when not plugged into shore power.

    Opinion: I really hope that the 12 volt battery system is less prone to maintenance in the Model 3 compared to the Models S and X. I also hope that its location is such that a swap out does not entail alot of trim disassembly .

    Thoughts?
     
  2. MelindaV

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    agree on expectations/hopes that changes are made to how the vehicle's systems deal with the 12v battery... however, my understanding is the 12v (at least in the current models) is not one you can pick up at auto parts stores - and - much more involved in it's connections than just a + & - connection. So most all current owners leave it to the service centers to change it out when needed.

    [​IMG]
     
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    • Mike

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      That photo alone kind of tells the story. I understand that the Model S was, from an initial design point of view, cobbled together 'no matter what'.

      The battery is a glass mat style of lead acid battery . The blogger I followed was able to aquire one relatively easily via the online aftermarket .
       
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      • AutoMcD

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        This is disappointing to see that they are using lead-acid. That is definitely a weak link in terms of maintenance.
        They make a lithium ion 12v swap-in, If the 3 comes with this I'm definitely replacing it.
        Lead acid has a 5-8 year life no matter how carefully you treat it.
         
      • Mike

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        I am not a subject matter expert, I'm just remembering what the original blogger said about this type of solution.

        Apparently, L-Ion 12 volt battery cannot accept a charge until it is at certain minimum temperature. The whole concept of "cold soaked" 12 volt battery seemed to really spook the original blogger ref "solutions" to the 12 volt battery problem.

        Like you, my impulse was, why not just use a L-Ion 12 volt battery?

        There must be some reason why Tesla is not using it (yet).....
         
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        • AutoMcD

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          Honestly my expectation is a 12V buck converter to make a constant 12V straight off the main battery pack, with one of those 0.5F audio capacitors for good measure. That there is another battery system at all seems a bit odd to me.
           
        • garsh

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          I think you want a separate electrical system. Something that can run some tests to make sure that the high voltage battery pack is in good shape and won't explode or catch on fire when you turn it on. I know this is the approach that Nissan took with the Leaf. I assume Tesla did something similar, but I haven't looked into it.
           
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          • garsh

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            Oh, and since an electric car doesn't require "cranking power" from the 12v battery, it's been hypothesized (but not tested, to my knowledge) that you could "start" a Nissan Leaf in a pinch (a dead 12v battery) by using 8 D-cell batteries in series.
             
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            • Badback

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              Has anyone asked Tesla why they did it this way?
               
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              • Mike

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                I was hoping that by bringing up this issue here, someone listening in who has a communications link with Tesla would ask that very question.
                 
              • @gravityrydr

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                From
                http://www.teslarati.com/understanding-tesla-12v-battery-service-warning/

                "The 12V battery maintains power for critical systems when the main battery pack is damaged or disabled. It powers the hazard lights, airbags, door locking and unlocking operations, as well as other critical components of the Model S. The 12V battery also ensures that electronics are “awake” and listening to the key FOB in order to automatically lock and unlock the vehicle based on proximity. It also allows the car to maintain its 3G connection for remote access when the rest of the vehicle is powered off. If the 12V battery happens to fail, it will isolate the main battery pack from the car and prevent charging. This is a safety feature of the Model S designed to help protect first reponders in the event of an accident."
                Makes sense, but still seems odd to me. Curious how Tesla will handle this on the Model
                 
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                • Michael Russo

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                  @garsh , I bet you'll be happy to turn (in...) that Leaf... :p
                   
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                  • AutoMcD

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                    I suppose that makes sense, I still don't have to like it! lol
                     
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                    • teslaliving

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                      I've got over 70K miles on my Model S in just over 2.5 years and have only had one battery replacement. The car gave sufficient warnings (low charge) and I was never out of service. In the early days there were more reports of issues with the 12V battery but recently there haven't been many complaints about it. My feeling from my own experience and in reading lots of Tesla news is that the 12V battery issue is mostly under control now.
                       
                    • Mike

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                      Hopefully the trend you speak of continues.

                      For s***s and giggles, I'd love to see the actual failure rates on a chart.
                       
                    • arnis

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                      My 50 cents:

                      Li-ion is not that beneficial for 12V battery.
                      At -25C it should enter hibernation/protection mode (not going to start). As it has very small thermal capacity
                      Li-ion 12V battery might get cold soaked within one night at -27C to freeze it.
                      And it will definitely not charge at that temperature. Will require heating elements, complexity for management system.

                      If I had to choose something, I would use sealed AGM battery. Not large. 30Ah.
                      And definitely add independent monitoring sensor and add software that controls charging independently.
                      Idea is to keep SOC above 60% at all times. Charge it to 90% and stop. Even if car is not used for long periods.


                      It's possible to add secondary mini DC-DC converter into the main pack (12V output only, so no orange cables).
                      With maximum output power of 5-10A. This inverter should have it's own firmware and logics. This would allow 12V battery
                      size to be reduced in size considerably (50-70%?) and would keep SOC of AGM battery in ideal range easily (60-80%).
                      With correct state of charge AGM battery lasts at least 8 years. This is how long BMW can keep OEM battery even if
                      start-stop cranking is used tens of thousands of times.
                      This mini DC DC converter could, potentially, be used with solar roof option to send excessive electricity into HV pack without
                      enabling contactors using 12V line.
                       

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