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Better 12v battery tech exists: LiFePO4

Discussion in 'Model S: Battery & Charging' started by Aargau, Dec 9, 2014.

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  1. Aargau

    Aargau Member

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    I love how Tesla is innovating in their cars; not just with batteries and drivetrains, but with their display, the door handles, the GPS enabled air suspension, etc.,


    Just curious if anyone has insight into why Tesla sticks with lead acid or gel batteries instead of a newer technology.

    LiFePO4 batteries are pretty standard on yachts, and like LiIon allow for much deeper drain and faster recharge. They are a bit more expensive, but lighter weight and more energy dense.

    Here's an example of one:

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    and here's a supplier for marine systems:

    Balqon Electric Vehicle Manufacturer
     
  2. strider

    strider Active Member

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    But Tesla's don't need deep-cycle. The 12V is there to keep a few systems "warm" when the car is off and to provide power for critical systems in the event of an accident that damages the main pack. It actually gets cycled very little unless the car sits for very long periods. Also, based on my experiences w/ RC vehicles, Lithium Polymer batteries can catch fire quite remarkably if they are mishandled by improper charging or physical damage. Cars are in accidents all the time. If a boat is in an accident of sufficient strength to damage the battery the occupants will likely be evacuating.

    I am a fan of Li-Ion 12V batteries to save weight and use them in my motorcycles but they need to be externally balanced as there are multiple cells - my bikes sit on tenders anyway so I just use a "smart" tender that also balances. Tesla would have to build this Battery Mgmt System and/or integrate it with the main pack BMS. I recently needed to replace the 12V battery on my Roadster as it gave up after 4 years. After much deliberation I decided to stick w/ Lead Acid as it "just works." Although heavy it's still a good technology for shallow cycle uses like in our cars.
     
  3. tom66

    tom66 Member

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    LiFePO4 is more expensive.

    It's also slightly less safe, requiring more considerations in crash and adverse scenarios.

    It requires balancing too, increasing the electronics/design cost (Tesla minimised this as much as possible by choosing over-the-shelf as much as possible.)
     
  4. Skotty

    Skotty 2014 Model S P85

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    I'm all for moving to something different. Lead-acid batteries have never been good and their quality seems to be slipping even further lately. I just replaced the 12v battery in my Jeep Wrangler a year ago and it's already dead and won't hold a charge. I doubt other options would cost more. That last car battery I bought was getting up close to $100 and it only lasted a year.
     
  5. woof

    woof Model S #P683 Blue 85 kWh

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    They could always switch to AXPW batteries! (ducks)
     
  6. JRP3

    JRP3 Hyperactive Member

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    Actually I think they do, otherwise they are using a larger capacity battery than necessary. They need a battery that can provide high cycle life and calander life. The current AGM lead acid doesn't seem to be cutting it.
    In fact it's always being cycled since the DC/DC keeps recharging it when ever it drops below a set voltage.
    LiFePO4 is not Lithium Polymer, and is in fact one of the safest lithium chemistries available, and one of the more durable ones.
    A BMS for a LiFePO4 pack is very simple and inexpensive, quality LiFePO4 cells tend to stay in balance on their own. I don't even use a BMS on the pack in my car. The only real issue I could potentially see is needing a bit of heating in colder climates, which may be the deal breaker, though some LiFePO4 may have overcome that issue.
     
  7. rtz

    rtz Member

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  8. spaceballs

    spaceballs Member

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    Same reason the Roadster 1.5 didn't have a 12v battery, then Later with 2.0/2.5 they add back in the 12v battery. Likely some requirement by government.
     
  9. green1

    green1 Active Member

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    Actually, it's the opposite, what the Tesla's 12V doesn't need is cranking amps. The trade-off is usually that you get higher cranking amps by not having a deep-cycle battery, which is why ICE use lead-acid. Deep cycle is better for all applications that don't need the cold cranking amps.

    Tesla did this in one of the versions of roadster, but they went back to having one, my understanding is that it was less reliable, or less efficient, to run the DC/DC inverter full time in lieu of the battery.

    - - - Updated - - -
    @spaceballs my understanding is that it was not a government requirement, but more of an engineering decision.
     
  10. Cosmacelf

    Cosmacelf Active Member

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    As far as lead acid batteries getting more unreliable goes, I have seen that in late model premium cars, like my wife's Infiniti QX56, the battery lasts two years tops. However in cheap econobox cars, batteries last a long time. My theory is that the luxury cars present a constant trickle drain on the batteries reducing their life. Which is what Tesla does to the batteries as well.

    So, yes, count me in for wanting to see a better engineered solution too. It is an unreliable failure point just waiting to brick your car. Either lithium batteries, or a DC-DC converter solution that doesn't compromise safety (my understanding is that they don't want the high voltage contactor to be engaged unless the car is turned on - but that doesn't make sense as the car can turn on its own AC to keep the battery cool and the AC uses the main battery pack...)
     
  11. tom66

    tom66 Member

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    FMVSS states that an electric vehicle must provide electrical power to the hazard lights for at least one hour after shutdown of the DC/DC or main battery pack:

    http://www1.eere.energy.gov/vehiclesandfuels/avta/pdfs/fsev/eva_proc/1999_eva_tech_spec.pdf

    Perhaps there was an early exception for the first Roadsters, but I believe it's a requirement for modern EVs where the primary low-voltage supply is derived from the main battery pack in some way (including hybrids like Prius and PHEVs like the Volt)
     
  12. Jaff

    Jaff Active Member

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    #12 Jaff, Dec 10, 2014
    Last edited: Dec 10, 2014
    I have the 50ah version of this Smart battery in my Roadster's trunk...I installed it back a few months ago to run the amps for my new sound system...I was going to install a 7ah version to replace the 8ah lead acid battery that the Roadster employs (located in the front of the car)...I decided against installing it after being warned that it might not be able to handle (for certain) the winter temperatures that we get up here...I think the "operating temperature range" of these batteries is not broad enough (at the low end) for critical systems use...(their website shows an operating temperature range of "-4 to 175 F)

     

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