TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

Model S + other EVs with 12V Lead-Acid starter batteries

Discussion in 'Model S: Battery & Charging' started by franknesss, Sep 10, 2014.

  1. franknesss

    franknesss Member

    Joined:
    Aug 23, 2014
    Messages:
    114
    Location:
    San Francisco, California, United States
    I was just wondering how the Model S and EVs keep their 12V batteries charged and prevent them from dying. How long can you NOT drive your EV before you get a dead starter battery like ICE cars. Is there a small alternator or does the 12V Lead-Acid get charged when the EV is charging?

    Thanks in advanced
     
  2. Saghost

    Saghost Active Member

    Joined:
    Oct 9, 2013
    Messages:
    2,939
    Location:
    Delaware
    Most EVs, including the Model S, use a DC-DC converter to power the 12V accessories and charge the 12V battery from the big battery. It serves the same functions as an alternator, but has no moving parts (except sometimes a cooling fan.)

    Some of them, including the Volt (not sure about the Model S) have a separate circuit in the charger module that provides 12V power while the car is charging, and newer Volts also have a "battery tender" mode that checks the state of the 12V battery and charges it periodically when left plugged in and off for extended period.
    Walter
     
  3. franknesss

    franknesss Member

    Joined:
    Aug 23, 2014
    Messages:
    114
    Location:
    San Francisco, California, United States
    Thanks for that answer!
     
  4. tezco

    tezco Sig P85

    Joined:
    Nov 9, 2012
    Messages:
    818
    Location:
    Colorado
    The S wakes up from time-to-time to check on the 12V. If power in the 12V is low, the DC-DC converter is turned on to charge the 12V. When this occurs too often (at the end if the 12V's life span), you will see a message on the dash informing you that the car needs service, (or service may be calling you on your phone).
     
  5. franknesss

    franknesss Member

    Joined:
    Aug 23, 2014
    Messages:
    114
    Location:
    San Francisco, California, United States
    So from what it sounds like, having a low charge on your 12V starter battery can have an effect on range, since it only charges when the DC-DC converter is on or has to turn on?
     
  6. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

    Joined:
    Mar 8, 2012
    Messages:
    12,751
    Location:
    Texas
    When the Model S is idle, it still wakes up every so often to check and charge the DC battery. The only time it would have a significant effect on the range is when the battery is about at the EOL and it has to be charged a lot. What's not known is how close to EOL does the message appear.
     
  7. franknesss

    franknesss Member

    Joined:
    Aug 23, 2014
    Messages:
    114
    Location:
    San Francisco, California, United States
    Well, from what it seems, that can't be the case either. Typical Lead-Acid batteries don't have a very long shelf life if not constantly charged. So, if the Lead-Acid is receiving it's charge from the DC-DC converter, that means it's drawing it's charge from the main battery, which i think could reduce the overall range. I'm not sure on how the Model S does it's 12V battery management, but I was just wondering if someone had a more definitive answer.
     
  8. WarpedOne

    WarpedOne Supreme Premier

    Joined:
    Aug 17, 2006
    Messages:
    2,651
    Location:
    Slovenia, Europe
    Regarding reducing range ... typical 60Ah Lead-Acid battery in a diesel car holds about 60x12V = ~700Wh. That is about 3 miles of range for a 100% charge.
    Battery in MS is smaller as it does not have to crank a heavy ICE also "range reduction" for one complete fill-up should be less then 3 miles.
     
  9. cynix

    cynix Member

    Joined:
    Jul 7, 2014
    Messages:
    652
    Location:
    Sydney, Australia
    Why don't they just use a Li-ion 12V battery?
     
  10. tom66

    tom66 Member

    Joined:
    Dec 17, 2013
    Messages:
    622
    Location:
    United Kingdom
    Cost & simplicity.
    Another thing to safety test and qualify - additional expense.
    Lead acid is fine for this application, eventually we'll probably see LiFePO4 batteries used.
     
  11. breser

    breser AutoPilot Nostradamus

    Joined:
    Aug 28, 2014
    Messages:
    2,310
    Location:
    North Bend, WA
    Another reason is it probably makes the 12V battery fairly easy to replace with 3rd party service if the car isn't near Tesla service. A dead 12V means you're not going anywhere. If the car needs a unusual battery you may be stuck until you can source that part.
     
  12. franknesss

    franknesss Member

    Joined:
    Aug 23, 2014
    Messages:
    114
    Location:
    San Francisco, California, United States
    Actually, i was curious if anyone has tried LiFePO4 batteries yet.
     
  13. spaceballs

    spaceballs Member

    Joined:
    Sep 17, 2013
    Messages:
    533
    Location:
    Sammamish
    I would connect voltmeter (with min/max function) up and see what is the highest voltage the 12v system charges the lead acid, i.e. connect up and leave running during drive. That will determine if you can use LifePo4 (btw always have BMS for it).
     
  14. TonyWilliams

    TonyWilliams Active Member

    Joined:
    Jun 11, 2012
    Messages:
    1,001
    Location:
    San Diego - Tesla powered Rav4 EV
    Folks have already put lithium 12 volt batteries in the Nissan LEAF, and I'm sure there are others. It's just very expensive for hopefully longer life and lighter weight.
     
  15. franknesss

    franknesss Member

    Joined:
    Aug 23, 2014
    Messages:
    114
    Location:
    San Francisco, California, United States
    Do you know if they have any results?
     
  16. wycolo

    wycolo Active Member

    Joined:
    May 16, 2012
    Messages:
    2,422
    Location:
    WY
    > Typical Lead-Acid batteries don't have a very long shelf life if not constantly charged. [frankness]

    Not the case. Once a month topping off is all a good quality 12v battery needs when not being used. The problem arises when people forget to do this so they leave a charger connected all the time. Better to use a charger that monitors the voltage and only charges when necessary. The newer micro-processor chargers do that. They also pulse charge which is good for battery health (reduces sulfation etc). Tesla does all these good things on both the MS and Roadster. If you monitor 12v power outlet it is usually between 14.3 and 14.7vdc.
    --
     

Share This Page