Why does the Prius have **TWO** batteries?

Discussion in 'Newbie Forum' started by mareakin, Apr 2, 2012.

  1. mareakin

    mareakin Junior Member

    Mar 29, 2012
    2010 Prius
    A hybrid system battery and another one just like a normal conventional ICE car battery to start the Prius.

    I am not critiquing the design, just trying to learn what their thinking was.

    It seems, that the more robust HV battery should power everything... without the conventional battery under the hood, you reduce a bit of weight and you get better weight distribution, and reduced cost..

    Of course, I'm an idiot, I'm sure they've got a very good reason for doing what they do.
  2. JimboPalmer

    JimboPalmer Friend to those who want no friends

    Apr 14, 2009
    Greenwood MS USA
    2012 Prius v wagon
    1) A ton of cheap automotive electrical parts are already 12 volts. No need to reinvent the wheel. (In line with this, the Gen 1, Gen 2 and Prius c all have different voltage for the HV battery, so you would need to stock a LOT of light bulbs if they were all at HV voltage)

    2) 12 volts is safe, by using 12 volts to power up the computers, then when the computers feel everything is safe, pulling in a relay to power the HV electrics, we are not risking High Voltage going where it shouldn't (through us).
    In case of an accident, everyone is happier if the car can power down the HV electrics

    https://techinfo.toyota.com/techInfoPortal/staticcontent/en/techinfo/html/prelogin/docs/2ndprius.pdf Page 16 of the PDF, Page 13 of the manual

    BTW, in all the Prius I am aware of, the 12 volt battery is under the hatch or back seat, not under the hood.
    Chilly36 and GrumpyCabbie like this.
  3. efusco

    efusco Moderator Emeritus
    Staff Member

    Nov 26, 2003
    Nixa, MO
    2004 Prius
    Hello Mareakin, welcome to Priuschat.
    First, if you saw the first reply (now deleted), I apologize for the rude response of that poster, it is NOT what we promote or tolerate here and your question is welcome, particularly in the newbie forum.

    Next, the 12v battery's main reason for existence is to reduce unnecessary drain on the HV battery. The 12V allows small trickle drainage to keep the memory of you radio, MPG, and other systems including the door sensors for SKS active. If not for the 12v there would be a continuous drain on the HV battery pack potentially discharging it down to a dangerously low level. If you kill your 12v by leaving the car for a month that's an easy inexpensive fix. Drain and kill the HV and you have a several thousand dollar repair bill.

    When you turn off your Prius the circuit for the HV battery is completely turned off from the rest of the car and therefore there is almost no discharge--so that battery can sit, unused, for 6 months, or potentially even longer, unused without being damaged or completely discharged. The other job of the 12v battery is to complete that circuit, putting the HV battery back On-line to allow you to drive the car.

    Hope that helps.
    Chilly36, gdswim, KK6PD and 2 others like this.
  4. Keiichi

    Keiichi Active Member

    Feb 1, 2012
    2012 Prius v wagon
    As the guys said here, you also have to understand that systems like the Tesla, where the car is purely electric... They require it to be charged as often as possible. If the battery were to be drained to complete discharge state, you risk bricking the car or damaging the battery.

    Toyota designed the car the way it is to try and stretch out the battery for as much as possible, hence why there is an Aux Battery for the other car functions, and why the hybrid system has its specific warranty, as required by the US warranty laws. If it didn't, it would not have the warranty it has. At least, that is what I have been told.
  5. mudmanrv

    mudmanrv Member

    Jan 16, 2012
    NE Indiana
    2012 Prius v wagon
    Everyone pretty well covered it. It's my understanding - the hybrid battery obviously powers the traction motors, and also the motor/gen2 which does the starting of the gas engine. both of those m1 and m2 motors are high voltage. I don't recall exactly - but I think it's something like 150volts or something. as others have already covered, headlights, dash lights, trip computer, etc. are all standard 12volt appliances... so they would either need a reducing transformer which is inefficient, and you would notice light "flicker" or dimming when you drew power for motion.... or keep the systems on 12volts as they are. when the engine runs, it charges the 12v system as well as the hybrid system.
  6. seilerts

    seilerts Battery Curmudgeon

    Mar 7, 2010
    Santa Fe, NM
    2005 Prius
    Just one more tidbit: custom EV conversions of any vehicle type almost always include a 12V DC-DC converter off the main pack. Some guys find it cheaper to have just a separate 12V battery than buying the converter. Having a high voltage system and a separate low voltage system is not only commonplace, it is a virtual requirement. The Prius also has a 12V DC-DC converter, but if that fails, then there would be nothing powering the computers/brakes/etc and such a failure would be dangerous. So the 12V battery serves the purpose of booting computers at startup, and providing a backup power supply in the case of DC-DC converter failure.
    1 person likes this.
  7. Dark_matter_doesn't

    Dark_matter_doesn't Prius Tinkerer

    Aug 8, 2009
    Seattle, WA
    2010 Prius
    RE weight distribution: the 12 V battery in the 2010 Prius is located in the far back passenger-side corner of the vehicle. It's actually more of a motorcycle battery in size rather than a regular car-size battery. You can easily lift up the right side cargo area cover (with the main cover open) to see the battery. You may have to take out the tray under the main cover.
  8. Rebound

    Rebound Senior Member

    Mar 11, 2010
    SF Bay Area, California
    2012 Prius Plug-in
    Plug-in Base
    And the main reason why the 12V battery is small is that it doesn't turn the starter motor. That high amp task is handled by the large traction battery.
  9. KK6PD

    KK6PD _ . _ . / _ _ . _

    Mar 24, 2008
    Los Angeles
    2014 Nissan LEAF
    Welcome to the group.
    Remember, there are no stupid questions, just ones you dumb enough not enough not to ask!
    One important note, NEVER volunteer to give anyone a "Jump" with your car!
    The risk of destroying your cars electrical system is too great a risk! Like mentioned, the Prius battery is small, just enough to start the electronics, that's it.
    If the person you are helping with a jump puts the cables backwards on his car, well that would/could be very bad!
    So ask away, that's why many of us are here!!! :welcome:
  10. Braddles.au

    Braddles.au DEFAnitely using an EBH

    Jan 25, 2012
    Canberra, Australia
    2010 Prius
    What KK6PD said.
    And, look at the manual for the terminals under the bonnet/hood where someone else would attach jumper cables to start your car. That is, don't connect the jumper leads directly to the battery in the rear. For a start, if the 12v battery is dead, it is not easy to open the rear hatch and remove everything to access the battery. Whereas opening the bonnet/hood to access the terminals is easy... once you've opened the front door.
  11. qbee42

    qbee42 My other car is a boat

    Mar 2, 2006
    Northern Michigan
    2006 Prius
    The 12V battery is useful for many reasons, as pointed out in the previous posts, but the main reason for having a separate 12V battery is safety.

    A lot of fuss has been made about the risks of high voltage electricity in hybrid vehicles. Dreadful predictions have been printed about first responders frying like giant bugs in a king sized zapper. Toyota was aware of the risks, and more over, aware of the perception of risk.

    The easiest and surest way to avoid high voltage contact is to turn it off, and that's what Toyota did. The HV battery has an internal contactor (relay or switch) that completely disconnects the the high voltage when the car is off.

    With the power switched off, some other means is needed to run accessories and re-close the contactor. This is where the 12V battery comes into play.

    Safety is the key. The other issues are just side benefits.

  12. RobH

    RobH Senior Member

    Sep 18, 2006
    Sunnyvale, California
    2006 Prius
    As best I can figure, the 12V battery is there because that's the way it has always been done. The HV battery is a new concept, and hasn't been fully integrated into the design.

    I seen no reason the 35 AH 12V battery capacity couldn't be reserved as part of the HV battery. There are plenty of reasons to keep 12V power available for lights and such, but a DC-DC converter could provide that off the HV battery. If there is a load that would drain a 12V battery, the DC-DC converter could just shut down the 12V before the HV battery goes too low.