How High Voltage HV Battery Works. Overview

Discussion in 'Knowledge Base Articles Discussion' started by BARSOMAUTOMOTIVE, Mar 12, 2012.


    BARSOMAUTOMOTIVE Prius Expert Whittier CA.

    Mar 10, 2012
    2004 Prius
    Toyota Prius High-Voltage Battery

    The sealed nickel-metal hydride (Ni-MH) battery technology developed for the hybrid system provides both high power density and excellent longevity. The hybrid system controls charge and discharge rates to keep the HV battery at a constant State of Charge (SOC).

    HV Battery Layout

    The HV Battery, Battery ECU and SMR (System Main Relay) are enclosed in a single case located in the luggage compartment behind the rear seat.

    Power Cable
    The power cable is a high-voltage, high-amperage cable that connects the HV battery with the inverter and the inverter with MG1 and MG2.
    In the '04 & later Prius, the power cable also connects the inverter with the A/C compressor.
    The power cable is routed under the rear seat, through the floor panel, along the under-the-floor reinforcement, and connects to the inverter in the engine compartment. The 12V DC wiring harness follows a similar route from the auxiliary battery to the front of the vehicle
    The power cable is shielded to reduce electromagnetic interference.
    For identification purposes, the high-voltage wiring harness and
    connectors are color-coded orange to distinguish them from ordinary low-voltage wiring.

    HV - Nickel-Metal Hydride Battery
    The HV battery pack contains six nickel-metal hydride 1.2V cells that are connected in series to form one module.
    In the '01-03 Prius, 38 modules are divided into two holders and
    connected in series. Thus, the HV battery contains a total of 228 cells and has a rated voltage of 273.6V.
    In the '04 and later Prius, 28 modules are connected for a rated voltage of 201.6V. The cells are connected in two places to reduce the internal resistance of the battery.
    The electrode plates in the HV battery are made of porous nickel and metal hydride alloy.

    For battery recycling information, please refer to the Warranty Policy
    and Procedure manual.

    HV Battery Pack
    HV Battery
    Main Components

    (’04 & later Prius)
    Battery ECU The battery ECU provides the following functions:
    • It estimates the charging/discharging amperage and outputs charge and discharge requests to the HV ECU so that the SOC can be constantly maintained at a center level.
    • It estimates the amount of heat generated during charging and
    discharging, and adjusts the cooling fan to maintain HV battery
    • It monitors the temperature and voltage of the battery and if a
    malfunction is detected, can restrict or stop charging and
    discharging to protect the HV battery.

    Battery ECU

    (’04 & later Prius)
    State Of Charge
    The battery ECU constantly monitors HV battery temperature, voltage and amperage. It also checks for leaks in the HV battery.
    While the vehicle is in motion, the HV battery undergoes repetitive
    charge/discharge cycles as it becomes discharged by MG2 during
    acceleration, and charged by the regenerative brake during
    deceleration. The Battery ECU estimates the charge/discharge
    amperage and outputs charge/discharge requests to the HV ECU to maintain the SOC at a median level.
    The target SOC is 60%. When the SOC drops below the target range, the battery ECU informs the HV ECU. The HV ECU then signals the engine ECM to increase power to charge the HV battery. If the SOC is below 20%, the engine is not producing power.

    Delta SOC

    The normal, low to high SOC deviation is 20%. If the Delta SOC exceeds 20%, this means that the HV battery ECU cannot correct or maintain the SOC difference within the acceptable range.


    The battery ECU outputs
    requests to the HV ECU so the SOC can be maintained at a center level.

    System Main Relay (SMR)
    The System Main Relay (SMR) connects and disconnects power to the high-voltage circuit based on commands from the HV ECU. A total of three relays (one for the negative side and two for the positive side) are provided to ensure proper operation.
    When the circuit is energized, SMR1 and SMR3 are turned ON. The
    resistor in line with SMR1 protects the circuit from excessive initial
    current (called `inrush' current). Next, SMR2 is turned ON and SMR1
    is turned OFF, allowing current to flow freely in the circuit.
    When de-energized, SMR2 and SMR3 are turned OFF in that order
    and the HV ECU verifies that the respective relays have been properly turned OFF.

    System Main Relay (SMR)
    The SMR connects and disconnects the power source of the high-voltage circuit. A total of three relays (one for
    the negative side and two for the positive side) are provided to ensure proper operation.

    Service Plug

    When the service plug is removed the high-voltage circuit is shut OFF at the intermediate position of the HV battery.
    The service plug assembly contains a safety interlock reed switch.
    Lifting the clip on the service plug opens the reed switch, shutting OFF the SMR.
    The main fuse for the high-voltage circuit is inside the service plug

    For safety reasons, you must always turn the vehicle OFF before removing the service plug.

    HV Battery Cooling System

    The battery ECU detects battery temperature via three temperature
    sensors in the HV battery and one intake air temperature sensor.
    Based on those readings, the battery ECU adjusts the duty cycle of the cooling fan to maintain the temperature of the HV battery within the specified range.
    The battery ECU keeps the fan OFF or running at LO if:
    • The A/C is being used to cool the vehicle.
    • Some margin is left in the temperature of the battery.

    Auxiliary Battery
    The Prius uses an Absorbed Glass Mat (AGM) 12V maintenance free auxiliary battery. This 12V battery powers the vehicle's electrical
    system similar to a conventional vehicle. The battery is grounded to
    the metal chassis of the vehicle and vented to ambient (outside) air
    with a tube.
    This battery is very sensitive to high-voltage. When charging the
    auxiliary battery you should use the Toyota approved charger, because a standard battery charger does not have the proper voltage control and may damage the battery. If the approved charger is not available you may use a trickle charger if the amperage is kept below 3.5 A.
    The battery should be removed from the vehicle during charging.
    However, it is safe to jump-start the Prius from either the battery or
    the jump-start terminal under the hood. This will allow the vehicle's
    charging system to restore the battery to normal SOC.
    If the vehicle will not be used for more than two weeks, disconnect the 12V battery to prevent it from discharging. Always make sure that all doors are properly closed and that the interior lights are OFF, especially overnight. These situations will quickly deplete the 12V battery.

    Auxiliary Battery
    In glass mat batteries, the electrolyte is trapped in separators to reduce the amount of hydrogen gas released when the battery
    is charged. Glass mat batteries are sealed and the electrolyte
    cannot be replaced.

    Auxiliary Battery
    Charging (’04 & later Prius)

    There is a remote access B + terminal in the main junction block under the hood, so it is no longer necessary to remove interior trim pieces to gain access to the battery.
    1 person likes this.
  2. cwerdna

    cwerdna Senior Member

    Sep 4, 2005
    SF Bay Area, CA
    2006 Prius

    BARSOMAUTOMOTIVE Prius Expert Whittier CA.

    Mar 10, 2012
    2004 Prius
    I am a new member, not allowed to put links.

    The information came from those links, I was looking through the forum but did not see those links.
    Thank you for the links.
  4. seilerts

    seilerts Battery Curmudgeon

    Mar 7, 2010
    Santa Fe, NM
    2005 Prius
    One more post and you are good to go.