Toyota Prius High-Voltage Battery Overview 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. NOTE For battery recycling information, please refer to the Warranty Policy and Procedure manual. HV Battery Pack Information HV Battery Main Components (â€™04 & later Prius) Battery ECU The battery ECU provides the following functions: â€¢ 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. â€¢ It estimates the amount of heat generated during charging and discharging, and adjusts the cooling fan to maintain HV battery temperature. â€¢ 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 (â€™04 & later Prius) State Of Charge (SOC) 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. SOC 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 assembly. NOTE 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: â€¢ The A/C is being used to cool the vehicle. â€¢ 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 (â€™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.