Battery Energy Control Module (BECM)

State of Charge 

The battery energy control module (BECM) uses a complex calculation method based on varying conditions over a range of operation to determine and update the state of charge (SOC) of the hybrid battery. The hybrid battery parameters used by the BECM include current, voltage, and temperature. These inputs are monitored by several sensors within the hybrid battery assembly.

Operating Functions 

The battery energy control module (BECM) is also referred to as the drive motor generator battery control module. In order to maintain proper state of charge levels and operate efficiently, the hybrid drive motor generator battery has several systems that need to be monitored and controlled. These systems include hybrid battery voltage measurement, hybrid battery current measurement, hybrid battery cooling and the high voltage interlock loop system. Additionally, the BECM operates the high voltage contactor relays at the request of the hybrid powertrain control module (HPCM). The BECM is located within the hybrid battery assembly.

Communication and Hosted Diagnostics 

The HPCM is the host controller for diagnostic trouble code (DTC) information for the following control modules:

• Accessory DC power converter module (APM)
• Battery energy control module (BECM)
• Motor control module (MCM) 1
• Motor control module (MCM) 2
• Auxiliary transmission fluid pump (ATFP) control module

The BECM will diagnose its own systems and determine when a fault condition is present. Diagnostics and system status is communicated from the BECM to the HPCM through the Hybrid LAN communication circuit.

In the event a hosted module communicates a fault condition, the HPCM will determine if hybrid operation is affected and notify the vehicle operator by requesting the MIL illuminate and/or by displaying a hybrid service required message. In addition, the HPCM will store the associated DTC information for retrieval by a scan tool. Some hosted modules may require an ignition cycle to clear certain DTCs from the HPCM.

Input Circuits - Voltage Measurement 

The BECM monitors the voltage values of the hybrid batteries using two different methods. The BECM monitors the sum of the voltage at 20 battery locations, and sums up these values to generate a calculated voltage value. The BECM also measures high voltage when the contactors are closed by monitoring the total bus voltage from the harness on the output side of the high voltage positive and negative contactor relays, this is the terminal voltage measurement.

Input Circuits - Hybrid Battery Current Measurement 

The BECM monitors current through the hybrid battery current sensor. The hybrid battery current sensor is mounted on the positive cable side internal to the hybrid drive motor generator battery assembly. The current sensor detects the amperage that flows in and out of the hybrid batteries. The current sensor inputs a voltage between 0-5 volts in proportion to the amperage. A reading of 2.5 volts is 0 amps.

Input Circuits - Hybrid Battery Temperature 

The hybrid drive motor battery is an air cooled component. The hybrid drive motor battery vent fan and relay are located inside the hybrid drive motor battery assembly and controlled by the BECM. The BECM monitors four hybrid battery temperature sensors, one air inlet temperature sensor, and one air outlet temperature sensor. These six temperature sensors are internal to the hybrid drive motor generator battery assembly. The four hybrid battery temperature sensors are located on the top of the hybrid batteries. The resistances of these temperature sensors increase with temperature. The BECM can adjust the vent fan speed based on vehicle speed and temperature.

Input Circuits - High Voltage Interlock Circuit (HVIC) 

The vehicle is equipped with a high voltage interlock circuit (HVIC) feature. The purpose of the HVIC is to detect attempted access to HV components when HV is present. The BECM sources and monitors current on a 2 wire circuit that runs through certain HV components and their access points. The BECM will take action based on the results of the measured current at both the input and output circuits. Additionally, the drive motor generator PIM monitors the HVIC and will also take action based upon the measured current. Refer to High Voltage Monitoring Systems Description .

Output Circuits - High Voltage Contactor Relays 

There are three high voltage contactor relays, the positive main HV contactor relay, the negative main HV contactor relay and the current limit relay, also called the precharge relay. The HV contactor relays, sometimes called contactors, open the high voltage battery negative and positive circuits and isolate the hybrid battery voltage from the rest of the vehicle. The BECM will close the HV contactor relays only at the request of the HPCM. Either the BECM or the HPCM can command that the HV contactor relays be opened in the event a system fault is detected. The contactors must be opened and closed under tightly controlled conditions in order to prevent damage to either the PIM or the HV contactor relays themselves.

In order to prevent damage to the PIM capacitors, the HV contactor relays are always closed in the following sequence:

1. The negative main HV contactor relay is closed.
2. The current limit relay is closed. The precharge high voltage circuit contains an in-line current limit resistor. With both the negative main HV contactor relay and the current limit relay closed, the high voltage is slowly brought to operating voltage. Slowly bringing the high voltage to operating level prevents surge damage of the PIM capacitors.
3. Once the BECM reads 95 percent of available buss voltage at the HV terminals, the BECM will close the HV positive contactor relay.
4. The positive main HV contactor relay is closed. The positive main HV contactor relay bypasses the current limit relay and resistor in the precharge circuit and allows high current flow in and out of the hybrid batteries to occur.
5. The current limit relay is opened once the system is at operating voltage.

In order to prevent damage to the HV relays, current flow through the relay switch contacts must be minimized prior to any attempt at opening them. The HPCM must ensure the system is in a low current flow condition prior to allowing the BECM to open the HV contactor relays. The HPCM will command that all high voltage components like the APM and the air conditioning compressor stop drawing current. Also, the HPCM will ensure that the motor/generators have stopped rotating and that they are neither consuming nor generating electrical current.

Output Circuits - Hybrid Drive Motor Battery Vent Fan 

The hybrid drive motor battery is an air cooled component. The hybrid drive motor battery vent fan and relay are located inside the hybrid drive motor battery assembly and controlled by the BECM. The BECM monitors hybrid battery temperature air inlet and air outlet temperatures. The BECM can adjust the vent fan speed based on vehicle speed and temperature. The BECM grounds the primary side of the vent fan relay to supply voltage to the vent fan. The BECM controls the vent fan speed through a separate wire with a pulse width modulated signal (PWM). The vent fan assembly internal circuitry converts the PWM signal into fan speed.