Energy management for hybrid drive system, function - GF08.30-P-1005FLM

ENGINE 276.952 in MODEL 212.095 as of model year 2014 

ENGINE 651.924 in MODEL 212.098/298 as of model year 2014 

The subfunction "Hybrid drive system energy management, function" is considered and described from the point of view of the hybrid system. Information about the 12-V energy management of the 12-V on-board electrical system and for requesting the 12-V alternator (for a diesel engine) are documented in the function description "energy management function".

Function requirements, general 

• Hybrid drive system "READY" (yellow or green)

Hybrid drive system energy management, general points 

The energy management module in the CDI control unit (N3/9) (for a diesel engine) or ME-SFI [ME] control unit (N3/10) (for a gasoline engine) coordinated the energy flows of the hybrid system and creates, with regard to the electrical factors, the interface to the battery management system control unit (N82/2), power electronics (N129/1) and to the electrical A/C compressor (A9/5).

To do this is exchanges information with all relevant control units via the CAN network. Within the CDI control unit or the ME-SFI [ME] control unit it communicates with the torque interface to coordinate energy recovery and use of energy.

The energy management model is also responsible for the following tasks:

• Calculation and calibration of SOC value (State of Charge) for charge level of high-voltage battery (A100g1)
• Implementation of the charging/discharging strategy while taking account of the boundary conditions from high-voltage battery, internal combustion engine and electrical machine (A79/1)
• Prognoses for capacitors and maximum available high-voltage battery output
• Controlling the energy exchange between the high voltage on-board electrical system and 12 V on-board electrical system

The CDI control unit or the ME-SFI [ME] control unit reads in the following signals as part of the energy management:

• Battery management system control unit, high-voltage battery voltage via the hybrid CAN (CAN L)
• Battery management system control unit, status via the hybrid CAN
• Battery management system control unit, temperature of the high-voltage battery via the hybrid CAN
• Power electronics control unit, charging voltage/current, signal via the hybrid CAN
• Power electronics control unit, status of the electrical machine via the hybrid CAN
• Power electronics control unit, status via the hybrid CAN
• Electrical A/C compressor (A9/5), status via the hybrid sensor CAN

Function sequence for energy management for a hybrid drive system 

The following tasks of the energy management are explained in more detail:

• Function sequence for calculating SOC value for high-voltage battery charge level 
• Function sequence for battery management 
• Function sequence for energy exchange of the high voltage and 12 V on-board electrical system 

Function sequence for calculating SOC value for high-voltage battery charge level 

Computation of the charge level of the high-voltage battery as the so-called SOC value (State of Charge) takes place on the basis of data about voltage, current flows and the temperature of the high-voltage battery transmitted from the battery management system control unit and the power electronics control unit via the hybrid CAN. The CDI control unit or the ME-SFI [ME] control unit uses them to calculate the SOC value and then presents this value as a percentage (0 to 100%) other control units that are integrated into the CAN network.

The SOC value serves, for example, as the basis for the charge level indicator in the IC (A1) and generally for control of all hybrid functions, dependent on the battery system charge level.

In order to compute the SOC value for the charge level of the high-voltage battery, the CDI control unit or the ME-SFI [ME] control unit evaluates the open circuit voltage of the high-voltage battery measured by the battery management control unit and the power electronics control unit and transmitted by the hybrid CAN and all currents flowing from and to the high-voltage battery.

Function sequence for battery management 

The battery management system control unit takes on complete monitoring of the high-voltage battery regarding temperature, voltage and currents, establishes safety limits for these values and transmits values and safety limits to the CDI control unit or the ME-SFI [ME] control unit.

The following function will be explained in more detail in the following:

• Function sequence for controlling the contactor 

Additional function requirements Controlling the contactor 

• 12 V on-board electrical system voltage above the minimum value (>9 V)

Function sequence for controlling the contactor 

The battery management system control unit monitors the interlock circuit and takes on actuation of the contactor. For discontinuity of the high voltage interlock circuit the battery management system control unit actuates the contactors for opening. The high-voltage battery is disconnected in this way by the high voltage on-board electrical system.

Furthermore, the battery management system control unit actuates all of the switching of the contactor requested by the CDI control unit or the ME-SFI [ME] control unit via the hybrid CAN.

Function sequence for energy exchange of the high voltage and 12 V on-board electrical system 

The energy management module in the CDI control unit or ME-SFI [ME] control unit regulates the energy flows in the high voltage on-board electrical system along with the voltage conversion and energy exchange from and to the 12 V on-board electrical system. To do this the CDI control unit or ME-SFI [ME] control unit communicates with the power electronics control unit via the hybrid CAN.

In order to ensure permanent supply with electrical energy, a DC/DC converter is integrated in the power electronics control unit. This is designed as a bidirectional DC voltage converter, which generates high voltage and 12-V direct voltage and transfers this between the high voltage and the 12 V on-board electrical system.

When faults occur such as, e.g. voltages outside the operating range, the power electronics control unit shifts into standby mode.

One differentiates between the following modes depending on the operating condition:

• Function sequence for charging the 12 V on-board electrical system (buck mode) 
• Function sequence for supporting the high voltage on-board electrical system (boost effect mode) 

Function sequence for charging the 12 V on-board electrical system (buck mode) 

In buck mode supply of the 12 V on-board electrical system with energy takes over the power electronics control unit from the high-voltage battery.

Additional function requirements for supporting the high-voltage on-board electrical system (boost effect mode) 

• An external onboard power supply is connected up
• Previously still no engine running in this ignition sequence
• Terminal 15 (>2 min)

Function sequence for supporting the high voltage on-board electrical system (boost effect mode) 

The battery performance of the high-voltage battery is very low at the start of the hybrid system (under the SOC), but the charge level of the rear on-board electrical system battery (G1/11) is very good or if an external power supply is connected, then a power transfer from the low-voltage side to the high-voltage side can serve to ensure starting capability.

If an external charger is attached and if the power electronics control unit measures an on-board electrical system voltage of more than 13 V in the 12 V on-board electrical system for an opened engine hood, then the 12-V on-board electrical system is loaded with a max. of 500 W. Energy is transferred over the rear on-board electrical system battery and the power electronics control unit into the high voltage system and the high-voltage battery is charged.