Hybrid Drive Systems: Description and Operation
Active Hybrid Voltage Supply
Active hybrid voltage supply
The hybrid car has two vehicle electrical systems driven by two different voltages:
- Conventional 12 V vehicle electrical system.
- High-voltage electrical system for voltage supply of high-voltage components (nominal voltage 316.8 V).
Brief component description
The following components for voltage supply are described:
- High-voltage battery unit with lithium ion battery and battery management electronics (SME)
- Electrical machine electronics (EME)
- Electric machine
- Electric A/C compressor (EKK)
- Auxiliary battery
- Intelligent battery sensor (IBS) for auxiliary battery
- Cut-off relay
- Starter motor generator
System wiring diagram: High-voltage system and starter unit.
High-voltage battery unit with lithium ion battery and battery management electronics (SME)
The high-voltage battery unit is used to load, store and supply electric energy for the high-voltage electrical system. It is installed in the luggage compartment behind the seat bench in the 5 Series and 7 Series and under the luggage compartment floor in the 3 Series.
The high-voltage battery unit contains the following components:
- Lithium ion battery:
The high-voltage battery consists of 8 cell blocks with 12 cylindrical battery cells each. The cell blocks are switched in series. The cooling system is the bearing structure in which the cell blocks are integrated. This cooling system consists of two refrigerant circuits that are connected to the refrigerant circuit of the vehicle air conditioning.
A cell monitoring system with a voltage sensor and temperature sensors obtains information on the state of the cells in each cell block. All 8 cell monitoring systems are connected to the battery management electronics (SME) via an internal Local Controller Area Network.
The nominal voltage of the high-voltage battery is 316.8 V, the useful energy output is 600 Wh.
- Battery management electronics (SME):
The control unit performs the following functions:
- Monitoring of the state of the lithium ion battery (temperature, voltage, state of charge, aging state, etc.).
- Activation of the equilibration (voltage equilibration of individual cells and cell blocks).
- Activation of the refrigerant shutoff valve in the air conditioning refrigerant circuit for cooling the high-voltage battery.
- Monitoring the high-voltage electrical system for insulation faults
- Signal source for the pulse-width modulated signal of the high-voltage interlock loop.
- Controlling the switch contactors for switching on the high-voltage system.
- Relay box:
Two switch contactors allow the disconnection of the high-voltage battery and the high-voltage electrical system. The high-voltage system can be switched-off in this way. A third switch contactor is used for preloading: Before the two switch contactors are closed, the high-voltage system is tested. A voltage and current sensor measures the voltage and current at the output of the high-voltage battery and at the input of the high-voltage electrical system. It is connected to the battery management electronics (SME) via an internal Local Controller Area Network.
WARNING: Safety information on the lithium ion battery
The high-voltage battery is a complex, high-voltage component: it is imperative that the handling and safety regulations are followed. In particular, lithium-ion batteries must not be overcharged or subjected to excessively high temperatures. Otherwise there is a risk of fire. The high-voltage battery unit must never be opened, otherwise there is a risk of fatal injury from high currents and voltages.
Electrical machine electronics (EME)
The electrical machine electronics (EME) incorporate the inverter for controlling the electrical machine, a DC/DC converter (2.4 kW) for transferring energy from the high-voltage electrical system to the 12 V electrical system, and the control unit. The EME is cooled via a separate cooling circuit with a coolant pump and expansion tank. The EME controls the starter unit (auxiliary battery with an intelligent battery sensor and cut-off relay).
Electric machine
The electrical machine incl. the clutch is between the combustion engine and the automatic transmission. The electrical machine consists of stator (inside) and rotor (outside). The rotor replaces the flywheel of the combustion engine. The electrical machine is designed as a permanently excited electric motor (synchronous) with a maximum output of 39 kW.
The electrical machine has the following functions:
- Electrical driving
- Support for the combustion engine (boost)
- Electrical braking (energy recovery)
- Alternator
For boosting (electrical acceleration), up to 210 Nm additional electromotive torque can be provided.
Electric A/C compressor
The electric A/C compressor is connected via the LIN bus to the IHKA control unit and enables the following functions:
- Ensuring air conditioning function during engine shutdown (MSA) or during electric driving.
- Ensuring optimum operating temperature for the high-voltage battery
- Independent air conditioning
As opposed to conventional vehicles, climate control of the vehicle is also possible with the engine switched off. The electric A/C compressor is connected via the high-voltage connections to the electrical machine electronics.
Auxiliary battery for starter unit
The starter unit enables comfortable starting of the engine from a standstill or while driving. The auxiliary battery supplies the energy needed to use the starter motor generator. The auxiliary battery is installed at the rear right in the luggage compartment. The auxiliary battery is a 40 Ah AGM battery.
Intelligent battery sensor (IBS) for auxiliary battery
The intelligent battery sensor (IBS) is secured to the negative battery terminal of the auxiliary battery and connected to the electrical machine electronics (EME) over a local interconnect network bus (LIN).
The IBS is a mechatronic component with its own microprocessor for monitoring the battery condition. The following measured variables are registered:
- Terminal voltage
- Charge current and discharge current
- Temperature of the battery terminal
Cut-off relay of starter unit
The auxiliary battery and the starter generator are connected to the 12 V vehicle electrical system via the cut-off relay. The cut-off relay is activated by the electrical machine electronics (EME) via a control line.
Starter motor generator
The crankshaft starter generator enables a comfortable transition from electric driving to driving powered by the combustion engine. The energy supplied by the auxiliary battery makes it possible to quickly start the engine with very low noise without requiring an additional starter torque in the electrical machine.
The crankshaft starter generator is connected to the digital motor electronics (DME) via a local interconnect network bus (LIN).
If there is a fault in the DC/DC converter or the high-voltage system, the starter generator can take over the supply of the 12 V electrical system.
System functions
The following system functions for the high-voltage system voltage supply and power management are described:
- Operating range of the high-voltage battery
- Starter unit
- High-voltage power management
- ECO PRO mode: Reduction of power consumption
Operating range of the high-voltage battery
The high-voltage battery is used to load, store and supply electric energy for the high-voltage electrical system. To provide the different drive functions (electric driving, boost, electric braking), the lithium-ion batteries of the high-voltage battery must be operated in a particular range.
- Operating range for ambient temperature: -40 °C to 80 °C.
An excessively high temperature has the following impact on the high-voltage battery: rapid aging, rapid self-discharging. An excessively low temperature reduces the charging ability and the performance of the high-voltage battery. Depending on the temperature of the high-voltage battery, the battery management electronics (SME) controls the refrigerant shutoff valve, promotes cooling via the refrigerant circuit of the air conditioning, or limits the performance of the high-voltage battery.
- Operating range for state of charge: 25 % to 70 %.
As soon the voltage of a cell drops below the minimum permissible voltage, the high-voltage battery is totally discharged. The cell is faulty. Conversely, a excessively high state of charge has the following impact on the high-voltage battery: rapid aging, rapid self-discharging. If the state of charge drops below 25 % or exceeds 70 %, the battery management electronics (SME) initiates various protective measures to the point where it opens the switch contactors (state of charge below 10 % or above 90 %). In this case, the switch contactors can only be closed by means of diagnosis for discharging or charging the high-voltage battery.
- Operating range for charge and discharge current:
The current limits for charging and discharging the high-voltage battery depend on the temperature. In case of limit violations, the SME initiates various protective measures to the point where it opens the switch contactors.
Starter unit
The starter unit enables comfortable starting of the combustion engine from a standstill or while driving via the starter motor generator.
The auxiliary battery is charged via the crankshaft starter generator. To avoid overcharging the auxiliary battery, the cut-off relay is closed when a certain engine speed is exceeded. However, to ensure that the cut-off relay is not loaded, the closing action must take place at the smallest possible load: To achieve this, the DC/DC converter in the EME adjusts the vehicle voltage to the voltage of the auxiliary battery. The cut-off relay then remains closed until the combustion engine is switched off again.
In case of a fault in the high-voltage system or in the intelligent battery sensor of the auxiliary battery, the cut-off relay remains closed.
High-voltage power management
The EME high-voltage power management system is an extension of the previous power management system on the engine control unit. The following functions are run by the high-voltage power management:
- Control of activation/deactivation of the high-voltage electrical system
- Controlling the power of the DC/DC converter.
- Output limit of the electric A/C compressor (EKK) depending on activate drive function (boost, recuperation) and the available power of the high-voltage battery.
- Control of switch-off inhibitors/switch-on requests for the automatic engine start-stop function (MSA) as follows:
If energy needs of the vehicle electrical system is too high, a switch-off inhibitor/switch-on request is set. A switch-off inhibitor is also set if the high-voltage battery temperature is outside a certain range (less than -15 °C or more than 46 °C).
- Controlling of the cut-off relay of starter unit.
ECO PRO mode: Reduction of power consumption
The ECO PRO mode achieves additional power savings through efficient air conditioning operation and a shift program of the automatic transmission. In the active ECO PRO mode, the following electrical consumers are switched off or reduced by power management:
- Switch-off of the mirror heating.
- Reduction of the seat heating.
Notes for Service department
General notes
WARNING:
Hybrid cars have an additional high-voltage electrical system which has its own safety regulations to be observed. Work on live high-voltage components is expressly prohibited. Prior to every operation which involves a high-voltage component, it is essential to disconnect the high-voltage system from the voltage supply and to secure it against unauthorized return to service.
1. Switch off ignition.
2. Remove the high-voltage safety connector.
3. Prevent the high-voltage safety connector from being reinserted.
4. Switch the ignition on.
5. Always wait 10 s until the "HV system switched-off" Check Control message (ID 636) appears in the instrument panel.
Handling the high-voltage battery in the workshop
See note for handling the high-voltage battery in the workshop
Vehicles in storage or parked for extended periods: Recharge the high-voltage battery at regular intervals
Just as with 12-volt batteries, the high-voltage batteries in stored vehicles and cars that remain parked for extended periods must be recharged at regular periodic intervals to avoid total discharge and the attendant damage. Recharge at the same recharging intervals prescribed for the 12-volt battery (Instructions for recharging the high-voltage battery booth vehicles).
Diagnosis instructions
NOTICE: Auxiliary battery: Register battery replacement
After installing a new auxiliary battery, the service function for registering battery replacement must be run. This is necessary to inform the electrical machine electronics (EME) that a new auxiliary battery has been installed in the vehicle. All measured data of the battery condition are reset.
NOTICE: Observe the service function
The diagnosis system provides the following service functions for the high-voltage battery and the starter unit:
- Start-up of the high-voltage battery
- Evaluating the battery state of charge
- Auxiliary battery: Register battery replacement
Path: Service functions > Body > Voltage supply > Hybrid vehicle
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