High-voltage battery unit

The high-voltage battery unit is used to load, store and supply electric energy for the electric motor and high-voltage electrical system. The high-voltage battery is comprised of several cell blocks which each have several cells. The cell blocks are switched in series.

60 Ah high-voltage battery unit

2013

94 Ah high-voltage battery unit

2016

120 Ah high-voltage battery unit

2018

Number of battery cells (lithium-ion battery) 96 (serial) 96 (serial) 96 (serial)
Number of cell blocks (12 cells each) 8 8 8
Nominal voltage 360 V 350.4 V 352.3 V
Voltage at 100 % state of charge 395 V 398 V 403 V
Voltage at 0 % state of charge 259 V 259 V 268 V
Capacity 60 Ah 94 Ah 120 Ah
Energy (nominal value) 21,6 kWh 33 kWh 42,2 kWh
Energy (usable) 18,8 kWh 28 kWh 38 kWh
Dimensions of the housing (length x width x height) 1660 mm x 964 mm x 174 mm 1660 mm x 964 mm x 174 mm 1660 mm x 964 mm x 174 mm
Weight approx. 233 kg approx. 256 kg approx. 273 kg

Brief component description

The following components for the high-voltage battery unit are described:

  • Control unit of battery management electronics (SME)
  • Safety box
  • Cell supervision circuit
  • Cell module
  • Refrigerant temperature sensor
  • Coolant duct with coolant duct and heating (depending on the options fitted)

Overview of high-voltage battery unit

Index Explanation Index Explanation
1 Case Covers 2 Cell supervision circuit
3 Wiring harness for cell supervision circuit 4 Cell module
5 Coolant duct with coolant duct and heating 6 Housing
7 Electronic connector 8 Connection of refrigerant line
9 Vent hole 10 Safety box
11 Battery management electronics (SME)

High-voltage battery unit 120 Ah: Additionally, a heat protection mat is provided between the cell modules and the housing cover.

System wiring diagram of high-voltage battery unit

Index Explanation Index Explanation
1 High-voltage battery unit 2 Cell supervision circuit
3 Cell module 4 Fuse
5 Control electronics for the heating (depending on the options fitted) 6 Safety box
7 Switch contactor for the preloading 8 High-voltage connection (positive terminal) of the high-voltage battery unit
9 High-voltage connection (negative terminal) of the high-voltage battery unit 10 High-voltage interlock loop of the high-voltage connection to the high-voltage electrical system
11 Switch contactor (positive terminal) 12 Switch contactor (negative terminal)
13 Connection to the circuit of the high-voltage interlock loop 14 Safety battery terminal
15 SME control unit 16 Bus connection (PT-CAN2)
17 Cell supervision circuit with sensors for the measurement of the cell voltage. 18 Cell block with several battery cells
19 Lithium-ion battery 20 Sensors for temperature measurement in the cell block
21 Heating of the high-voltage battery (depending on the options fitted) 22 Voltage and current sensor

Battery management electronics (SME)

Index Explanation Index Explanation
1 Connector of the high-voltage cable for the insulation resistance measurement 2 Communication connector for the safety box
3 SME 4 Communication connector for the cell supervision circuits
5 Communication connector for the vehicle 6 High-voltage cable (negative connection) for the first cell block
7 Connector for the high-voltage cable to the heating of the high-voltage battery 8 Safety box.

the battery management electronics (SME) continuously monitors the state of the battery cell and the required parameters for a secure operation of the system.

The control unit performs the following functions:

  • Control of the starting and shut down of the high-voltage system is made by request via the electrical machine electronics (EME).
  • Evaluation of the voltage and temperature measurement signals of all the battery cells and the current level in the high-voltage circuit.
  • Control of the cooling system for the high-voltage battery unit.
  • Determining the state of charge and aging condition (State of Health) of the high-voltage battery.
  • Determining the available power of the high-voltage battery, and if needed, the request of a limit for the electrical machine electronics.
  • Safety function (e.g. voltage and temperature monitoring, of the high-voltage interlock loop, monitoring of the high-voltage system for isolation faults.

The fault code entries of the SME can be organised into different categories that are dependent on the seriousness and the function that is still available

  • Immediate shut down of the high-voltage system.
  • Limited power and range.
  • Fault without effect for the customer.

Safety box

The safety box includes the following components:

  • Switch contactors:

    The high-voltage battery is connected / disconnected with the high-voltage electrical system with the help of two electromechanical switch contactors. The electromechanical switch contactors are activated by the SME. The rear power distribution box supplies the switch contactors with vehicle voltage (12 V) through the safety battery terminal.

    A third switch contactor is used for preloading: Before closing both electromechanical switch contactors, the connection on the high-voltage electrical system is checked.

  • Voltage and current sensor in the current path of the negative battery terminal:

    A voltage and current sensor measures the voltage and current at the output of the high-voltage battery and at the connection of the high-voltage electrical system. The voltage and current sensor is connected to the battery management electronics (SME) via an internal Local Controller Area Network.

  • Safety fuse in the current path of the positive battery terminal.
  • Control electronics for the heating (depending on the options fitted):

    Depending on the equipment, heating is installed in the high-voltage battery unit. The control electronics is in the safety box and is connected with the voltage and current sensor via the LIN bus.

Cell supervision circuit

The SME supplies the cell supervision circuit with a supply voltage of 5 V. In addition, a cell supervision circuit can be supplied directly from the cell module. A cell supervision circuit monitors the state of the 12 lithium-ion batteries:

  • Measuring and monitoring the voltage of each individual battery cell.
  • Measuring and monitor the temperature at several points of the cell block.
  • Communication of the measured variables on the SME.
  • Carrying out the adjustment process for the cell voltage of the battery cells.

The cell supervision circuits are connected to the SME via an internal Local Controller Area Network.

Cell module

Index Explanation Index Explanation
1 Cover 2 High-voltage cable (positive connection) for the next cell block
3 Feed line to the cell supervision circuit 4 Cell supervision circuit
5 Cell module

A cell block is comprised of 12 in-series-connected lithium-ion batteries. The battery cells are pressed together with pressure plates. Temperature sensors are also installed in the cell block. A lid is used for touch protection.

Each cell module is labelled with a unique 28-digit serial number: e.g. 7625066 07 72883817 130902 00001 

  • 7625066: seven-digit part number.
  • 07: two-digit change index.
  • 72883817: eight-digit supplier number.
  • 130902: six-digit production date (year, month, day).
  • 00001: five-digit serial number.

Refrigerant temperature sensor

The refrigerant temperature sensor measures the temperature of the refrigerant with coolant ducts at the output of the heat exchanger. The refrigerant temperature sensor is connected to the SME.

Coolant duct with coolant duct and heating (depending on the options fitted)

The high-voltage battery unit is cooled by refrigerant. For this reason the air conditioning refrigerant circuit is extended to include the high-voltage battery unit. A heat exchanger made of flat aluminium pipes is located in the high-voltage battery unit under the cell block and is connected with the refrigerant circuit of the Air conditioning.

Depending on the equipment, a heating with 1000 W power is also installed along the coolant ducts. The heating is connected via a high-voltage cable on the safety box and is supplied from the high-voltage system, if the high-voltage system is active (closed switch contactor).

System functions

The following system functions for the high-voltage battery unit are described:
  • Controlling the switch contactors for switching the high-voltage system on and off.
  • Monitoring the high-voltage system for insulation faults.
  • Controlling the cooling and heating of the high-voltage battery.
  • Equilibration of the individual battery cells (equilibration).

Switching on and off the high-voltage system

Starting the high-voltage system takes place via interaction between the control units of the EME electrical machine electronics and the SME battery management electronics. During this the EME assumes the role of the master, the SME is the carrying out slave. The associated commands are transmitted as bus signals via the PT-CAN2.

The EME requests starting of the high-voltage system when either terminal 15 is switched on or a request for stationary cooling or charging is present. Starting takes place in several steps:

  • Testing the high-voltage electrical system (preloading):

    Checking if the high-voltage battery unit and the entire high-voltage electrical system is operational. In addition, the circuit of the high-voltage interlock loop must also be closed.

  • Increasing the voltage:

    Due to the capacities in the high-voltage circuit (link capacitors) a very high switch-on current would flow, which could damage both the link capacitors and the switch contactor over time. This is why the voltage is slowly increased.

  • Closing the contacts of the switch contactor

When shutting down the high-voltage system, the regular shut down and the fast switch-off are differentiated. During the regular shut down, the protection of the electrical components and the checking of the high-voltage system are paramount. For example, the contacts of the electromechanical switch contactor should only first be opened, once the current level has fallen to a value close to 0 A, as they are otherwise heavily stressed.

The quick switch-off of the high-voltage system is always performed when the voltage in the high-voltage system must be lowered to a safe value as quickly as possible due to safety reasons:

  • Accident:

    Depending on the seriousness of the accident, the switch-off is requested via the bus signal or forced by separating the safety battery terminal from the positive terminal of the 12V battery hart. In the 2nd case, the voltage supply of the electromechanical switch contactor is automatically interrupted and as a result their contacts open automatically.

  • Overload current:

    The current level in the high-voltage electrical system is monitored with help of the voltage and current sensor. If too great of a current level is detected, the SME initiates an abrupt opening of the switch contactor.

  • Short circuit.
  • Critical state (undervoltage, overvoltage or excess temperature at a battery cell).
  • Interruption of the circuit of the high-voltage interlock loop.

Monitoring the high-voltage system for insulation faults

The insulation monitoring determines whether the insulation resistance between the active high-voltage components (e.g. high-voltage cables) and the vehicle ground is above a required minimum value. If the isolation resistance drops below the minimum value, there is a risk that the vehicle components may be subject to a hazardous voltage.

The insulation monitoring responds in two stages. Even if the insulation resistance drops below the first threshold value there is still no direct risk to people. This is why the high-voltage system remains active, no Check Control message is issued, but the fault status is stored in the fault memory. When dropping below a second low threshold value of the isolation resistance, not only a fault code entry takes place, but a Check Control message is also issued, which asks the driver to find a workshop.

Controlling the cooling and heating

In order to maximise the service life of the high-voltage battery unit and achieve the best possible performance, it is operated in a defined temperature range. As a general principle, the high-voltage battery unit is operational in the range from -20°C to +45°C. These temperature limits, however, refer to the actual temperature of the battery cell, not the outside temperature.

Starting at 32 °C (temperature of the battery cell) the SME introduces a cooling of the high-voltage battery and opens the combined expansion and shutoff valve. This allows refrigerant to flow to the high-voltage battery. The cooling of the high-voltage battery takes place independently from the cooling of the passenger compartment, as it is regulated via a separate combined expansion and shutoff valve.

In the temperature range where a cooling of the high-voltage battery unit is not necessary, the combined expansion and shutoff valve for the high-voltage battery unit remains closed.

Preheating/precooling for charging the high-voltage battery:

Heating is installed depending on the equipment. The control electronics is in the safety box and is connected with the voltage and current sensor via the LIN bus. The heating is only activated during the charging procedure (charging plug is plugged into to the high-voltage charging socket), to bring the high-voltage battery into the optimal temperature range.

If below 10 °C, the heating of the high-voltage battery is activated, and if above 24 °C, the cooling of the high-voltage battery is activated.

Equipment Heat pump Combined expansion and shutoff valve Heating in the high-voltage battery unit
Standard equipment Not installed Can be activated electrically by SME; states: completely opened or closed Not installed
with optional equipment SA 494 Not installed Can be activated electrically by SME; states: completely opened or closed Installed
With optional equipment SA 4T9 Installed Electrically adjustable opening 0 to 100 % via heat pump controller Not installed

Equilibration of the individual battery cells

If one or more battery cells clearly show a lower cell voltage than all the other battery cells, the useable energy content of the high-voltage battery would be limited as a result. This is because the energy withdrawal is determined by the 'weakest' battery cell: If the voltage of the weakest cell has dropped to the discharge limit, the discharge procedure must be ended, even if the other battery cells still have enough stored energy. If the discharge procedure was continued anyway, the weakest battery cell would be permanently damaged as a result. This is why a function is provided for adjusting the cell voltage to an approximately equal level.

The SME regularly wakes up from its rest phases and compares all the cell voltages to one another. Since the adjustment of the cell voltages can only take place via a targeted discharge of the individual battery cells, those with a clearly higher cell voltage, compared to the weakest battery cells, are the ones which are selected. By a request via the Local-CAN to the cell supervision circuits belonging to these battery cells, the discharge is started and performed, until the voltage level has been adjusted. The discharge current flows via an ohmic resistance, which is integrated in the respective cell supervision circuit.

The adjustment of the cell voltages is thus a lossy process, but still useful and necessary for maximising the range and service life. The adjustment of the cell voltages takes place completely automatically during the vehicle standstill.

Notes for Service department

General notes

Warning!

Special safety regulations must be observed for vehicles with a high-voltage system.

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 unauthorised recommissioning

See the following document: De-energise the high-voltage system.

Note!

Internal high-voltage battery unit fault: Repair work on the high-voltage battery unit must only be carried out by specially trained experts. If necessary, hand over the vehicle to the next qualified BMW i service partner.

Note!

The serial number and position number of the cell block and cell supervision circuit is stored in the SME. This allocation must be maintained during repair. Or when replacing a cell block or cell supervision circuit, note down the serial number and position number of the newly installed component for the start-up before the assembly.

See the following document: Composition of the high-voltage battery unit.

Diagnosis instructions

Note!

When replacing a cell block, make sure that the voltage of the new cell block corresponds with the voltage level of the rest of the cell blocks before installing the new cell block. The diagnosis shows the appropriate nominal voltage according to the instructions for replacing a cell block.

Note!

Follow the instructions for service functions!

The diagnosis system provides several service functions for the high-voltage battery unit:

Path: Service function > drive > high-voltage battery unit

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