Battery Modules - Packs - Systems

The high and low environment temperatures, due to the seasons and the geographical location, reduce the range of electric vehicles. Research activities are performed at Fraunhofer IISB to address this main issue by developing innovative solutions using a combination of electrical and thermal energy management:

• Reuse of the thermal energy dissipated by the power electronics (e.g., DC/AC inverter), by the energy converters (e.g., electric drives), or by the range extender (e.g., internal combustion engine)
• Using the charger also for thermal conditioning of the vehicle subsystems (e.g., batteries, electric drives, passenger compartment)
• Storing the heat/cold in subsystems (e.g., phase change materials for storing heat/cold in the battery compartment)

An example of a smart battery system developed for the hybridization of an Audi-TT is shown. Eight battery modules, each implementing 36 Lithium Iron Phosphate (LiFePO4: LFP) cylindrical battery cells from A123 Systems connected in a 12s3p configuration, provide a total nominal voltage of 317V for a total energy capacity of 2.2kWh.

The battery system is air-cooled, placed in the under-floor of the luggage compartment, and contains all the necessary control and power electronics (i.e., high-voltage battery with battery monitoring and management, battery charger, boardnet converter, galvanic isolation monitoring, switches) to be operated in an autonomous way.

A smart battery pack based on pouch cells was developed for a Citroen AX Electric. This car is a demonstration platform used by students for their thesis (TechFak EcoCar). The battery pack consists of 6 battery modules, each integrating 12 LiFePO4 pouch cells of 45Ah. The modules are connected in a redundant way, using a two string topology, each string having 3 modules connected in series. The energy capacity is 11.5kWh and the nominal pack voltage is 115V. The maximum continuous current is 90A, with a maximum peak current of 320A. The battery pack is water cooled and uses an innovative cooling system based on carbon plates coupled with newly developed very low cost printed temperature sensors. The monitoring is done by battery monitoring circuits from Linear Technology. The maximum charging voltage per cell is 3.65V, but it is set electronically at a maximum of 3.55V. With this value, the stored energy will be reduced by about 1% but the number of charging/discharging cycles (lifetime) will be increased by 33%.

The state-of-the-art centralized battery monitoring architecture has major drawbacks, which obstruct the cost-efficient development and production of battery systems:

• The assembly and contacting of the battery cells for monitoring their voltage and temperature is very expensive, even if fabricated in high volumes.
• The high amount of connectors is a source of failures and causes reliability issues.
• Different applications, such as different types or models of the product, have different requirements and specifications. In most cases, this leads to a full redevelopment of the battery module and the battery monitoring circuit boards for each new type and model.
• The redevelopment of battery monitoring boards increases the time-to-market, the development and the production costs, which makes the centralized battery monitoring architectures economically not viable for the mass market.

These drawbacks were the reason for designing a more flexible battery monitoring and management system demanding less manufacturing and assembling effort.

The novel smart battery cell monitoring concept based on distributed battery monitoring (patent pending) consists of single battery cells with integrated electronics allowing bidirectional contactless data transfer to the battery management system. The proposed distributed battery cell monitoring uses a differential capacitive galvanic isolated data transmission bus to communicate with the battery management system, thus providing following advantages:

• The smart battery cells provide all the necessary electronics to be fully monitored and balanced.
• The monitoring and balancing functions are integrated into each smart battery cell, thus improving the reliability, simplifying the assembling and reducing the contacting effort.
• The effort required for contacting the smart battery cells is reduced to two power contacts (battery poles) and two galvanically isolated capacitive differential contacts to the data bus used for communicating between the integrated battery cell monitoring electronics and the battery management system.
• The cell temperature measurement, cell voltage measurement, cell pressure measurement, cell current measurement, cell balancing current measurement can be provided by the monitoring electronics implemented in the smart battery cell without any additional contacting effort needed when constructing the battery modules and the battery pack.
• Smart battery cells enable much shorter time-to-market when developing novel battery packs, since no more complex application specific battery monitoring circuits and boards need to be developed.
• Smart battery cells dramatically improve the usability and the modularity during battery modules and battery pack design.
• The hardware of the battery management system can be standardized and reused in new and different developments simply by maintaining and improving the firmware (i.e., embedded software).
• Costly and failure prone galvanically isolated CAN transceivers are no longer needed for the data transmission bus between the battery modules and the battery management system.
• The electronics integrated into the smart battery cells allows protection against counterfeit.

The new smart battery cell monitoring concept not only solves the issues of state-of-the-art centralized battery monitoring architectures, but also adds modularity and flexibility, and alters the traditional role of the battery manufactures, automotive manufacturers and automotive suppliers.

A battery system using this technology is currently in development at Fraunhofer IISB. The system specifications are listed hereafter.

Specifications of the battery system:

• Total energy capacity: 21.5kWh
• Number of battery modules: 7
• Maximum battery voltage: 400V
• Minimum battery voltage: 225V
• Maximum discharging current: 300A (continuous)
• Maximum charging current: 180A (continuous)

Specification of one module:

• Weight of one module: 51kg
• Battery module topology: 16s2p (32 cells)
• Energy capacity of one module: 3kWh

Specifications of the selected battery cells:

• GB-Batteries of type GP30EVLF
• Cell chemistry: Lithium Ferro Phosphate
• Nominal cell voltage: 3.2V
• Charging (continuous): 90A (3C)
• Discharging (continuous): 150A (5C)
• Discharging (peak): 300A (10C) for 5s
• Maximum charging voltage: 3.65V
• Minimum discharging voltage: 2.00V
• Internal resistance: lower than 2mOhms
• Specific energy: 87 Wh/kg
• Specific power: 427 W/kg
• Energy density: 178 Wh/L
• Power density: 876 W/L
• Size: 30 x 176 x 95 mm
• Weight: 1.1kg