|Design of the modular batteries. Source: KIT. Click to enlarge.|
At the upcoming IAA International Motor Show in Frankfurt, Karlsruhe Institute of Technology (KIT) will present a modular battery concept for the efficient operation of an electric bus; an electric city bus demonstrator will illustrate the concept. The development of the e-city bus demonstrator was carried out within the scope of the project Competence E and was funded by the German Federal Ministry of Economics and Technology.
The key modules of the demonstrator are a drive train with a high-torque electric motor, a high-voltage network, a battery management system, and a novel modular battery system with lithium-ion cells made in Germany. At the IAA, the demonstrator developed for drive tests will present options for the design of the electric drive train of buses.
Modular battery system. The battery system consists of flat modules that can be stacked to reach the dimensions and electric characteristics desired. The modular concept—and more specifically, the flat modules—allow the capacity, voltage, and size to be adapted to various needs. The number of cells and thereby the length of a single module can be varied.
An advantage of this flexibility is the ability to keep a module under 60 V (14 cells) since modules with higher voltages must be handled by special trained persons. Another advantage of small modules is that they can be placed in smaller spaces.
Any number of modules with the same length can be stacked. Within one stack the modules can be connected in parallel, in series, in series-parallel or in parallel-series. This is possible because of an adaptable connection concept. To build larger distributed battery systems (e. g. for stationary energy storage), multiple battery packs can be connected.
Automated joint processing can be used because of the easily accessible conductors of the cells. This also allows for detachable plug-type and clamping connections—currently in development—to be implemented. Coolant can be easily passed through a cooling channel next to the conductors. Electrical connections and cooling channels are far away from the outer surface of the battery and therefore from the possible impact areas. In case of a crash, the cell additionally absorbs crash energy before electrical contacts and cooling channels are damaged.
It is also possible to heat the cells using a surrounding heating mat, for example if the temperature is lower than 5 °C and the cell won’t charge. The change in volume while charging and discharging of a pouch cell is compensated by an evenly compressible foam layer. The foam layer also helps to fix the cells between modules and distributes forces evenly. This prevents sliding of cells up to 30 g and reduces mechanical strain on the conductors.
|Modular components. Source: KIT. Click to enlarge.|
Various spaces in the different types of vehicles can be used for accommodating the energy storage system. The battery management system and drive control developed for the KIT demonstrator allow for driving operation taking into account the current performance limits of the system and its components.
|Modular batteries (orange) can be integrated easily in the free space of the vehicle. Click to enlarge.|
E-city bus demonstrator. The bus is driven by a permanently synchronous motor. The motor transmits the driving torque through a differential with a fixed gear ratio directly to the wheels of the rear axle. The transmitted power can be up to 160 kW at 650 VDC. The installed power allows the bus to drive 107 km/h (66 mph) on a flat track.
The high continuous torque at low engine speed allows a bus with a weight of 9 tons to climb hills with a gradient of 15 % at speeds up to 25 km/h (15.5 mph). While the final bus will be operating at 750 VDC, the voltage level is 450 VDC at the first expansion stage. The voltage level in operation influences the max. power output, since the occurring current within the components has to be limited.
For the operation of the permanently synchronous motor, the constant current of the battery is changed in 3 phases of alternating current by the inverter. Besides the electric drive system, additional high voltage auxiliary consumers are connected to the battery using a power distribution unit (PDU). A DC-DC converter to supply low voltage to the electronics, the compressor for the braking system, the cooling pumps, the fan and the control units.
The vehicle control unit (VCU) communicates with the other control units (BMS, Motor Control Unit) and transforms the driver’s input—given by the gas and braking pedal position—into a torque request for the electric drive system. The torque request is determined and reduced based on the limits of the components.
Using the demonstrator, KIT researchers say, enables the innovation potential of KIT’s research results to be validated and interaction of the components can be analyzed experimentally under the simulated operating conditions.
Competence E. The Competence E project covers all research aspects from the battery material to the electric drive. With an open technology platform for battery-electric vehicle drives and stationary energy storage systems, the systemic approach is aimed at developing industrially applicable solutions and their production methods.
Due to integration along the value chain, battery systems with an energy density of 250 Wh/kg are anticipated to be manufactured at costs of €250/kWh (US$330/kWh) by 2018.