Traction battery and Battery Management System for minibuses by VDL Bus & Coach

The indisputable ecological benefits of electromobility are frequently challenged by economic reservations. However, it is particularly easy to clear electrical commercial vehicles from such reservations. In particular, buses of all sizes are put into operation for hours at a time, and frequently only travel relatively short distances. The major ecological effects through the use of a battery-electric drive on such buses can hardly be achieved on any other type of vehicle. 

The VDL MidCity Electric and its field of application

Taking this into account, a bus manufacturer based in the Netherlands, VDL Bus & Coach, is to extend its zero-emission electric bus series by the fully-electric VDL MidCity Electric. The innovative vehicle caters for the abrupt rise in the demand for quiet, clean and efficient minibuses. This marks a major step forward, not just for the manufacturers but also for their suppliers, and of course for the operators and passengers.  

The bus for 14+5+1 passengers is to display its strengths in cities, but increasingly also in rural areas. The familiar Mercedes Sprinter in the 5.5-tonne design is to serve as the basis for this fully-electric bus, but its conventional diesel drive line is to be removed during the retrofitting process and replaced with an electric drive. The battery capacity can total 87 kWh, and permits a range of up to max. 220km, which covers all typical operation profiles.  

Thanks to braking energy recovery (KERS Kinetic Energy Recovery System), the VDL MidCity Electric saves a lot of energy, as in particular the frequent stops and accelerations typical for bus operations are particularly favourable on consumption in an electric vehicle.  

Between tours, the batteries can be recharged in a charger installed in the vehicle and a charging connector according to the European standard Type 2. This accords with the charging infrastructure already available for cars today. 

Battery technology by Kreisel Electric

Together with the Austrian company Kreisel Electric GmbH, VDL has developed the particularly innovative fully-electric drive line. The core of this is the unusually compact and reliable battery technology by Kreisel, a technology which is currently unparalleled.

The Kreisel batteries are liquid-cooled and yet still especially lightweight – they weigh only 4.6 kg for every kWh of energy they contain. The term cooling frequently used in connection with this is insufficient for the purpose. In reality, this comprises innovative thermal management which also units heating and air conditioning. In this way it is possible to produce an ideal temperature of between 25° C and 29° C inside the battery, even in extreme temperatures. The cells respond accordingly with an increased lifetime, and the feared losses in range and power capacity during heat and cold are no longer of any consequence.  

The decisive innovation in the Kreisel batteries is the patented construction with direct circulation around all the battery cells using a liquid medium. This permits direct and therefore rapid cooling or heating of the individual cells. The special construction and the use of the liquid medium even reduce the construction volume and increase the inherent safety of the battery system. 

Battery Management System by STW

In its e-vehicles, Kreisel Electric relies on the tried and tested Battery Management System mBMS by Sensor-Technik Wiedemann (STW). The mBMS was especially developed for installation in traction batteries with lithium-ion cells. For this purpose, it unites all required functions in a configurable module consisting of electronics and software components. For the battery manufacturer, it is therefore no longer necessary to develop the required electronics themselves, as the mBMS already encompasses all electric functions in its compact construction, and can easily be configured via data records. 

Only the completeness of the scope of function turns the mBMS into a system. This includes in particular a comprehensive range of sensors for cell voltages, cell temperatures, currents, coolant temperatures, battery voltages and insulation values. Due to the sensor data obtained, the mBMS decides whether battery switch-on is possible without risks or whether it is necessary to shut the system down for safety reasons.  

If such cases occur, the mBMS makes the decision autonomously, and actively triggers disconnection of the battery from the consumer – this disconnection is considered a “safe condition”.  

Lithium-ion batteries conceal an intrinsic risk potential, which has to be controlled through a combination of design and electronic measures. These safety functions are a major component of a Battery Management System. Their implementation is subject to the applicable regulations and provisions for functionally-safe systems as determined in the IEC 61508 Standard and the application-specific standards derived from it. 

The mBMS possesses a separate application controller which is completely decoupled from the functionally-safe components and which executes the complex algorithms for condition determination. This includes the prediction of the current power capacity (“power prediction” – PP), the charging condition (“state of charge” - SOC) and the aging condition of the battery (“state of health” - SOH). The algorithms, on the other hand, are fed by the configuration tables, which the battery manufacturer has obtained from characterisation data for the cells. However, a further basis is the currently-determined temperatures, voltages, currents and internal resistances.

A whole series of additional functions such as the alignment of the state of charge for the individual battery cells (“balancing”) or the controlled and adaptive pre-charging of the capacitors in the power electronics (“pre-charge”) complete the system.

All of these electronics functions have been realised so that they accord with the conventional standards regarding high-voltage safety and electromagnetic compatibility. The components cover a wide current and voltage range and accord with the standards of the automotive industry. Therefore the manufacturer Kreisel Electric can depend on the fact that the battery systems they deliver accord with the required standards.

Relevant Products

The lithium-ion cells connected by Kreisel Electric for high-performance batteries are highly sensitive. Therefore, it is important to continuously monitor temperatures, capacities as well as discharging and charging currents. This last must also be controlled using the STW Battery Management System.