Electric Drive System for Mobile Machinery

Deutz has developed an electric drive system that is suitable for various mobile applications thanks to its modular design. The focus is not only on charging performance and battery conditioning, including capacity extension, but also on functional safety requirements.

In order to achieve effective climate protection, both the EU with the Green Deal and the German government with the revision of the Climate Protection Act in June 2023 have set themselves the goal of becoming greenhouse gas neutral by the middle of the century. To this end, the registration of new vehicles powered by combustion engine is being limited and the transition from fossil fuels to regenerative forms of energy is being driven forward. What has long been established in law for the transport sector will in the long term also apply to the offhighway sector. With this in mind, Deutz has developed an electric drive system for mobile machinery. Thanks to its modular design, the E-Deutz System (EDS) can be used for machines in the fields of agriculture and construction, material handling, or airport aprons. Exemplary applications are wheel loaders, forklifts, telescopic handlers, pushbacks, baggage tractors, or concrete pumps. After the first prototype vehicle was built in 2020, the EDS was ready for series production at the end of 2023. Some of the system's features and development steps are described in the following.

Basic Design

The EDS comprises various subsystems. The basic configuration consists of a High-voltage Battery (HVB), a 12-V onboard power converter (High-voltage Boardnet Converter, HBC), an inverter with an electric motor, an Onboard Charger (OBC), a system control unit (Powertrain Control Unit, PCU), and a Power Distribution Unit (PDU), to which all High-voltage (HV) components are connected and fused.

The EDS has the option of using up to two motors, which are operated as a drive or work propulsion. While the drive powertrain, for example, directly drives the wheels of a vehicle via a gearbox and a differential, the work drive is used to operate a hydraulic pump, which is used in mobile machinery, for example to raise or lower a telescopic arm or to tilt the tines of a fork via hydraulic rams. It is also possible to connect custom-specific high-voltage consumers to the so-called DC-PTO, which is an high-voltage interface for supplying air conditioning compressors or auxiliary units with electrical energy from the DC link.

Charging Power

The EDS has had a type-2 charging socket since the first prototype. In the first evolutionary stage, the charging power still had to be reduced to 3.6 kW on the OBC side. Nevertheless, the EDS already communicates with the charging station via the Control Pilot (CP), recognizes the connected cable via the so-called Proximity Pin (PP), and identifies the maximum charging power. The OBC is responsible for converting AC voltage into DC voltage.

Due to the low charging power, an option for faster charging of the HVB was developed. Thanks to a higher-performance OBC, it has been possible to charge the EDS via three phases with up to 22 kW from the second evolutionary stage onward. This means that the HVB with a gross capacity of 42 kWh can be charged from 20 to 80 % within 1 h. With this increase in charging power, however, it is necessary to reduce the charging current under certain thermal conditions from this stage onward. To prevent damage, the maximum possible charging current is dynamically adjusted if, for example, the cell temperatures of the HVB reach critical ranges. Communication between the HVB and the OBC is handled by the PCU. As part of the distributed software in the overall system, the signals from all components converge on the PCU. This evaluates the information in its entirety and sends the setpoints to the components in accordance with the programming and operation. As the HVB would switch off to protect itself if the maximum charging currents were violated, the interaction between the components must take place within certain tolerances and times.

In the current stage of evolution, it is now also possible to charge the EDS with up to 150 kW. For this purpose, a Combined Charging System (CCS) charging socket, a DC disconnection box as a safety component, and a power line communication module have been integrated into the EDS, allowing the system to communicate with a DC charging station that is connected directly to the HVB via the DC link [1]. The signals relevant for charging are then exchanged via the communication: Essentially, these are the target voltage and the maximum current. This can again significantly reduce the charging time and thus maximize the system's operating time.

Overall, the EDS is able to operate all charging modes (1 to 4) according to the specifications of IEC 61851-1. In addition, a special solution for charging in non-European countries on alternating current with up to 22 kW has been realized, for which no CP/PP communication is necessary.

Figure 1

Development of the charging unit on the E-Deutz system (© Deutz AG)

Battery Conditioning

High-charging currents at low temperatures can lead to accelerated ageing, known as lithium plating. Lithium ions cannot diffuse quickly enough through the top layer and deposit as metal. This leads to a loss of capacity, an increase in internal resistance, and the risk of lithium dendrites forming, which can cause an internal short circuit and thus a fire [2]. As a result, the Battery Management System (BMS) reduces the maximum possible charging currents if the cell temperatures are too low. A high-voltage heater is integrated into the EDS to condition the HVB so that it can still be charged at cold temperatures and to speed up the charging process overall. The system was subjected to tests at different ambient temperatures in the in-house cold chamber. The high-voltage heater is integrated in such a way that cooling water with up to 7 kW is heated and pumped through the HVB. In order to speed up the charging process and minimize power consumption at the same time, the charging power must be optimally distributed between the HVB and the heater.

Figure 2 shows the initial (shortly before the charging process) maximum charging current in relation to the value at 0 °C without heating (blue bars). As soon as the charger supplies this maximum current, the BMS reduces this (peak) value by approximately half after a short time to a charging current with which the HVB can be continuously charged at the temperature level. At 0 °C with a heater, the same current is reported, at -10 °C the current is already 80 % lower, and from -20 °C the charging current is already at 0 % or 0 A. Charging without a heater would therefore no longer be possible. The red bars show the relative charging time, also based on 0 °C without a heater, which was achieved in order to charge the HVB from 0 to 100 % State-of-Charge (SoC). The result shows that the heater can extend the range of use of the battery down to -30 °C and shorten the charging time. In these tests, the charging time is significantly lower than at 0 °C without a heater, by 14 and 7 % respectively. As soon as a temperature is exceeded during the charging process at which the continuous charging current corresponds to the maximum current determined by the OBC, the heating phase is terminated and the entire available charging power is fed into the battery. Further heating of the HVB cells, which can then still be observed, can be explained by the charging losses of the HVB.

Figure 2

Charging time and initial charging current with and without heater at cold ambient temperatures (© Deutz AG)

Dual Battery

A clear requirement for the battery capacity to be used is derived from the very different application scenarios at customers. Therefore, the number of batteries and the corresponding capacity can be individually adapted to the application. As shown in Figure 3, the battery of the first generation from the E-Deutz modular battery system has a gross capacity of 42.2 Wh. By connecting the batteries in parallel, the maximum charging and discharging currents are increased and the total capacity is doubled to 84.4 kWh.

Figure 3

Capacity increase through parallel arrangement of several high-voltage batteries (© Deutz AG)

The technical solution to this is to configure one battery as the so-called main HVB and all others as secondary HVBs. The configuration is specified externally via the wiring in the cable harness. On the software and hardware side, all batteries in a network are identical. The BMS is designed in such a way that up to 30 HVBs can be linked together in a network (parallel and/or serial). Communication between the batteries is realized via an internal battery CAN. The individual batteries use this to transmit their values to the main HVB and in turn receive its commands. These values primarily concern the current voltage and hence the relaxed state of charge of the individual batteries, the current possible charging and discharging currents, and temperatures. The battery network only communicates with the surrounding systems via the main HVB, which provides the information on the corresponding sum of the currents/voltage and error states. This means that the communication interface always remains identical, regardless of the number of connected batteries. The main HVB also handles the voltage balancing of different battery voltages during operation and an associated diagnostics management system.

Functional Safety

All software functions involved in the system can cause the system to react with critical behavior even in the case of a small error. That is why the EDS was developed from the very beginning with relevant aspects of functional safety in mind. The basis for this is ISO 26262, which was originally intended for the automotive domain, but is also considered state-of-the-art for functional safety due to its relevance and scope.

In order to be able to develop the EDS as an easy-to-integrate Safety-Element-out-of-Context (SEooC [3]) even without specific customer requirements, a comprehensive hazard and risk analysis was carried out, taking into account nearly all potential customer applications. It resulted in five safety goals that were taken into consideration during development: prevention of unintended torque, prevention of unintended speed, avoidance of a thermal event of the HVB, avoidance of overvoltage at the low-voltage interface, and prevention of motoring torque after activation of the emergency stop. While some safety goals are completely covered by the EDS (such as “preventing a thermal event of the HVB”), others can only be fulfilled to a certain extent.

To prevent unintended acceleration or torque, for example, the functional chain usually begins in the driver's cabin with an accelerator pedal, Figure 4. This is then evaluated by the Vehicle Control Unit (VCU) and converted into a setpoint value for the torque based on the desired driving behavior. The EDS receives the target value in the PCU and forwards it to the inverter, taking into account the current system status (for example SoC of the HVB). The torque is then set via the motor's phase currents.

Figure 4

Functional chain to prevent unintended torque (© Deutz AG)

In this example, the EDS as SEooC has its boundaries at the interface between the VCU and PCU. However, the entire chain was taken into account during development, for example in the allocation of fault handling time intervals, measurement tolerances, or hardware metrics. For seamless integration, the SEooC is fully described in the safety manual with respect to its boundaries. All assumptions made during development and requirements for the integrating system to achieve the defined safety goals can also be found there. Therefore, for the integration, only the safety goals and the assumptions and requirements made need to be compared with the target application. In this example, it is only necessary to ensure that the target torque value sent to the EDS is correct. This procedure and the underlying processes are ISO 26262 certified, which significantly reduces the effort involved in certifying the customer's product.

System Efficiency Test

At each evolutionary stage reached during the development process, specific energy efficiency tests are carried out for documentation purposes using defined load cycles. For such a test of the EDS, the HVB is first fully charged, run through a cycle until it is completely empty, and then fully charged again. During the entire process, the voltage of the DC link and the currents are measured. The focus is particularly on the currents at the OBC, the HVB, and the drive unit (DRU) or work unit (WRU) respectively. In addition to the mechanical variables of speed and torque of the load unit (dyno), the power of the AC current at the input of the charger (net) and the power at the low-voltage output of the DC/DC converter (HBC) are also measured. The latter is mainly consumed by water pumps and fans as part of the thermal system (THS), Figure 5.

Figure 5

System diagram for calculating the individual efficiencies and the overall efficiency (© Deutz AG)

With the help of the values obtained in this way, it is possible to calculate the individual efficiencies of the components on the one hand and to determine the overall efficiency of the system on the other. The overall efficiency is then the ratio of the mechanical benefit to the input charged from the grid. Figure 6 shows the overall efficiencies for a working propulsion and driving propulsion, respectively. The efficiency of the motor is highly dependent on its operating range. Operation at the typical duty cycle compared to the traction drive does occur in a range where the efficiency of the inverter-motor combination is slightly worse, but this alone does not explain the difference in the calculated efficiencies. Instead, the advantage can mainly be explained by the recuperation that is possible with the driving system due to the thrust drive. Since the working hydraulics are usually taken over from a conventional system with an internal combustion engine, recuperation is usually not possible here. During recuperation, the braking energy is absorbed by the motor and stored in the battery. This increase in charge level and energy content enables a longer drive so that a higher mechanical energy can be achieved.

Figure 6

Overall efficiency with operating ranges for working and driving propulsion (© Deutz AG)

Conclusion and Outlook

With the E-Deutz system, customers can be offered an electric drive system that can be integrated as a complete solution for a wide range of applications. Depending on the engine power or battery capacity needed, the system can be adapted to many requirements. During development, all state-of-the-art standards are respected (such as ISO 14990, DIN 1175, VDE 0100, ISO 13849 and ISO 26262). Thanks to compatibility with the established Serdia diagnostic tool, the existing global service network can be accessed to service the electric driveline. The next upcoming milestone is the release of the second generation of the high-voltage battery. It will have a modular design based on a cell module, deriving both the 48-V variant and the 360-V variant.

References

• [1] DIN EN 61851-23 (2017) Konduktive Ladesysteme für Elektrofahrzeuge - Teil 23: Gleichstromversorgungseinrichtung für Elektrofahrzeuge (IEC 69/523/CD:2017). Berlin: Beuth, 2017

• [2] Münnix, J.: Einfluss von Stromstärke und Zyklentiefe auf graphitische Anoden. Aachen, Rheinisch-Westfälische Technische Hochschule Aachen, Dissertation, 2017

• [3] ISO 26262-10:2018 „Guidelines on ISO 26262“. Vernier: ISO, 2018