# Analysis and Synthesis of Architectures for Automotive Battery Management Systems
## 6 Exemplary Synthesis of a Powertrain Domain Focusing Start-Up and Shut-Down of the HV-System
### Detailed Function and Interaction Analysis  
In today's batteries there are three main contacts in the BMS that are needed to operate the battery safely and to connect or disconnect the HV-system, as shown in Figure 4. Another contactor is located in the power electronics to actively discharge the HV-capacitor when shutting down the HV-system. The basic function of all contactors is to safely connect and disconnect electrical circuits. The HV-contactors are electromechanical switches located in the BMS's junction box in a distributed system. They need to switch under high voltages or currents safely and with a high number of duty cycles. Due to arcs, which do occur while the contactors are connecting or disconnecting the circuit, the component lifetime is limited. To minimize the risk of damage by arcs, the contactors often become vacuumed and overmolded. The main switches of the HV-system can be loaded with high currents for a long time because these are connected under normal operation (vehicle is driving). Full galvanic isolation at least in two points (redundancy) is an important safety criterion in HV-systems today. The third contactor in the BMS is the switch of the precharging resistor. This switch is only briefly loaded with higher currents when the system is starting up until the DC-capacitor in the power electronics has reached a sufficient SOC. When the system is shut-down, the DC-capacitor must be discharged in a short time (seconds) to avoid electrical accidents. A low-value resistor is connected via another contactor in the power electronics. Here, higher currents are transmitted for a short time. The high-value passive resistor in parallel to the DC-capacitor is permanently connected and serves as a redundancy. It can discharge the capacitor over a longer period (up to minutes) in the event of a fault in the low-value resistors circuit. In a sum, this is why contactors and resistors are relevant parts in the design process of a centralized PD in terms of their function, price, size and also weight. A detailed function and interaction analysis demonstrates that by integrating components and functions into a central architecture, potential savings can be made in starting up as well as in shutting down the HV-system. However, a component that is suitable for both operations must be designed as a compromise. In order to check the feasibility, simulations are set up, which can also be used for the HiL test process later. For this reason, simulations should be performed on real-time capable systems. In this example, the pre-charge resistor is integrated into the power electronics DC-link to achieve a reduction in costs and overall building size of a centralized powertrain system. The in-rushing current into the HV-system while starting up the battery system is limited by the pre-charge resistor. Whereas the electromechanical contacts are physically disconnected once they are open, semiconductors either need a further small physical switch or a physical gap, in order to ensure a safe disjuncture. To do so, the current in the pre-charge circuit needs to be limited to a value, where it is safe to operate for the semiconductor. The current systems that are available on the market today mostly use electromechanical contacts and passive pre-charge resistors with values between 1-1000 \(\Omega\)[55; 56]. Depending on the powertrain's voltage level, these values may be higher in some pre-charge circuits. Furthermore, current systems provide two different discharge resistors, which are connected in parallel to the DC-capacitor. To make sure the capacitor becomes quick and fully discharged after a system shut-down, two different discharge resistors are connected in parallel to the capacitor. One possible simplification is to use one small valued resistor as a pre-charge and as an active discharge resistor. It needs to be switched in series or in parallel in the DC-circuit,depending on the operation mode. In general, there are various schematics to realize such an electrical circuit, as shown in Figures 10-12.