# Battery Management System Hardware Concepts: An Overview
## 6 Safety and Reliability  
One of the main purposes of using a BMS is to minimize the risks associated with the operation of lithium-ion cells in battery packs. In the following section, some examples of typical safety functions of a BMS are given. Afterwards, design aspects related to the safety of the BMS itself are discussed. According to [59], the main hazards inside lithium-ion battery packs are exposure to high voltage, arcing, fire, vented gas combustibility and vented gas toxicity. In combination with suitable battery pack design, the safety functions of the BMS should minimize the likelihood of occurrence and severity of these events. Insulation monitoring (further discussed in the next section) and interlock circuits can be used to ensure the high voltage safety of the battery pack. This also lowers the risk of arcing events due to buildup of pollution or condensation inside the battery pack, which also affect the insulation resistance because they can be detected at an early stage. The BMS hardware design should follow relevant standards [60; 61] for proper creepage and clearance distances for the PCB as well as all connectors. Regarding the communication interfaces with other control units or auxiliary power supply (low voltage vehicle electrical system), care has to be taken to ensure galvanic isolation from the high battery voltage. During design, it has to be assured that the selected isolation device fulfills the criteria for \"reinforced isolation\", as it has to provide sufficient protection against electric shock [62]. Traditionally, optocuplers have been used for galvanic signal isolation. In recent years, so-called \"Digital Isolators\" ICs have entered the market and promise superior performance [63], especially in terms of reliability and ease-of-use. These ICs exhibit an internal isolation barrier and capacitive or inductively coupled signal transmission circuits. The risk of fire can be lowered by placing temperature sensors inside the pack and reacting appropriately to critical temperature values. In addition, sensorless temperature detection methods using indirect sensing methods, like electrochemical impedance spectroscopy [64; 65; 66] can be used additionally. New methods to acquire temperature along wire harnesses within complex electrical systems can also be utilized to add safety [67]. As the examples above show, in many applications, contactors are used to allow disconnecting the battery pack from the system. As all contactors have a finite break current rating, a melting fuse should be used additionally. The operating characteristics of both fuse and contactors have to be taken into account to make sure that a fault current can be interrupted in all cases. Parasitic capacitance and inductance inside the battery pack, as well as from the application, have to be taken into account for the selection of the fuse rating because of their significant influence on the rise time and magnitude of the short circuit current [68]. The intrinsic safety of the cells depends on the strict compliance to the defined safe operation conditions of the cells. The BMS has to inhibit the charging of the battery outside a defined temperature window. Especially lithium plating at low temperatures is a serious risk, as it can lead to the formation of dendrites and subsequent internal short circuits of the cells [69]. In addition, no operation of the battery pack after a deep discharge should be allowed by the BMS because of the possibility of short circuits due to corrosion phenomena [70]. Some diagnostic algorithms are proposed in literature to detect internal short circuits [71; 72] at early stages. As the hardware and software implementation of the BMS get increasingly complex, the possibility of software errors and sensor faults also rises. One critical issue is the integrity of the cell voltage sense cables between the battery cells and the cell supervisory unit. A sense line cable break cannot immediately be detected by measuring the voltage alone [73].