Introduction to the basic components of electric vehicles - battery system

　　An essential component of any electric vehicle (EV) is the battery. The battery must be designed to meet the requirements of the motor and charging system used by the vehicle. This includes physical constraints, such as efficient packaging within the vehicle body to maximize capacity. As a contributor to the weight of an EV, designers must also consider the location of the battery in the vehicle, as they affect power efficiency and vehicle handling characteristics (this is often why you often see batteries placed under the vehicle floor). Here is an overview of some of the specifications, safety considerations, and management systems that go into EV battery design.

　　Electric vehicle batteries are typically made up of hundreds of small, individual cells arranged in a series/parallel configuration to achieve the desired voltage and capacity in the final battery pack. A typical battery pack consists of 18 to 30 parallel cells connected in series to achieve the desired voltage. A battery pack rated at 400V typically has 96 series cells. Common rated pack voltages for current vehicles range from 100V-200V for hybrid/plug-in hybrid vehicles to 400V-800V and higher for electric vehicles. This is because higher voltages allow more power to be transmitted with less loss over the same diameter of copper cable.

　　The disadvantages of higher voltages include the need for higher voltage-rated components throughout the system. They also prevent the use of low-voltage DC fast-charging stations without incorporating some type of DC-DC boost converter in the on-board charger. On the other hand, common battery capacities range as follows: hybrid electric vehicles: 0.5 to 2 kWh; plug-in hybrid electric vehicles: 4 to 20 kWh; electric vehicles: 30 to 100 kWh or more.

　　Batteries represent multiple challenges for safety when designing, as well as the high voltages that are permanently present inside the battery. There are usually fuses inside the battery pack, before the output connectors, usually at the positive and negative terminals. Special high-current sealed relays called contactors connect the internal fuses to the battery itself.

　　Contactors have features such as sacrificial contacts to prevent increased resistance due to contact pitting. They also typically include an auxiliary contact to detect internal welding, which can occur if the contactor is intentionally or accidentally opened while high current is passing through it. Contactor coil power is typically passed through an HVIL or High Voltage Interlock loop that circulates between all high voltage components in the system along a high voltage cable (usually included in each connector), preventing the contactor from receiving power to close unless all high voltage connections are securely plugged into the battery.

　　The pre-charge contactor closes before the main contactor to allow a small current to flow into the system through a large resistor. This limits the inrush current into all the large capacitors in the system and allows the battery management system to detect a short before the high current path is completed. Isolation is usually continuously monitored on both sides of the main contactor, and if the isolation from either side of the high voltage system to the chassis drops to less than 500 ohms/volt, a fault will occur.

　　Tesla also includes a new safety device in their Model 3 and newer battery packs called a pyrotechnic fuse. This device can be blown open by a small pyrotechnic charge if the contactor becomes welded, which allows them to use less robust contactors. Discharge resistors are sometimes included along with the contactor at the battery output to allow the system to actively discharge to a safe voltage after shutting down.

　　The battery pack needs to be monitored and kept balanced, and specialized circuit boards are included in the battery pack to perform this task. These boards must include an independent communication interface because the ground reference voltage of each board will be hundreds of volts different from each other and the main BMS (battery management system). These boards monitor the voltage and temperature of each module and the temperature of the interconnects between modules. They also contain a small set of resistors to perform the balancing task.

　　The battery blocks within a battery pack must be kept within a few millivolts to allow maximum power transfer in and out of the pack. Due to natural variations in battery manufacturing, some battery blocks charge or discharge slightly faster than others. To combat this, balancing is performed during the charging process to draw a small amount of power from the highest voltage block, placing it closer to the other blocks. These block monitoring boards also provide additional battery pack safety features by very accurately monitoring the temperature of the cells and interconnect points within the battery pack. In the case of a damaged cell, for example, this means that a failure, or even a fire, can occur before serious damage occurs.

　　Finally, the battery management system, or BMS, manages the tasks of monitoring and controlling various aspects of the battery pack. The current shunts report a variety of information to the BMS, including the total charge incoming and outgoing. Voltage measurements before and after the contactors allow monitoring of the battery pack system voltage. The contactor control and economizer circuits manage contactor closure and minimize quiescent current through the coil after the contactor is pulled in. The BMS also communicates continuously with the block management board to monitor cell voltage and temperature and control balancing.

　　The entire system and connector temperatures are monitored to detect any high resistance connections caused by loose connectors or bolts. System and package isolation are also continuously monitored, and other possible redundant safety features can also be integrated. The BMS also provides a communication interface to the rest of the vehicle via automotive Ethernet or CAN bus, where the BMS communicates with the inverter, charger and other systems. It calculates and provides charge and discharge current limits, battery pack health status and charge status, and notifies other systems when contactors must be opened, so ideally they can open without load.