HV battery stacks
For practical reasons, it is better to arrange batteries for
electric cars in several separate battery packs, each containing from six to twenty-four cells. Smaller sized packs allow the maximum use of the available space in an irregularly sized electric vehicles battery enclosure which can fit into a typical electric vehicle. The use of battery packs also allows more freedom in selecting parallel/serial combinations to create different voltage/current profiles for different traction motor drivers. Additionally, if there is a single cell failure, it only affects one battery pack out of many and the electric car batteries can still function.
Cell Balancing
The cell voltage and energy capacity vary slightly between cells, so simply daisy-chaining all the cells together when
charging the batteries for electric vehicles will create imbalances between the cells with some fully-charged while others still needing more charge. The over-charged cells can get hot which could damage the cells and the battery pack. In worst case scenarios, the battery pack could catch fire. To avoid this situation, cell monitoring ICs are used to individually monitor and control the charging (and discharging) profiles to ensure that all the cells are used to their full capacity without being damaged by undervoltage, overvoltage or overtemperature conditions. During the charging process, individual fully charged cells can be bypassed to allow the other cells in the stack to continue to be charged. This balancing process continues until all cells are equally fully charged. During discharge, the same balancing circuit can ensure that all the cells are equally discharged.
Battery Management Systems
The electric vehicles battery packs are stacked together to form the required EV battery voltage and communicate with a central battery management system (BMS) via a communication bus, usually CAN-bus which is prevalent in the
automotive industry. The BMS monitors overall charging and discharging profiles and calculates the state of charge (SoC) and state of health (SoH) of the HV stack. It also monitors each pack current, voltage and temperature to ensure safe operation. As the number of cells in the HV battery increases, the amount of data that needs to be collected and processed also increases, but the system loop time requirements remain fixed. The CAN-bus must operate with high data rates (up to 25Mbps) and low propagation delays (100 – 50ns).
Safety Isolation
All
EVs must have a mechanical safety switch or contactor to disconnect the HV battery in an emergency. If the switch is placed in series with the high voltage output of the battery, then there still could be sufficient current flowing between the battery packs in the stack under fault conditions to cause a fire, so it is usually placed in the middle of the stack. This unusual arrangement (Figure 1) maximises the overall safety but requires isolated communication lines to ensure that current cannot bypass the safety switch via the data bus connections.
Fig 1. Battery stack with isolated data bus
Increasing fault tolerance with isolated DC/DC converters
The majority of cell balancing ICs incorporate an internal voltage regulator that uses the battery voltage to power both the IC and the isolated side of the data communication port, while the BMS controller powers the non-isolated side (red traces in Figure 1).
However, with high voltage battery stacks consisting of many battery packs arranged in a parallel/ series configuration, it increases system reliability to isolate each battery string communication bus separately using isolated CAN-bus transceivers. In this case, isolated power is also required for the isolated CAN-bus side (Figure 2).
Fig. 2: Separate isolated power and bus connections for paralleled battery stacks
RECOM offers the
R05CTE05S isolated 5V-to-5V module specially designed for isolated bus transceiver applications. It provides 1W of power in a compact 16 SOIC SMD package over a temperature range of -40°C up to +125°C, making it ideal for installation inside the battery compartment. The isolation grade is 3kVDC/1 minute, meaning that it can be used with 800V or higher battery stack voltages with ease, allowing for any new EV battery technology developments in the future as electric vehicle battery manufacturers are constantly looking to improve their products. To improve system fault tolerance, the output is protected against continuous short circuits, over currents and over temperature. An under-voltage lockout function means that the converter only starts up once the supply voltage exceeds 3.3V avoiding data corruption issues during the power up sequence of the BMS system.
