Cars whose wheels are driven by battery-powered electric motors, continuously or intermittently, have become a hot topic. These "green" vehicles rely on batteries of series-connected cells to obtain sufficiently high voltage to operate the motor efficiently.
Such high-voltage (HV) stacks are used in all-electric vehicles (EV) " as well as hybrid electric vehicles (HEV), which rely on an internal combustion engine (ICE) for charging and (in many cases) shared propulsion.
EVs must be plugged into a power source for charging; some newer hybrids are designed as plug-in hybrid electric vehicles (PHEV) and are intended to run as EVs essentially but they do utilise an ICE for range extension.
HV stacks are already used in many industries and applications outside of the transportation industry " typically in uninterruptible power supplies (UPS) to store energy from the grid in dc form; as emergency dc supplies in 48V communications equipment; as emergency supplies in crane and lift systems; and in wind turbines for feathering the blades in an emergency. Although we discuss here the use of battery stacks in vehicles, the underlying issues are common to all types of stacks.
Battery stacks for transportation can typically involve 100 or more cells, providing hundreds of volts. Since it is generally accepted that more than 50 V or 60 V can prove lethal to human beings, and even lower voltages can damage electronic equipment " and considering the stability concerns about cells using some types of electrochemical reactions " safety is a key concern. Accepting that such stacks are inherently dangerous and that it is also necessary to communicate with the cell monitoring electronics that are usually located within the battery enclosure, some method of communicating safely & reliably with the monitoring electronics is going to be necessary.
The original equipment manufacturer generally specifies the physical packing of the cells into enclosures called packs, which typically contain from 6 to 24 cells in series. With a large number of cells in a pack the physically larger packs are more awkward to fit into typical vehicle spaces and become unwieldly. The cell-monitoring integrated circuits associated with the cells are physically close to the monitored cells and are powered by the cells themselves.
Whether it is essential to monitor the voltage of each cell depends on the cell chemistry. For instance, the behaviour of HV stacks based upon NiMH chemistry is very well understood and generally no effort is made to measure individual cell voltages; it is sufficient to measure the total voltage of all the cells within any given pack. However, for a stack based upon Li-ion chemistry, it will be necessary to monitor the voltage of each cell to detect an over- or under-voltage of any individual cell in the string. It is not generally necessary to measure the temperature of each Li-ion cell but the facility to do so should be available. The electronics for monitoring a NiMH stack are considerably simpler than those for a Li-ion stack. Figure 1 shows a common approach to building and monitoring an HV stack.
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Fig 1: Serial cell monitoring and isolation in a battery stack.
