Battery Management System Controls 99 ŶĞƌŐLJŵŽǀĞĚƚŽ ůŽǁĞƌ^KĐĞůůƐ ƵŶƟůĂůůĐĞůůƐĂƌĞ ĂƚƐĂŵĞ^K Figure 10 Active cell balancing. active balancing is more expensive and can require more space within the pack. The electron- ics necessary to do this must also be attached to each cell, or integrated in the slave boards for a group of cells. There have been a lot of evaluations of both systems over the past few years; however, current studies do not show the long-term benefits of using an active balancing system.
In short with the current level of technology, there does not appear to be any major system benefits that can be achieved and those minimal benefits do not outweigh the added cost for an active balanc- ing system. Additional BMS Functionality The other functionality of the BMS is no less important than balancing. In fact while it will significantly affect the life of a pack, the energy storage system could operate without any form of balancing. On the other hand, monitoring the temperature of both the cells and the pack, as well as the voltage of the cells and the pack is critical for maintaining the safety of the system.
The core job of the BMS is to ensure that the battery system does not allow the cells to operate outside of their safe operating range. This includes monitoring the pack current, cell and pack voltage, and the cell and pack temperatures. Monitoring the pack current enables the system to determine how much power is instanta- neously available for both discharge and charge (regeneration). Since driving the cells either over or under their maximum and minimum voltages can result in catastrophic failure, it is vital that the BMS has capability to monitor every cell in series within the pack (cells in parallel will be treated as a single cell in most BMS systems).
These data can then direct the system as when to begin or stop a charge or discharge event. Finally, monitoring and manag- ing the temperature of the cells and the overall pack is another vital piece of data as contin- ued operation outside of these limits can not only reduce the life, but drive the cells into thermal runaway. The BMS is responsible for telling the system to send more cooling or heating to the cells.