Battery Basics: Part 2 - The Battery Management System

 

The robustness and potential of the Lithium Iron Phosphate (LiFePO4) battery is indeed state of the art. However, if the battery packs are not properly monitored they run the risk of overcharging. If a LiFePO4 battery cell is charged above its maximum voltage tolerance, the voltage begins to denature the chemical composition of the battery, reducing its capacity. In extreme cases, improper management can even cause batteries to explode. Therefore, in battery packs containing multiple cells, it is imperative to have a Battery Management System (BMS) to balance the individual voltage levels and keep the pack within its Safe Operating Area (SOA). The SOA is denoted as the upper and lower limits of voltage, temperature and current.

An advanced BMS typically provides the following functions:

  • Prevent discharging and overcharging by keeping the battery pack within allowed voltage values
  • Regulate the temperature of the battery pack and override the system to maintain these limits
  • Record and constrain the current both in and out of the battery
  • Balance the voltages between cells to maintain an efficient use of capacity
  • Continuous estimation of each cell’s State of Charge (SoC), Remaining Useful Life (RUL) and State of Health (SoH)
  • Transmit data to server and receive external commands from server

Figure 1: Diagram of a BMS module and the purpose of each component. Source: İlker Aydın and Özgür Üstün from İstanbul Technical University

                Energy Management Software (EMS) is a critical component of modern BMSs. The architecture of the EMS provides data management and gives full control over the operation and balancing of the BESS at the system, array, stack, module and cell levels. The software is able to autonomously provide cell-level balancing and monitoring services, but a secure server can also provide manual remote access as well.In the case of LiFePO4 batteries, a BMS is critical because the cells are placed in series. This means that once a cell reaches its maximum charge, almost zero current is able to flow through the cell. A BMS is able to bypass the charging current from full cells through parallel wiring to each cell, and it contains diagnostic software to provide cell-level management and sub-second recording of cell voltage, temperature and State of Charge. Thus, each cell is able to continuously remain within 10mV of its optimal state. 

The BMS can be dissected into a series of layers, each with their own level of control over the entire system:


Figure 2: Block diagram of BMS communications. Source: Powin Energy.

Together, these layers of the BMS provide reliability and efficiency to the entire battery storage system. It can be considered the most important component of the BESS because it ensures the safe regulation that is required for its reliable use.