Challenges of Designing the Ideal BMS

By Nate Wennyk | Sep 9, 2016

What makes an ideal battery management system? The first thing that should come to mind is safety. There are many videos and articles out there that show lithium-based batteries venting and/or bursting into flames when pushed past their operating limits. A BMS solution must prevent the battery pack from entering into an unsafe condition. But safety is only the beginning of the story. Users also want to maximize the life of their battery pack, and try to maintain the capacity of the battery pack as it ages. This is where the idea of an “ideal” BMS solution can start to be defined.

The ideal BMS solution provides accurate fuel gauging, and cell balancing to ensure that a battery back is completely empty or completely full. There are two main design challenges to create the ideal BMS solution:

  1. Designing the internal battery pack topology to allow for monitoring of each cell.

  2. Including a mechanism for the BMS to balance the cells.

There are three main topologies for cells in a battery pack:

Battery_topologies.png
Battery Pack Topologies

The simplest topology that allows for individual cell monitoring is a single string of series-connected battery cells. A more complicated topology is necessary when the capacity of a battery pack needs to increase but the overall voltage needs to remain the same. This requires a parallel connection of battery cells. If the parallel connection occurs at the per-cell level, an approximation is made that each cell in the parallel string contributes equally to the overall health of the parallel string. This approximation has the undesirable consequence of reducing the accuracy of the BMS.  A parallel connection at the total pack voltage is better, as it allows the BMS to read the state of each individual cell. It also opens the door for limp-home modes of operation. (More on that later!)

Just as there are multiple cell topologies, there are also multiple mechanisms for cell balancing. Cell balancing brings each cell into alignment and ensures that each cell within the battery pack has equal state of charge (SoC). The Society of Automotive Engineers released a whitepaper in 2001 that describes different cell balancing solutions shown below:

Cell_Balancing_Methods-1024x209.png

Cell balancing methods (source)

The simplest method to modify the level of charge of a battery cell is to apply a load to that specific cell. This will lower the level of charge of highly charged cells to match the average level of charge. This balancing method will provide the ideal BMS with a mechanism to ensure that the battery pack can be fully charged. However, it doesn’t necessarily allow for the battery to be completely discharged in a useful way. An active cell balancing method such as charge shuttling using a switched capacitor or energy conversion using a transformer allow cells nearing their end of charge level to be charged with excess energy contained in other cells still in their range of operation. Refer to Nuvation’s recent article about cell balancing for further detail.

Thus, to overcome the two previously mentioned BMS design challenges, it’s necessary to:

  1. Use a series-connected cell topology or use parallel-connected strings of series-connected cells to ensure accurate cell monitoring.

  2. Implement an active balancing method to allow increasing and decreasing SoC of each cell to ensure balance is achieved and maintained from completely full to completely empty.