Learn the about the latest in lithium-ion battery technology.



Erik Stafl, president of Stafl Systems, explains two primary purposes of a battery management system for a lithium-ion battery pack: providing safe and reliable operation. The Battery Management System takes a number of inputs, including voltage, temperatures, and current among other inputs. The BMS then runs algorithms to create a series of outputs including state of charge (the fuel gauge), the state of health, the safe operating envelope ™, and any faults the battery pack may have.

Erik then describes how the BMS is used in a multi-cell application, using a system diagram.

Topics Covered

  • Purpose of a BMS
  • BMS Inputs
  • BMS Outputs
  • Current Sensors
  • BMS System Diagram
  • BMS Communication



Erik discusses BMS safety and fault management. He looks at the conditions the BMS monitors, using a system diagram to understand cell voltage reporting. Using a chart with voltage and temperature, Erik explains the safe operating area for the battery pack cells. The BMS monitors the minimum safe temperature, maximum safe operating temperature, minimum safe voltage and maximum safe voltage, depending on chemistry. The BMS will work to keep battery cells within the safe operating area and report any faults that threaten overtemperature or overcharge, which can lead to thermal runaway. The BMS will also prevent overdischarge or under temperature that can also threatened the battery cell.

The BMS manages the cell performance and communicates this information to the external application. The BMS can also disconnect the from the external application to protect the battery pack to prevent unsafe operation, triggering a fault.

Topics Covered

  • BMS safety
  • Safe Operating Area
  • Over Voltage Fault (OV Fault)
  • Over Temperature Fault (OT Fault)
  • Under Voltage (UV Fault)
  • Under Temperature (UT Fault)
  • Dendritic Growth
  • Thermal Runaway
  • Fault Reporting
  • BMS Disconnect



The state of charge is defined as the capacity remaining divided by the total capacity of the battery pack. Erik provides an example of how the state of charge is calculated. Using a discharge curve, he explains that this curve is not linear. Given this, the true state of charge is often different from the energy remaining. Looking at State of Charge (capacity) and the State of Charge (energy) we can understand the differences between these values and how to calculate the most relevant information.

Coulomb counting is discussed next. Coulomb counting integrates amp hours and time to calculate the capacity removed from the pack. This provides a basis for SOC. The current sensor has drift and measurement error, so it is also important to have an open cell voltage lookup.

Topics Covered

  • State of Charge
  • State of Charge (Capacity) (SOCcapacity or SOCc)
  • State of Charge (Energy) (SOCenergy or SOCe)
  • Fuel Gauge
  • Coulomb Counting
  • Depth of Discharge
  • Open Cell Voltage Lookup



State of health, or SOH, is commonly defined as the total capacity of the battery pack today divided by the total beginning of life capacity, given in units of Amp Hours. Erik explains how this is calculated by the Battery Management System and why understanding state of health is important for a system. State of health explains the degradation of the battery over a number of discharge cycles.

Erik goes on to look at State of Health impedance as well as the more common State of Health capacity. Over time, the ESR, or impedance, of the cell will grow. This increasing impedance decreases the state of health. A battery management system looks at both of these calculations to understand how a battery pack ages. This changes how the range of a battery pack is calculate for an electric vehicle.

Topics Covered

  • State of Health
  • Discharge Cycles
  • State of Health Capacity
  • State of Health Impedance
  • ESR
  • Estimated Range Remaining
  • Range Calculations
  • Battery Capacity Fade Effect



How does a battery pack report the available maximum charge and discharge currents to a system? The Safe Operating Envelope is a BMS calculation developed by Stafl Systems. In this video, Erik Stafl from Stafl Systems explains how this calculation works and what it means when calculating the capability of a battery pack.

Topics Covered

  • Safe Operating Envelope
  • Discharge Cycles
  • Maximum Charge
  • Maximum Discharge
  • Battery Management System


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