Stafl Systems
Insights Video Series

Learn about the latest in lithium-ion battery technology.

Erik Stafl, founder of Stafl Systems, walks you through the latest innovations in Battery Management System technology in this series of detailed, easy to understand videos.

Table of Contents

 

Series 1
Series 2
1a.

Introduction to
Battery Management Systems

 

Summary

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
    • 1b.

    BMS SAFETY AND FAULT MANAGEMENT

     

    Summary

    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
    • 1c.

    STATE OF CHARGE

     

    Summary

    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
    • 1d.

    STATE OF health

     

    Summary

    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
    • 1e.

    SAFE OPERATING ENVELOPE

     

    Summary

    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.

    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

  • Safe Operating Envelope
  • Discharge Cycles
  • Maximum Charge
  • Maximum Discharge
  • Battery Management System
    • 2a.

    Lithium-Ion Battery Cell Selection for Battery Packs

     

    Summary

    In this video, Erik Stafl, President of Stafl Systems, discusses the different lithium ion battery cell types and form factors. Three primary form factors are common in the industry today: cylindrical cells, pouch cells, and prismatic cells. This video discusses 18650 and 21700 cells and the benefits and downsides of using a commodity cell, including thermal performance and heat distribution. Pouch cells are thin pouches with anode and cathode layers stacked inside the cell and can be easier to incorporate thermal rejection methods. Erik then discusses the differences in cycle life with the different cell types.

    This video discusses and defines key figures of merit, including specific energy (Wh/kg), energy density (Wh/L), power capability (W/kg or W/L), and rate capability plots (voltage and depth of discharge).

    Topics Covered

  • Lithium-Ion Battery Cell Types
  • Cylindrical Cells
  • Pouch Cells
  • Prismatic Cells
  • Cell Sizes - 18650 and 21700
  • Thermal Performance
  • Specific Energy, Energy Density and Power Capacity
    • 2b.

    Lithium-Ion Battery Cell Chemistries:

    Understanding the Differences in Cell Selection

     

    Summary

    In this video, we discuss the different lithium-ion battery cell chemistries, including NMC, NCA, LCO (high capacity, high energy); LFP; and LTO. Erik Stafl draws a chart looking at cell voltages and depth of discharge to examine the differences in cell performance. NMC cells generally have a more defined voltage curve than an LFP cell, making it more difficult to estimate the state of charge on an LFP cell. LTO cells are generally lower voltage.

    This video explores the trade offs between the cell chemistries, comparing energy density and cycle life, and where each chemistry may be the best fit. The safety difference is also important, and is inversely correlated to the energy density of the cell.

    Temperature is the final topic discussed in this video, examining the temperature ranges at which each cell chemistry performs best.

    Topics Covered

  • NMC Battery Cells
  • NCA Battery Cells
  • LCO Battery Cells
  • LFP Battery Cells
  • LTO Battery Cells
  • Energy Density
  • Battery Safety
  • Cell Thermal Performance
    • 2c.

    Lithium-Ion Battery Pack Architecture

     

    Summary

    In this video Erik Stafl, President of Stafl Systems, discusses the architecture of lithium-ion battery packs, starting with the primary factors to consider. He then shows examples of series and parallel configurations. When cells are in series, the voltages are added and the capacity stays the same. When cells are added in parallel, the voltage remains the same and the capacity of the cells are added together. This is an important concept: series cells add voltage, parallel cells add capacity.

    Using a real-world example, Erik shows a basic battery pack configuration with a series and parallel pack targeting both a voltage and energy capacity requirements.

    Topics Covered

  • Energy Storage (Wh, kWh)
  • Voltage Range (48V, 350V and 700V+ are some examples)
  • Size (Physical Size)
  • Weight
  • Power Levels (C-Rate)
  • Series Cells
  • Parallel Cells
    • 2d.

    Disconnect Switches for Lithium-Ion Battery Packs

     

    Summary

    Every battery pack consists of a group of cells, a battery management system (BMS) and a disconnect switch. What is the purpose of the disconnect switch? How do they work, and what types of disconnect switches are best for a given pack? Where do these go in the system? Understanding these fundamental disconnect methods are critical for battery pack safety.

    In this video, Erik Stafl, President of Stafl Systems, reviews this key part of the battery pack design. When a BMS detects an unsafe state it will open a switch to prevent current flow into the pack.

    There are two primary disconnect switch types: Electromechanical - Relay (contactor), and Solid state - NMOSFETs. The different switch types and architectures are described in detail, with a focus on designing safe systems. At the end, Erik discusses the manual service disconnect switch, which is used when servicing a battery pack. 
    2e.

    Pre-Charge Circuits for Lithium-Ion Battery Packs

     

    Summary

    Pre-charge circuits are an important safety and functional feature for high voltage battery packs. Why is this, and how do these circuits work? In this video, Erik Stafl, President of Stafl Systems, describes the use of pre-charge circuits in high-voltage battery system design.

    Pre-charge circuits are used because the power source connected to the battery has a high capacitance. If we did not have a pre-charge circuit, there would be a very large inrush current - upwards of thousands of amps - that can weld contactors shut. We want to avoid this inrush current. 
    More videos coming soon!

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    Proven, Reliable
    Systems Engineering

     

    At Stafl Systems, our systems engineers work on a variety of different powertrains and vehicles to maximize performance and reliability.