Battery Management Systems and Their Role in Safety and ......Things to look for in a BMS Provides...
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Battery Management Systems and Their Role in Safety and Reliability Part 2 presented by Chris Ewert from Ewert Energy Systems, a designer of BMS systems for large format lithium batteries
Transcript of Battery Management Systems and Their Role in Safety and ......Things to look for in a BMS Provides...
Battery Management Systems and Their Role in Safety and Reliability
Part 2 presented by Chris Ewert from Ewert Energy Systems, a designer of BMS systems for large format lithium batteries
Outline
What is a BMS?
Why use a BMS?
What can and can't a BMS do?
What is cell balancing
Common battery pack failure modes
Design tips
What is a BMS?
Battery Management Systems are used to monitor and protect lithium ion battery packs.
Monitors the voltage of each individual cell in series and protects the battery pack.
Often interfaces with other devices.
Primary Functions of a BMS
Over & Under Voltage Protection (Each Cell)
Thermal Protection
Over-current Protection
State of Charge Calculation
Cell Balancing
Fault detection
Secondary Functions of a BMS
Current limit calculations
Digital or analog interfaces with application
Cell Health Monitoring
Data logging and analysis
Additional calculations (range, cell resistance)
Peripherals such as fan management, isolation fault monitoring, etc.
Why use a BMS?
Lithium cells are very intolerant to over-charge or over-discharge! Each cell's voltage must be monitored continuously.
Maximizes cell lifespan.
Maintains balance for maximum capacity.
Application must often be aware of state of charge.
Advanced warning before most failures.
Why use a BMS?
Enforce maximum charge or discharge currents at various temperatures
(for example, many lithium cells cannot be charged safely below a certain temperature.)
What can a BMS do?
From: Why Lithium Ion Batteries Fail and What You Can Do about it – A Design and Manufacturing Perspective ; Vidyu Challa, DfR Solutions Webinar.
You can get a copy of the presentation by emailing Vidyu at [email protected]
What can't a BMS do?
Cannot protect against internal shorts or manufacturing flaws within cells.
Cannot extend the lifespan or capabilities of a cell beyond how it was originally manufactured.
Protect against wiring shorts or loose / bad connections.
Provide proper protection when improperly designed, integrated or installed.
Common Battery Failure Modes
Low capacity cell(s)
High resistance cell(s)
Cells developing high internal self discharge
Cell manufacturing defects
Other physical abuse (shock & vibration, thermal, impact, improper compression)
What is a low capacity cell?
Cell
Voltage
Amp Hours
Example of High Resistance CellC
ell
Vo
ltage
Time
What is cell balancing?
Cell balancing adds or removes energy from high or low cells
Balancing is performed typically at one location in the battery (high or low side)
Having balanced cells maximizes the usable capacity of a battery pack up to the capacity of the weakest cell.
Active balance can slightly increase usable range, but only for certain applications.
What is an out of balance cell?
Amp Hours
Cell
Voltage
Example of Out of Balance CellC
ell
Vo
ltage
Amp Hours
Common causes of battery failure
Low quality cells
Allowing over-charge
Allowing over-discharge
Use at elevated temperatures
Charging below minimum temperature
Using cell above specified max. currents (or prolonged use at max. currents)
Detecting Faults
Monitoring cell internal resistance
Monitoring cell capacity
Cell temperature
Isolation fault monitoring (for large packs)
BMS should monitor it's own sensors for faults
Application Design Tips
BMS Must be able to stop all charge and all discharge current if it determines that is necessary
Ensure equal current is pulled from each cell. Do not tap the battery pack at different locations for different voltages.
Ensure sufficient electrical isolation to prevent shorts.
Always have backup shutoffs for safety.
Battery Pack Design Tips
Select the best cell for the particular needs
Prevent breaks in stack (fuses / disconnects)
Busbars
EMI considerations
Keep all cells the same temperature
Thermal management for shorts
Design the battery pack with extra margins!
Test The Application!
Always test the application.
Battery pack cycling is recommended. Simulate as many characteristics as possible including temperature and C-rates.
Cell datasheets sometimes overstate capabilities of cells.
Parallel Strings
Avoid whenever possible
Configuration not recommended when it can be avoided!
Additional safety systems needed for this configuration
Standard Cell Configuration
Parallel Cells
Things to look for in a BMS
Provides all basic cell protections including thermal protections
Ensure the BMS can detect faults with the battery and with itself
Accuracy, Speed and Reliability of BMS
Doesn't drain cells while in standby
Handles failures safely (study the failure modes)
Will the BMS identify which cell is defective
EMI considerations
Compatibility with your cell voltage and amperage
Features for your specific application
Things to look for in a BMS
Provides all basic cell protections including thermal protections
Ensure the BMS can detect faults with the battery and with itself
Accuracy, Speed and Reliability of BMS
Doesn't drain cells while in standby
Handles failures safely (study the failure modes)
Will the BMS identify which cell is defective
EMI considerations
Compatibility with your cell voltage and amperage
Features for your specific application
Questions?
Chris Ewert (Part 2)
Ewert Energy Systems
(630) 868-3173
www.orionbms.com
Vidyu Challa (Part 1)
(301) 640-5809
DfR Sales Contact:
Mark Musitano
(301) 640-5809
www.dfrsolutions.com
