Battery Management Systems for Electric Vehicles.ppt

Battery Management Systems for Electric Vehicles
Comparison:Lead vs. Lithium in EVs
Charging
Lead-acid batteries charges well in a long string
Over voltage in a cell is not good, but generally passes the current to the next cell in an equalization cycle with little damage.
Cell balancing can be done with a sophisticated charger (IUIa cycle)
Lithium batteries OK in a string, but over voltage on a individual cell can do serious cell damage.
Individual cell charging is solution, or
Balancing cells and charge in a string.
Discharging
Lead can tolerate discharging to 0% State of charge (SOC) with some cycle life damage.
Lithium will have serious damage when discharging below , can pletely ruined.
Lead-Acid Discharge Curve
teryMaintenance/
6V Lead Acid Battery Discharge Curve
Battery
Voltage
State of Charge
Lithium Discharge Curves
Lithium Batteries have a fairly flat discharge curve with sharp shoulders
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Lithium BMS Challenges
Must not Over-Charge an individual cell
Must not Over-Discharge an individual cell
Must not let cells get too hot during charge or discharge
ENTER THE LITHIUM BMS
Many thoughts and discussions on what constitutes a Battery Management System (BMS):
Monitor and Detect Cell Over-Charge, and cut off charger
Monitor and Detect Cell Over-discharge and alert operator, or cut off system power.
Cell Balance for string charging
Temperature Monitoring
Remaining State of Charge determination
This is done in your cell phone & laptop, why not in your car?
High voltages and high currents make it difficult
Sparse BMS technology availability has held up Lithium conversion projects.
BMS Topology: Distributed
Put voltage monitor and discharge balancer on each cell, with munications for charger cutoff and status.
Advantages: Simpler design and construction and its potential for higher reliability in an automotive environment.
 
Disadvantages: Large number of mini-slave printed circuit boards which are needed a