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As presented to . EVAOSD. July 2009 . Battery Management Systems for Electric Vehicles. Comparison: Lead vs. Lithium in EVs. Charging Lead-acid batteries charges well in a long string

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As presented to


July 2009

comparison lead vs lithium in evs
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 2.0V, can be completely ruined.
lead acid discharge curve
Lead-Acid Discharge Curve

6V Lead Acid Battery Discharge Curve



State of Charge

lithium discharge curves
Lithium Discharge Curves
  • Lithium Batteries have a fairly flat discharge curve with sharp shoulders

lithium bms challenges
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
BMS Topology: Distributed
  • Put voltage monitor and discharge balancer on each cell, with digital communications 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 and the difficulty of mounting them on some cell types.

bms topology modular
BMS Topology: Modular
  • Several Slave controllers consolidate data to a master

Advantages: Does not need printed circuit boards connected to individual cells.

Disadvantages: Master-Slave isolated communications can be challenging in an EV.

bms topology centralized
Centralized Master Control UnitBMS Topology: Centralized

Advantages: Single installation point. No complex inter-vehicle communications

Disadvantages: Typical EV batteries are distributed in the vehicle, requiring

wiring to a central location.

Single source for balancer heat generation.

li ion bms market options
Li-Ion BMS Market options
  • Investigate BMS solution for highway capable EV conversion
  • Needs to support typical DC system:
    • 160 AH prismatic LiFeP04 (3.2V),
    • 250A + systems
    • 40-48 cells (128 to 153 volts)
    • Must monitor
    • Should manage, report and balance
bms honorable mention
BMS Honorable Mention
  • Lithium Balance – No published specs or pricing
  • Gary Goodrum – DIY BMS Ckt, 24 cell on Endless Sphere, Low current device for bikes
  • Metric Mind – Custom BMS, no pricing for BMS products
  • Boundless – creates custom battery packs.
  • Hot Juice Electric BEQ – Balance only
  • Manzanita Micro – Partial solution, 4 cells for $250
  • Open Source BMS projects – no resolutions
small print
Small Print:
  • Company: A few other companies are getting ready to offer Li-Ion BMSs, but are not yet ready to be listed here.
  • Class:• Simple: analog technology, just able to detect that some cell's voltage is too low or too high• Fancy: sophisticated digital technology, able to measure and report every cell voltage, and to calculate SOC
  • Topology: See previous slides
  • Number of cells: this is the acceptable range in the number of cells in series. The number of cells in parallel does not matter.
  • Balance: The BMS is able to remove energy just from the most charged cells, to allow the other cells to reach the same level of charge.
  • Temperature: The BMS is able to measure and report individual cells' temperature.
  • Current sense: The BMS includes a current sensor or at least an input for a current sensor, to measure battery current. This enables the BMS to react to excessive current, and to calculate the SOS or DOD.
  • "Fuel gauge": a.k.a.: "Gas Gauge". The BMS calculates the SOC (State Of Charge) or DOD (Depth Of Discharge), by integrating the battery current.
  • Communications: • Wire: separate wires are used, each with a single, specific function, such as to turn on the charger relay.• CAN: CAN bus, common in vehicles and European industrial equipment.• RS232: serial point-to-point communication, usually used only for initial set-up and testing, but some time also available for communication during operation.
  • Case: Whether the BMS controller is enclosed (metal or plastic case), or it is an open PCB assembly. Unless otherwise noted, any cell-mounted boards are assumed to be open PCB assemblies.
  • Price: from manufacturers' websites or discussion with their clients.
hardy ev flex bms


Hardy EV Flex BMS
  • Centralized BMS Architecture
  • Miniature In car display and operator alerts
  • Battery monitoring for over-voltage, under voltage
  • 3 versions in production
    • Up to 36 cells - For NEVs and small EVs
    • Up to 48 cells – For DC systems
    • Up to 84 cells – Prius plug-in conversions and AC systems
  • Temperature monitoring
  • Adjustable voltage and temperature thresholds
  • Cell balancing with built-in thermal management
  • Full diagnostic self test identifies faulty wiring
  • Internal Log allows identification of problem batteries
  • USB Log Option for detailed cell monitoring logs
  • Current monitor option for state of charge determination
  • Works with charger up to AC: 25A 240V
  • Priced for EV conversions: $891 for 48 cell system
    • Data logger option $50
    • Current Monitor option $60
contact information
Contact Information

Mark Hardy

Hardy EV, LLC