Engineer shares how to build an electric vehicle from the ground up: Lead-acid vs Lithium-ion batteries

-July 06, 2011

This article is a continuation of "Engineer shares how to build an electric vehicle from the ground up: Design choices."

Lead-acid vs lithium-ion batteries

The original batteries were lead acid, weighing in at 800 lbs. However, over the three-year span of the project, the cost of Li-ion batteries dropped in half and their cycle life doubled.  The new 350-lb Li-ion batteries are from Thunder Sky. Thirty 200AH cells form a 100V, 200Ah pack (20 kWh), which gives around 90 miles range!

Santini does not have a battery balancing circuit right now, but the cells stay in balance pretty well when new. Stay tuned for the next steps for further improvements. See Figure 5.

 

20kWh Li-ion batteries
Figure 5: New 20kWh Li-ion batteries. Each cell now has a wire running to a data logger, so all the cells can be monitored during discharge as well as charging.


Lead-acid batteries pros and cons

  • One of the lowest energy-to-weight and energy-to-volume battery designs in existence, making them very big and heavy for the total amount of power they can put out.
  • Very high surge-to-weight ratio, meaning they can deliver a big jolt of electricity all at once. This makes them perfect for applications that need a big, sudden surge of power, such as car starters.
  • Inexpensive to produce.
  • Not very good in roles that require a steady, low, or middling supply of electricity over a long period of time.
  • Long recharging times.


Lithium-ion batteries pros and cons

  • High power-to-weight and power-to-volume ratio, so are light and small as compared to lead-acid.
  • Special types have high surge capacity, but those are more expensive. Regardless, an EV does not need "cranking" power to start the car like a gas engine. The AC motor only draws higher currents during acceleration.
  • Do not have frequent maintenance needs like lead-acid.

See Figure 6 for a summary of Li-ion's advantages.

  Lithium-ion battery advantages---lighter, smaller, high energy density
Figure 6: Lithium-ion battery advantages---lighter, smaller, high energy density. (Courtesy of Micro Power Electronics)

A lead-acid discharge curve is shown in Figure 7 and a lithium ion in Figure 8.


6V lead-acid battery discharge curve
Figure 7: 6V lead-acid battery discharge curve. The X axis shows state of charge, Y axis shows batter voltage. (Courtesy of Trojan battery company)

 
Lithium batteries have a fairly flat discharge curve with sharp shoulders.
  Figure 8: Lithium batteries have a fairly flat discharge curve with sharp shoulders. State of charge (X-axis) vs battery voltage (Y-axis). (Courtesy of  EnerDel)

Continue reading: How it drives

 

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Steve TaranovichAbout the Author

Stephen Taranovich has 40 years of experience in the electronics industry. He received his MSEE from Polytechnic University, Brooklyn, NY, and his BEEE from New York University, Bronx, NY. Steve is also chairman of the Educational Activities Committee for IEEE Long Island.


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