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Battery/Ultracapacitor System for Small Electric Vehicles

Configuration of an EV with battery pack and ultracapacitor. From Li et al.(2009) Click to enlarge.

Researchers at the Illinois Institute of Technology and Allborg University (Denmark) are studying a hybrid battery/ultracapacitor system for small electric vehicles. They presented a paper on their work at the recent SAE 2009 World Congress in Detroit.

In the system, the batteries function as the main energy storage source of the vehicle, supplying average power to the load. The ultracapacitors are used to meet the peak power demands during transients. In their study, the researchers connected the battery pack and ultracapacitor bank in parallel to the DC link with bi-directional two quadrant buck-boost converters. To control the system, they propose a power flow management methodology based on load demand.

The vehicle target was a small electric vehicle (similar to a neighborhood elecric vehicle, NEV) with a speed limit of 40-50 km/h (25-31 mph), maximum power of 40 kW, and a weight of about 800 kg. The battery pack comprised two parallel strings of 14 series-connected 12 V NiMH batteries (Saft NHE 10-100). The ultracapacitor was a Maxwell BMOD0063 module with a nominal voltage of 125V.

The battery pack had a minimum allowable state of charge of 20%. The minimum allowable state of charge of the ultracapacitor was 25%.

In this topology, the stress and oscillations of the battery current is reduced which results in the size and cost reduction of the battery and increases the lifetime. Presented results show that the power management based on hybrid system reduces the stress in battery current without sacrificing the performance.

The desired performance is achieved by the power flow control of two sources in acceleration and regenerative braking modes while providing a reduced size of battery with longer life time and less subjection to stress and oscillations.Li et al. (2009)


  • Zhihao Li, Omer Onar, Alireza Khaligh, Erik Schaltz (2009) Design, Control, and Power Management of a Battery/Ultra-Capacitor Hybrid System for Small Electric vehicles (SAE 2009-01-1387)



How about a couple of low power DC motors to drive the front wheels direct from the supercapacitors, with batteries/inverter and a larger AC motor to drive the rear axle?

To get the energy from the wheels -> motor -> inverter -> DC converter -> batteries then back again might lose you 1/4 to 1/3 of your braking energy.

If your only going from wheels -> motor -> caps and back again, you should get much better round trip efficiency.

Supercaps will be very useful to buffer the batteries during fast charging


I like this battery/cap solution. Sounds similar to CSIRO's UltraBattery:


A sophisticated costly system.

As the costs of high power DC/AC/DC converters and super caps come down this can become a good mix. Reduced battery and/or ICE size.

But the money and weight for the extra parts (super cap, Converter) can presently be just put into batteries for longer range.

It's a tradoff.


Can power be pushed from the battery to the super cap when the super cap state of charge is low and the vehicle velocity is low?

Can power be pushed from the supercap to the battery when the super cap state of charge is high, and the vehicle velocity is high?

Would this be worth the weight/complexity/cost?

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