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NREL/GM Project Finds Ultracaps Perform as Well or Better Than NiMH Pack for Energy Storage in Mild Hybrid Vehicle

NREL simulation results showed similar fuel consumption vs. energy window trends for various drive cycles, with approximately half of the fuel savings being provided by a window ≤ 50Wh. Source: Gonder et al. Click to enlarge.

A National Renewable Energy Laboratory (NREL) project, funded by GM, found that an ultracapacitor-based energy storage system (ESS) in a Saturn VUE BAS (belt alternator starter) mild hybrid performed at least as well or better than the NiMH battery system in the production hybrid. Jeffrey Gonder, a senior engineer at NREL, presented the results at the SAE 2010 Hybrid Vehicle Technologies Symposium in San Diego.

The project grew out of analysis NREL performed for the United States Advanced Battery Consortium (USABC) suggesting that a hybrid vehicle can achieve significant fuel savings with a high-power, but low-energy on-board storage system. Approximately half of fuel savings on various drive cycles were achieved with an “energy window”—the in-use min-max energy required for a particular cycle—of ≤ 50Wh.

Significant fuel savings could be achieved with a 150 Wh high power ESS, the earlier study found, with with fuel savings tapering off at energy windows > 200 Wh.

Reasons for the large total nominal energy in present production HEVs, the study concluded, included:

  • Infrequent drive cycle use (e.g., long up/downhill grades)
  • Achieving longer cycle life from reduced SOC swings
  • Energy comes along with sizing for power requirements (particularly at cold temperatures)
  • Power dominates cost in HEV (high P/E ratio) batteries

That earlier study concluded that ultracapacitors should be considered as a low-energy, high-power ESS and can bring potential benefits such as long cycle and calendar life; good temperature performance; and low long-term projected costs.

For the Saturn VUE study, NREL selected off-the-shelf Maxwell 48 V, 165 F modules (each with around 35 Wh usable). NREL tested single and dual (in parallel) module configurations, providing a lower (35 Wh) and upper (70 Wh) boundary to the 50 Wh window.

Vehicle testing took place using the stock as well as the one- and two-ultracapacitor module configurations, both on-road and in a climate-controlled chassis dynamometer test cell. The ultracap modules were paired with a spare Energy Storage Control Module (ESCM); the stock NiMH pack remained in the vehicle, and the testers could toggle between it and the ultracaps.

The Saturn VUE BAS hybrid has a 25% higher window-sticker fuel economy than its conventional counterpart; all three configurations showed comparable results on the FTP, Highway and US06 cycles.

Dyno testing, FTP Cycle. Click to enlarge.   Dyno testing, Highway and US06 Cycles. Click to enlarge.

Areas for future exploration, Gonder said, could include support for a larger motor and/or a smaller ultracap module; study of more cycles; and use of a different vehicle platform.


  • Gonder, J.; Pesaran, A.; Lustbader, J.; Tataria, H. (2009). Fuel Economy and Performance of Mild Hybrids with Ultracapacitors: Simulations and Vehicle Test Results (Presentation).



An ultracap buffer would allow a smaller battery pack to provide the same performance as a larger one and reduce the duty cycle on the more battery.

Roger Pham

All well and good for a mild BAS hybrid.

But, for a full hybrid, considerably higher energy buffer will be needed because full hybrid uses Atkinson-cycle engine which offers significantly less power than a conventional Otto-cycle engine. This requires significantly higher electric motor boost at high acceleration in order to perform on par with other non-hybrid vehicles.
A full hybrid uses its battery energy extensively for powering the AC during stop during the summer, thus saving gasoline that otherwise would be used wastefully to run the engine to power the AC compressor.

50 Wh capacity is not enough for full braking energy recuperation at highway speed or down an incline in a 3000-lb car. It will take at least 150-200 Wh capacity for that purpose.


So a 200 Wh Ultracap module (50 Wh per wheel if running 4 wheel motors) would be enough to reduce the cycling load on the propulsion battery pack as it would avoid regen charge reaching the batteries at all?


I think the Ultracap module could of course, reduce the cycling load on the propulsion battery pack and, if the regen was not too big, could avoid regen charge reaching the batteries at all (but why).

The tradeoff might favor smaller Ultracap capacity to simply REDUCE max current into the batteries since the motor/generator is only so big.

I think tradeoffs, so far, favor putting the money into more batteries rather than caps AND electronics that can handle the wide variations in voltage.


I agree that is works for the BAS. If you notice, the Malibu hybrid is not offered in 2010 because of battery problems from Cobasys. So if the can use ultracaps, this may solve the problem.

Henry Gibson

Driving slower on motorways and using a smaller engine will get even better results at very low cost. ..HG..

Henry Gibson

Use EFPOWER bipolar cheap lead acid batteries for hybrids or a CSIRO version of the same for low cost and high performance. ..HG..


Caps should be able to capture regenerative braking energy better than a small pack of batteries. So, I would say the BAS would be a bit better with caps.

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