The nickel metal hydride (NiMH) batteries used in most hybrid electric vehicles (HEVs) provide satisfactory performance, but are quite expensive. In spite of their lower energy density, lead acid batteries would be much more economical except they are prone to sulfation in HEV applications. However, sulfation can be greatly reduced by a circuit that uses an ultracapacitor in conjunction with the battery. The resulting system will provide much cheaper energy storage if ultracapacitor prices can be reduced to levels predicted by some manufacturers. (c) 2005 Elsevier B.V. All rights reserved.