The performance of a standard, production VRLA cell has been improved by paralleling with combinations of supercapacitors. The supercapacitors act to lower the source impedance of the combination providing a much-enhanced performance. Transient voltage swings are suppressed and the rms voltage is supported. Low-temperature performance may also be improved due to the characteristics of the supercapacitor. These enhancements are described for various combinations of cell and supercapacitor under realistic hybrid vehicle power demands. A simple model is proposed and used to demonstrate supercapacitor size requirements for optimal current in the parallel cell. (c) 2005 Elsevier B.V. All rights reserved.