Modem applications, such as wireless communication systems or hybrid electric vehicles operate at high power fluctuations. For some applications, where the power frequencies are high (above some 10 or 100 Hz) it is possible to filter the high frequencies using passive components; yet this results in additional costs. In other applications, where the dynamic time constants are in the range up to some seconds, filtering cannot be done. Batteries are hence operated with the dynamic loads. But what happens under these dynamic operation conditions? This paper describes the fundamentals of the dynamic characteristics of batteries in a frequency range from some MHz down to the mHz range. As the dynamic behaviour depends on the actual state of charge (SOC) and the state of health (SOH), it is possible to gain information on the battery state by analysing the dynamic behaviour. High dynamic loads can influence the battery temperature, the battery performance and the battery lifetime. (c) 2005 Elsevier B.V. All rights reserved.