The prediction of the state of charge of batteries is a classic topic of considerable interest. The present work aims to predict the state of charge of lithium ion cells using a simple diffusion model. The extent of lithium ion diffusion into the cathode is given as a measure of the state of charge. Similarly an attempt has been made to correlate the impedance response of the cell to its state of charge. Coin cells were assembled. Charge-discharge studies and impedance measurements at various states of charge were carried out and the results are compared with the values predicted theoretically. (C) 2004 Elsevier B.V. All rights reserved.