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.