1. A cell monitoring apparatus comprising: a memory to store input state data representing initial parameters for estimating parameters of a rechargeable cell; a processor operably coupled to the memory and supporting an estimator unit to receive, when in use, the input state data; wherein the processor and the memory are arranged to execute code representing a linear time-invariant state transition model and a non-linear observation model modelling the rechargeable cell using at least a non-linear open circuit voltage, an internal resistance, a time-invariant distortion voltage across a reactive component block, and a distortion current component constituting an error in respect of measurement of current flowing through the reactive component block; the estimator unit for performing extended Kalman filtering in respect of the state transition model and the observation model using the input state data in order to generate output state data; and the processor is arranged to evaluate a criterion associated with at least part of the output state data and to generate, when in use, a control signal in response to evaluation of the criterion.