In this article, a nonlinear finite time observer is designed for multicellular converters. The aim is to estimate the capacitor voltages by taking into account the hybrid behaviour of the converter. This article extends the validity of the strong Lyapunov function, proposed in Moreno and Osorio (Moreno, J., and Osorio, M. (2008), 'A Lyapunov Approach to Second Order Sliding Mode Controllers and Observers', in Proceedings of the IEEE Conference on Decision and Control, New Orleans, USA, pp. 2856-2861), in order to deeply study the reaching time estimation and robustness of the homogeneous finite time observer given in Perruquetti et al. (Perruquetti, W., Floquet, T., and Moulay, E. (2008), 'Finite Time Observers: Application to Secure Communication', IEEE Transactions on Automatic Control, 53, 356-360). The proposed approach enables the stabilisation of the observation errors in spite of the presence of perturbations and uncertainties. Some simulations and comparisons with the super-twisting sliding mode observer highlight the efficiency of the proposed strategy.