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The study of negative-ion production and its optimization is of fundamental importance in the thermonuclear research field. Indeed, negative-ion beams are favorites for the generation of neutral particle beams used in fusion plasma heating because of their higher neutralization efficiency with respect to positive ions. This paper concerns the development of a 0-D model for multicusp negative-ion sources. The state-to-state approach used to describe heavy-particle kinetics has been applied to electrons; the Boltzmann equation is replaced by a set of master equations similar to those describing the heavy-particle kinetics. This approach allows solving in more self-consistent way, as a whole differential set, heavy particle and electron kinetics. The model has been improved with respect to our previous one by introducing the calculation of effective surface to describe the ion-wall loss. Comparisons between our theoretical results and experimental data are presented for different discharge conditions pointing out the effect of some parameters on the negative-ion kinetics. Finally, results for a mixed discharge H2-Cs are reported.