In this paper a survey is given of the pros and cons of the existing models for the lead-acid battery. Next, a diffusion model is proposed which explains inter alia the occurrence of sharp lead sulfate walls observed earlier by Haebler et al. The model also leads to an expression similar to Peukert's law. Finally, in order to account for secondary effects in the lead sulfate distribution the model has been extended to include a second internal diffusion mechanism.