The monodomain equations model the propagation of the action potential in the human heart: a very sharp pulse propagating at a high speed, which computation require fine unstructured 3D meshes. It is a non linear parabolic PDE of reaction diffusion type, coupled to one or several ODE, with multiple time-scales. Numerical difficulties, such as unstructured meshes and stability are addressed here through the use of a finite volume method. Stability conditions are given for two time-stepping methods, and two example sets of ODEs, convergence is proved and error estimates are computed.