We have studied the onset of dendritic growth in symmetrical lithium/polymer/lithium cells, at current densities ranging from 2 x 10(-2) to 3 x 10(-1) mA cm(-2). We observe that dendritic growth starts at a time t(cc) which follows a power law as a function of the current density, very close to Sand's law. However, in our experimental conditions no Sand behavior is expected. We attribute this surprising result to the existence and/or formation of local inhomogeneities on the surface of the electrodes. We discuss a preliminary model accounting for the observed behavior.