Experimental evidence is given that the n.t product rule for prediction of aerosol charge distribution equilibrium behaviour in a neutraliser fails at high particle concentration or low particle size. Charging equations are solved numerically to map equilibrium behaviour as a function of two non-dimensional groups, the non-dimensional ion concentration, and the non-dimensional neutraliser residence time. A 3D model then captures the physics of unsteady particle charging inside a neutraliser, and experimental results match predictions from the non-dimensional model. Finally, a new tool supplementing the n.t product rule is proposed for practical use.