EMF (epsilon) is the work done per unit charge by non-electrostatic forces inside the battery: epsilon = W/q. It equals the terminal voltage when no current flows (open circuit). With current I flowing: V_terminal = epsilon - Ir (discharging). During charging: V_terminal = epsilon + Ir (terminal voltage exceeds emf). Short circuit: R_ext = 0, I_max = epsilon/r (maximum current, dangerous). For cells in series: total emf adds, total internal resistance adds — epsilon_eq = epsilon_1 + epsilon_2, r_eq = r1 + r2. For identical cells in parallel: epsilon_eq = epsilon (same), r_eq = r/n (reduced). Mixed grouping (m rows of n cells each): I = mn*epsilon/(nR + mr), maximized when nR = mr (external R = total internal r).