EMF (epsilon) is the work done per unit charge by non-electrostatic forces inside the battery: epsilon = $\frac{W}{q}$. 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, $I_{max}$ = $\frac{epsilon}{r}$ (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), $r_{eq}$ = $\frac{r}{n}$ (reduced). Mixed grouping (m rows of n cells each): I = mn*$\frac{epsilon}{nR + mr}$, maximized when nR = mr (external R = total internal r).