Series grouping (n cells, each emf epsilon, internal resistance r): Total emf = nepsilon. Total internal R = nr. I = n$\frac{epsilon}{R + nr}$. Best when R >> r (external resistance dominates). Parallel grouping (n identical cells): Total emf = epsilon. Total internal R = $\frac{r}{n}$. I = $\frac{epsilon}{R + r/n}$. Best when R << r. Mixed grouping (m rows, n cells per row): I = mn*$\frac{epsilon}{nR + mr}$. For maximum current: nR = mr, or n/m = $\frac{r}{R}$. This gives $I_{max}$ = (mn)^$\frac{1}{2}$ * epsilon / (2sqrt(rR)). The mixed grouping is tested when the problem specifies a fixed total number of cells.