In how many ways can a combination of cells be done? Explain.

(N/A) Cells can be combined in three primary ways to achieve different voltage or current requirements:
$1$. Series Combination: Cells are connected end-to-end. The positive terminal of one cell is connected to the negative terminal of the next. The total electromotive force $(EMF)$ is the sum of individual EMFs $(E_{eq} = E_1 + E_2 + ... + E_n)$,and the total internal resistance is the sum of individual internal resistances $(r_{eq} = r_1 + r_2 + ... + r_n)$. This is used to increase the total voltage.
$2$. Parallel Combination: All positive terminals are connected together,and all negative terminals are connected together. The equivalent $EMF$ is given by $E_{eq} = \frac{\sum (E_i / r_i)}{\sum (1 / r_i)}$,and the equivalent internal resistance is $\frac{1}{r_{eq}} = \sum \frac{1}{r_i}$. This is used to increase the current capacity and reduce internal resistance.
$3$. Mixed Combination: $A$ combination where rows of cells in series are connected in parallel. This is used to obtain maximum power output in an external circuit.