Two cells of emfs $\varepsilon_1$ and $\varepsilon_2$ with internal resistances $r_1$ and $r_2$ are connected in series with the negative of one joined to the positive of the next. The equivalent emf and internal resistance are

1. For cells in series aiding, the same current passes through both. 2. The terminal potentials add: $\varepsilon_{eq} = \varepsilon_1 + \varepsilon_2$. 3. The internal resistances are in series, so $r_{eq} = r_1 + r_2$. 4. The parallel form (B) and the subtraction (D, which needs opposing cells) do not apply. _Source: NCERT Class 12 Physics Ch 3 "Current Electricity", p.15_