Two cells of emfs $\varepsilon_1$, $\varepsilon_2$ and internal resistances $r_1$, $r_2$ are connected in parallel (positives joined, negatives joined). The equivalent internal resistance is

1. For cells in parallel, the internal resistances combine like resistors in parallel. 2. The rule is $1/r_{eq} = 1/r_1 + 1/r_2$. 3. Solving gives $r_{eq} = r_1 r_2/(r_1 + r_2)$. 4. The sum $r_1 + r_2$ (A) is the series result, not parallel. _Source: NCERT Class 12 Physics Ch 3 "Current Electricity", p.16_