In charging a battery of a motor car,which of the following effects of electric current is used?

Two batteries with different $emf$ and different internal resistances are connected as shown in the figure. The voltage between the two terminals $A$ and $B$ is ........ $volt$.

Two identical cells each of emf $1.5 \,V$ are connected in parallel across an external resistance formed by two $20 \,\Omega$ resistors connected in parallel. $A$ voltmeter connected in the circuit measures $1.2 \,V$. The internal resistance of each cell is ................. $\Omega$.

Two identical cells each of emf $E$ and internal resistance $r$ are connected in parallel with an external resistance $R$. To get maximum power developed across $R$, the value of $R$ is

$A$ galvanometer is connected in series with an unknown resistor $R$ and two identical batteries of $1.5\, V$ each. When the batteries are connected in series,the galvanometer shows a current of $1\, A$. When the batteries are connected in parallel,it shows a current of $0.6\, A$. The internal resistance of each battery is:

$n$ identical cells are joined in series,with two cells $A$ and $B$ in the loop having reversed polarities. The $EMF$ of each cell is $E$ and the internal resistance is $r$. The potential difference across cell $A$ or $B$ is (where $n > 4$):