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

$A$ battery consists of a variable number '$n$' of identical cells,each having an electromotive force '$E$' and internal resistance '$r$',connected in series. The terminals of the battery are short-circuited,and the current '$i$' is measured. Which of the graphs below shows the relationship between '$i$' and '$n$?'

Three identical cells,each of emf $10 \ V$ and internal resistance $3 \ \Omega$,are connected in parallel across terminals $A$ and $B$. The net emf across $A$ and $B$ is .......... $V$.

Two batteries with different $e.m.f.$ are connected in parallel. Consider the following statements: $(i)$ The equivalent $e.m.f.$ is less than the individual $e.m.f.s$. $(ii)$ The equivalent internal resistance is less than the individual internal resistances.

Twelve cells,each having emf $E$ volts,are connected in series and are kept in a closed box. Some of these cells are wrongly connected with positive and negative terminals reversed. This $12$-cell battery is connected in series with an ammeter,an external resistance $R$ ohms,and a two-cell battery (two cells of the same type used earlier,connected perfectly in series). The current in the circuit when the $12$-cell battery and $2$-cell battery aid each other is $3 \text{ A}$,and it is $2 \text{ A}$ when they oppose each other. Then,the number of cells in the $12$-cell battery that are connected wrongly is:

$n$ identical cells,each of $e.m.f.$ $E$ and internal resistance $r$,are connected in series. This combination is connected in series with an external resistor $R$. What is the current flowing through $R$?