The battery of a mobile phone is rated as \( 4.2 \, \mathrm{V} \), \( 5800 \, \mathrm{mAh} \). How much energy is stored in it when fully charged?
\( 24.4 \, \mathrm{kJ} \)
\( 48.7 \, \mathrm{kJ} \)
\( 87.7 \, \mathrm{kJ} \)
\( 43.8 \, \mathrm{kJ} \)