do you know how old the battery is?
i was getting that, i left the battery flat and discharged over the winter period.. with the freezing temperatures... it caused the battery to freeze.. then it dead-celled. was running at 10 volts :/
if its not that...
the alternator could be on its way out. charges the battery during the day when your using less power (no lights) and at night your using slightly more power so the alternator cant charge the battery sufficiently .
duh... grammar fail
hope this helps.
![:D :D](./images/smilies/icon_biggrin.gif)