citreon saxo 1.0?

Happy to advise Jo - there are a few of reasons that your battery is not holding its charge.
1) Its old and internally discharging
2) There is an obvious drain on the system ie the interior lights or sidelights have been left on unintentionally
3) The cars battery charging system - the alternator - has developed an internal fault and is discreetly draining the power away overnight.
However, all is not lost. Pop over to a local motor factor and they will test the battery condition for you (in the hope of selling you a new one, naturally). They can also check the health of the alternator very quickly too. However, if no fault is found, then its a good idea to disconnect the battery overnight. If it holds its charge, then its a fault of (2) or (3) and further investigation is necessary. Hope this helps!

John
July 2009