A fully charge battery runs the car all day no problems every thing works ,charging light goes out as it should. Switch off the engine and leave it overnight . the next morning the voltage drop is sufficent to prevent the starter turning enough to start the engine. put a battery charger on for a couple of hours and you are good for the day.

How old is the vehicle,
how old is the battery? is it the original battery - battery could be worn out and need replacing

You either have a dying battery that just wo not hold a charge anymore or you have a short somewhere.

Turn off everything and then disconnect the negative cable on the battery. Tap it to the post and if you get a spark, you have a short in your wiring somewhere.

You can also leave it off all night and reconnect it in the morning. If it still has a charge, it is a dead short in the wiring. If the battery is still dead, time to get a new one.

It may be as simple as checking the boot light goes out on closing the boot, or a internal light stays on.
But if they check out OK.
It may be a faulty alternator, that charges up the battery while you drive. Get this checked out at a local garage.
It may also be the battery, any car shop can check that it holds its charge.
Good luck mate.

Dead cell in the Battery. Try Bat aid tablets first and then buy a new one if that does not work

Try switching off the headlamps when you get home at night.

(Failing that get a new battery). ;-)

replace the battery, it has a dud cell it will never get better

See an autoelectrician,they will ascertain what the problem is,this is your best and cheapest option.Why buy another battery if you dont need to ?

a battery is only good if filled to the required level,if its a sealed battery ,the other fault could be the boot light is staying on,a common fault on peugeot 306

get new battery

Sometimes the relay does not open when you turn ignition off. burnt contacts keep them closed.

Your Battery has a dodgy cell !