We have had our C3 battery & alternator replaced, twice within the lst year, & still the battery seems to discharge - only getting about 3 days of normal driving from the car.
The work was done by our Citroen dealer. He said that if that did not work that we should bring the car to an auto electrician.
(i charge the battery over night, every night now. I notice that there appears to be about 2 ohms resistance between the chassis & the engine. does this sound correct? Where is the earth strap between engine & body? Perhaps it needs to be replaced or the nuts tightened?

Tony, November 2014

connect a multi meter across the battery,set on volts,start the engine,rev the engine ,you should get a reading of about 12.75 around that,as your reving the engine,it may go up to 13,75 volts,not a prob,if you get no readings like these then its the alternator or associated wiring ,its not rocket science mate mm

mm, November 2014

You can get a car voltmeter from Amazon for under £2 - it plugs into the 12V cigar lighter outlet.

Mine (the battery) sits at around 12.2 Volts when cold - 9V when it goes flat but not often - and about 14.3 V when the engine's running and the battery is being charged.

If you had one you could see what's happening to your battery as you drive along.

From what you say either the battery is not being charged - the battery charging light (battery not charging) should tel you that - or something is taking power from it when it should not - overnight.

Be sure that you see the battery light when you turn on the ignition and that it goes out as the engine runs. The light itself is an essential part of the alternators circuitry - if the lamp fails or the wire that runs from it to the alternator is broken there will be no excitation current to the alternator and the battery will go flat.

Wikipedia says it this way...
The field windings are supplied power from the battery via the ignition switch and regulator. A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator.(which is why the indicator is on when the ignition is on but the engine is not running). Once the engine is running and the alternator is generating power, a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power.

Clarkson, November 2014