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
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