The gauge on the contol panel will be a voltmeter - probably calibrated to show battery charge; mine shows the voltage and I have to work out the charge state from that
In my (limited) experience, analogue voltmeters are not very accurate in showing battery charge - but if it's gone from 'full' to 'half full' then that's a pretty good indication that something's wrong.
The state of charge of a battery and the voltage across the terminals varies between battery type - lead-acid, gel, calcium etc; however you should be able to get an idea with your meter. If you set the meter to DCV and put the red lead on the battery positive terminal and the black lead on the negative one then if the battery is fully charged you should read something like 12.6 volts; half charge will be about 12.3 volts; anything less than about 12 volts and its pretty much fully discharged. All this should be done with no load on the battery - all switches off.
If you get a reading above about 12.5 volt then it might well be the calibration of the battery state meter that's wrong.
Cheers
Mike