[wazzz]

So it sounds like what I said above, battery is good but charging system is bad and when running the engine, only the battery is used, hence the voltage drop after some time.
It could be as simple as a dead bulb in the gauge. If that bulb is blown or missing, the alternator does not "boot" (can't find the word for it but it needs this bulb and some current flow through it, otherwise it does not supply voltage).