Alternator / Drive Belt / Battery Issue

[Oo_xrsx_oO]

Twice in the last week I have been driving and my dash lights (have had headlights on) faded to the brightness of the low setting and the volt gauge steadily dropped to about 11 volts. The battery light has not come during driving.

Car felt the same drive wise (if I didn't have lights on or a volt gauge I would not have known this happened) and then volts went back up to normal around 14+

Anyone had this happen before or have an idea of what it could be? Possibly battery as the car does take a bit longer to kick over on start up? Maybe have the alternator and battery tested?

Car is due for a service soon and I have to take it to ford : due to some warranty issues that need looking at. Just worried it will be one of those things that will be fine as soon as a mechanic looks for the problem.

Thanks