Battery icon flashing inconsistently

[Lappo]

Hi,

I have a 2004 1.4 MK5 Golf in which I've recently put a new stereo (Sony WX-GT90BT). Installation went fine and I disconnected the negative terminal of the battery as suggested to avoid any issues with blowing fuses etc, however now, the battery icon on the dash flashes, mainly when the car is idling however it does still come on sometimes when driving.

Below is a short video of what I mean.

Has anybody had this problem before or could perhaps shed a light on why this may be happening? I have completely disconnected and reconnected both terminals to ensure it is not down to poor connection.

Also, the battery itself is fairly new, just a few months old.

Thanks in advance,

Jack

[ChazGTI]

I would imagine its a problem with the alternator. Check it visually and try listening for it whining (bearing) and also test your battery voltage with the engine running. Should be around 14V if the alternator is working properly.

[r5gtt]

Either your alternator clutch pulley is warning you it's going to go pop or your alternator voltage regulator is!. I had the same in my last car as it was the regulator packing up.

Take the belt off and check that the pulley only turns one way and not the other. If it turns both ways it's goosed 
Here's a little video not mine but you'll get the gist.