Hesitation at lights and red battery warning light

[belfry]

I just got a call from my daughter who is driving my 3.4 996 tip Tronic. She took it to the country for the weekend and while she was driving back into London tonight she said on two occasions she pulled up at the lights with her foot on the brake, And the revs dropped slightly and a red battery light flashed up for half a second. Then the car ran fine. The same happened about five minutes later, after she had driven for about two hours.

Any thoughts about what might be going on? I thought about the alternator belt slipping?

[belfry]

Good thinking about the strap. Over the years I have had to regularly remove and clean it up in order to improve the hot start starter motor speed. It’s always started however when warm can sound quite sluggish even with a new battery.
I will check this.