MG Car Repair: Rover 414SI charging light

Question
Hi. Barry , i wonder if you could help me ? The charging light comes on infrequently on dash  and stays on for long periods but is inconsistent and goes out at higher revs 2500- 3000.this started about twonwerks ago  . The multimmeter read 13. 75  volts across battery terminals  when light was off and about 9-  10 volts when light was on and even much lower briefly .Alternator looks like fairly well used , pulley belt looks good fit . Are there easy repairs to alternator or can it be a regulator thing  or maybe something else . ?
Tankyou for your time , regards
Aidan Bremner

Answer
Hi Aidan

This sounds more like a battery problem to me.  

The simplest test is to turn the car headlamps on and then get someone to start the car while you watch the headlamps.  If the lamps go very dim or the car turns over very slowly, then you have a worn out battery.

I would then go to a reputable garage and get them to test both the battery and the alternator.  

Do not allow them to replace the alternator without getting the battery tested, because there is a high risk that a bad battery can blow a new alternator.