Is alternator going bad?

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Hello everyone, 

I have just purchased a 2010 Smax 2.0 tdci with 90k miles. In December on the first cold morning the battery died, I tested the battery after a charge and it was reading 11.7v so I put a new battery on it and all seemed OK. 

Today I turned the engine over and the battery light stayed on for about 5 seconds, so I decided to check the alternator. These are the readings I'm getting. 

battery 12.6v. battery with engine idle 14.8v.  Under load with everything turned on the voltage slowly drops, after about 2/3 minutes the voltage drops to about 12.8v and then jumps back up to 14.8v and it remains at this. I've done this test 3 times and get the same results each time. 

Is this normal or the sign of bad alternator or regulator? Could this have damaged the previous battery

Is it worth trying to service the alternator or just put a new one on? 

Any help or advice much appreciated 

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