Need some quick advice re alternator

[VolvoRob38]

Hi all,

Im just about to start replacing my alternator, belt, and tensioner after my electrics goes a bit crazy a few weeks ago with several error messages, one of which was Power System Service Urgent.

The consensus was alternator has packed up, plus pretty common on C30's. I tested my battery which showed 11.64v with engine off, and 11.65v with engine running, so looks like the alternator isnt charging the battery.

I forgot to disconnect my battery while Ive been waiting for the parts to arrive and so expected to be jumping the car this morning to move it round the back where I can do the work. However.... I got in the car, all electrics came on, and the car started on the first trun of the key. A little laboured but it has been sitting for over a week.

The only different thing I did do last week was use my key to lock the car instead of the central locking, this was so I didnt activate the alarm which I had my suspicions was draining the battery, after reading that the lithium alarm battery lasts 10-15 years and when its dead will start to draw power from the main battery, draining it.

The fact that the car started just now with the alarm being deactivated for the past week makes me think it could be the alarm draining the battery while my car sits there week in week out, and possibly not the alternator. Although my multimeter says that it is... I guess it could be both I suppose

Before I start dismantling my car I could do with some helpful friendly advice

TIA