When the car was running in the mot centre for some reson was getting 11.87v or there abouts

But the other day after i charged it , running i was getting 13.86 or something and when turned off it was 12.75 volts , when start the car the battery lights always on , give it a rev and it goes out and you see my batt voltage guage shhot up to around 13ish ? always done this tbh

But checked direct from alternator and was getting 14.2 from the positive on it to the negative of the battery

Little blue wire on the alternator and connecton on everythigs all been re-done and cleaned gotta be one or the other im thinking , no good with this sort of thing