Try doing a volt drop check on the alternator cable mate as this can have an effect if there is high resistance on the cable.

With the engine running switch on lights, heater, heated rear screen etc, now with your multimeter set to volts, put one probe on the positive terminal of the the battery and the other on the main cable connection on the back of the alternator and see what you get. You shouldn't have any more than .5v reading. More than that suggests your cable isn't as good as it should be and you need to replace/rewire it

I checked mine a little while ago and was getting 14.3v at the battery.