The answer is both and the answer is also neither because of host of things could be causing the voltmeter to be dropping as you change the fan.
What’s the real issue you’re trying to solve first. the slow hard cranking, or the fact that the voltmeter moves? Let’s work the issues in order of priority. if you want to completely rule out the alternator do the test Jerry posted above if you wanna rule out the battery, put a new one /different one in. And see if it changes anything.
