I'd check the brushes / voltage regulator anyway as it may have been making decent contact for a while but because it's worn won't do it consistently.
https://www.micra.org.uk/threads/alternator-tear-down-repair-help.63589/
Unlike in that thread I've changed brushes / regulators a couple of times on my old camry before the alternator eventually died at about 450,000kms some 5 or 6 years after first changing the brushes, it may be possible to carefully clean up the slip rings with emery tape or a file etc but I just popped them in... Only done it in a camry not a micra though...
