I bought a new charger and new batterie, nitecore i2 (two bays), and AmazonBasics AAA. I charged in pairs but nearly everytime i do so, one of the batteries is popping during or right after the charge. When i charge single, no popping. When i charge in pairs, there is popping. Which is faulty, and what should i return?
If the battery is making a noise, it's probably damaged.
Whether it was badly made or the charger damaged it is hard to tell, but I would place a small bet on it being the battery. The Nitecore charging stations (at least on the larger 4-bay ones) have independent charging circuits.