Ventilation in a battery bank?

Hi there I have a cupboard which contains a battery bank in my shed there is 10 battery's 220ah each and I wanted to ventilate the cupboard for when I install the battery's I was going to put a 4" bathroom extractor fan in but that would involve buying a 4" hole saw and it would need to Be Masonry and they are expensive so I was thinking of drilling 20x 10mm holes.I was told the hydrogen and oxygen will find its own way out so would this work