The end of the handle contains a vent, which I think is there so that part of the air flow goes around the batteries, cooling them (more on that later)

The vent cover is held in place by two teeth, shown in the photos by the arrows

Though the teeth are at the narrow/rounded edges of the vent cover, I recommend prying it open by working the straight edges because then you can use the pry tool with maximum advantage and avoid damaging the rounded edge

In contrast to the main cover, the vent cover doesn't require much force to open