What Pachyrhinosaurus comes with a battery cover? The only one that I can think of on the top of my head is the Dino-riders one.
In that case yes, there are no listings of a single battery cover for that toy, but upon closer inspection (I'm not quite sure, but possibly) the battery cover for the Pachy is the same as for the Triceratops, just a different color.
So, in theory, you can get a Triceratops cover, paint it yellow, and use for the Pachyrhinosaurus.