You should run a ground wire from the filler cap ring to the frame with any cell. Think about this, your plastic cell is an insulator for your filler cap ring, therefore you need the ground. On your aluminum cell, did you put the rubber gaskets between the filler cap rings and the aluminum to seal it up? They are an automatic insulator for the filler cap ring. Yes it may touch the metal, or yea the bolts may touch the metal some where, but not enough to make a "good ground". Put a ground wire on it and you know it is grounded and there fore no arc flash threat.

That's my $1.250.