If you still accept suggestions, I think that it should affect immaterial shields too. Just the way that Fire Shield affects immaterial creatures, Battering Ram should also be able to affect immaterial shields. Being immaterial only protects from targeting, but Ram's ability is passive, just like Fire Shield. There is physical contact.
(Immaterial DR shields also block damage, this strenghtens my point.)
EDIT: But here comes an interesting question. Should Battering Ram affect shields with 0 DR?