will this work in an mb-d12 on a d800?
According to what I read online it will. To be absolutely certain you may wish ti check with Nikon support.
I see no reason why it should not work - if the batteries works at all ... Mine stopped working possibly after exposure to cold. Something the originals did not have a problem with though.
It's been working great with my D4s for 6 months. I switch it out with another battery as a matter of professional practice but I have never exhausted it with 600 to over a 1000 photos. So I think it might handle 1500 to 2000 photos. Maybe more; the charger only show 50% needed charge.