I'm using about 15-30 on my ship of the B-12K rechargeable battery tank from the near future electrics mod, 5 fuel cell arrays, and some battery packs.
Ok that's why it won't work most likely. I've just started exotic matter right away and just assumed that it would work. I'm going to test that out.
EDIT: ok I just tested it out and it says exotic matter full but it still wont allow me to start the engine.