I would also suspect the Wfco as they are lower end not so great chargers. However it could be that either one or both together are the problem in that the design is marginal in terms of generating a "good" sine wave or rejection of a "not so good" sine wave. With the battery connected apparently the Wfco acts differently and the gen stalls. I suggested AC noise which can be viewed with the right equipment.
ScottG posted some tests indicating it just might also be your gen.
At any rate there are many posts concerning Wfco poor battery charging.
