Speaking as an electrical engineer who has designed battery charging systems for exotic battery packs in the past I would suggest that Boeing may be trying to reproduce conditions that are difficult to do on the ground. If I was working on the problem I would want to insurument the holy ()*^ out of the entire system and then put it through simultaneous stresses- altitude, temperature, g loading, load dumps from the engines, etc. Maybe then I could get a clue as to what might be going on. The easiest way to do this might be a flight test.