Mr Battery meet Mr Circuit Breaker
I'm a professional electrical engineer. I don't work in aviation. I have just read the NTSB Interim and I am horrified.
We (the overall engineering sector) have know for a long time that Li Ion batteries don't play well with others. They are have an *inherent* tendency to get hot and catch on fire if they are at all mis-treated (over-charge, over-discharge, sudden changes in rates of charge).
The standard solution is always to provide an old-style circuit breaker (or fuse) and an old-style over-temp breaker. By old style I mean "mechanical".
Boeing chose to use a "contactor" do both jobs, based on software calculations performed by the BCU. This is just plane (no pun intended) poor engineering. I have written thousands of lines of controller code in my career, I would never support a decision to protect a Li Ion battery with a breaker which makes decisions based on software (Even if I had written it).
This is a blatant failure of good engineering governance and a failure to apply good sense. (And putting a firebox around what is essentially a bomb still leaves you with a bomb, I don't know of any material which would have contained the fire which happened at BOS).
So I guess Boeing need a good, safe, sound, sensible solution. My suggestion is a slow-blow circuit breaker and an over temp fuse, placed in series. I know that this combination may limit the performance envelope of the battery but I think that's what needs to be done.