I gather that if a lithium battery is not charging it pretty much won't be a hazard. Is that right?
I don't know, and the NTSB seems to avoid making that claim. Here's what they said about the S-Trust fire:
"A thermal runaway occurs when a cell overheats and combusts; it is a chemical reaction that can occur to any type of battery cell if it is damaged, shorted, overheated, defective, or overcharged. It is possible, based on the battery remains’ location among the charger remains, that one of the batteries had been left in the charger, which could have led to overcharging. However, a crewmember told investigators that the batteries were not in the chargers before the fire. Further, investigators were not able to find the missing cells, and, due to the explosion, the extensive heat from the thermal runaway reaction, and subsequent fire on the bridge, the battery cells may have been completely consumed. Therefore, investigators could not examine the first cell that exploded to determine the exact cause of the initial thermal runaway."
So, if a Lithium-Ion cell can cook-off even while not on the charger, which is apparently possible, then I guess the next logical question is whether that's more likely with Li-Ion cells than it would be for any other battery types.
It does seem that the best practice when charging handheld electronics powered by Lithium batteries is to do it where any incident will be noticed right away. For sure that means
not overnight in an unattended space, which I have for sure done plenty of times.
Of course I can't help thinking about the Conception incident in light of this incident aboard a major commercial vessel, whose crew was awake and on-duty, and included a trained fire team, as you'd expect aboard any oil tanker.