Boeing has submitted paperwork for a fix to FAA. They are improving the battery assembly and enclosing the battery assembly inside a fireproof battery box with over board vents, so that should the "improved" battery catch fire again the fire will be contained inside the battery box and the smoke vented overboard.
The "improved" battery is only medium convincing. This is a replaceable cell battery. Eight separate cells, each yielding a little less than four volts are packed inside a metal box. Wired in series this gives a 28 volt battery, the standard aircraft battery voltage for the last 70 years or more. There is a battery monitor, an electronic black box that checks each cell, jumper straps to tie the cells together in series, a wiring harness for the battery monitor. Changes involve wrapping each cell in tape, lock washers on the terminal straps, more shrink tubing to insulate the wiring harness. The battery monitor will be reprogrammed to alarm more readily. These are quality control measures that are a good idea in general, but don't sound like a real fix. The real problem is that for some reason battery cells now and then decide to catch fire. Once a single cell catches fire, it will light off its neighbor cells since they are all packed cheek by jowl inside the battery assembly.
In going over all the paperwork generated, it was revealed that Securiplane, the maker of the 787 battery charger, never tested their charger on a real battery. Due to a previous battery fire in their lab, they decided testing with real batteries was too dangerous. All testing of the charger circuitry was done on simulated batteries instead of the real thing. That's surprising. Anyone with real flight line experience will tell you that simulators are never exactly like the real thing.