Here's how I expect the Dreamliner to be fixed and restored to airworthiness.
The battery that has twice caught fire is a Lithium battery because that particular battery chemistry holds the most energy for its weight and size. Weight's more important in this case. Any battery can get an internal short circuit as a result of wear while in normal operation, and that may be what happened to the Dreamliner batteries that caught fire. A short circuit leads to a point that's white-hot within the battery. What made this so severe as to ground the entire line of aircraft is that Lithium batteries burn very well once the heat of the short circuit gets them going, and they're really difficult to put out.
This is because the chemicals they are made of contain both the fuel and the oxidizer - the two critical elements necessary to make a fire. Most fires use the air as an oxidizer, and can thus be put out by anything that gets the air away from them - and this is how most fire extinguishers work. Spray some water on a fire, and it puts the fire out primarily by getting the air away from it for a moment. The cooling effect of the water also helps, by getting the fuel below the "flashpoint" temperature at which it will burn.
But this doesn't work with lithium. Since the oxygen is coming from inside the battery, most extinguishers have no effect. And it's hard to cool the battery because coolants like water can't get inside of the battery easily.
The Tesla cars use lithium batteries too, but they use many more cells which are much smaller in size, and have air gaps and steel barriers internal to the battery so that a fire won't spread. In contrast the Yuasa battery used in the Dreamliner is very simple, with only 8 large cells. Elon Musk of SpaceX and Tesla suggests that the Yuasa design will lead to thermal runaway, in which one cell heats up due to a fault and then its heat causes the other cells to heat up as well, causing the entire battery to become hot enough to ignite a fire.
This battery starts up the plane's auxillary power unit (APU), a small jet turbine that drives a generator. The APU can then make enough power to run the plane's electrical equipment and start the plane's main engines. And everything on that plane is electrical. Without the APU, the plane can't start up on its own, and will need to be connected to external power whenever the engines aren't running. Having the airplane be able to start itself is a safety feature, not just convenience - engines can flame out, etc., and may have to be restarted in the air.
For a while, maybe for the life of this model, they will probably have to use a chemistry other than lithium. Maybe the old faithful lead-acid batteries, which are of course as heavy as the lead and water they're made of. Making the plane heavier means they might have to get rid of a few seats, or they might be able to fly a few hundred fewer miles on a tank of fuel than they might have with the lithium battery.
Or perhaps they'll make friends with Elon Musk and use the lithium batteries from Tesla cars or SpaceX rockets.
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