Several things.The Dreamliner has replaced almost all hydraulic and pneumatic systems with electrics, resulting in a huge weight and fuel savings. Pneumatics were largely financed by bleeding power off the engines, costing about 3% of generated power. Hydraulics required three separate lines for redundancy to every affected surface (4 for wings, 3 for tail- each times three) plus wheels, some doors, etc. hydraulic systems also require hydraulic reservoirs, of course (times three), plus tubing (times three) everywhere, plus gallons and gallons and gallons of hydraulic fluid. It's much lighter to run wires and have smaller, efficient electric motors move these flight control surfaces.If you are going to run on electrics then you need them powered even (especially) in in-flight emergency situations where your "generating plant" fails, hence the need for batteries. [Each Dreamliner has six such plants, earlier aircraft had a maximum of three.] There are two batteries in each plane, one forward near the rear of the cockpit, one aft behind the passenger compartment. They weigh about 63 pounds each, and are in insulated compartments precisely in case of an overheat problem. The Dreamliner is about 20% more efficient than previous models, largely due to weight reduction (carbon composite materials account for most, but not all of the savings), and since fuel is the #1 expense for airlines, that promises a fairly dramatic competitive advantage, all other things equal.The batteries produce a stable 235v, but (apparently) are subject to "runaway heat events", sometimes called "fires" when put under dramatic stress for short periods. Normally they serve as reserve power supply and also to condition power to the correct voltage & amperage.Given that each pound saved means about $20,000 a year savings in fuel costs, over the lifetime if an aircraft you are talking some truly significant cost savings. Which explains why the airlines are replacing seats with new ones that weight 3 pounds less, replacing flight manuals with iPads, and even washing the planes more frequently to remove dirt (true!) Li batteries are far more energy dense, which is why Boeing chose to use them. (They are a slightly different technology than the Li batteries in cars and cell phones, and supposedly (but perhaps apparently not) the overheating issues had been overcome. (This was all rigorously tested and approved by the FAA, which doesn't explain to me, at least, why we have two incidents so early in the deployment of the aircraft.)[I used to own a small publication for the airline industry and still have some access to industry pubs, so this is a brief summary of my last hour or two of reading. I haven't been following it all that closely except as a lay person generally interested in aviation technology.]
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