Monday, April 29, 2013

Batteries in Airplanes

Batteries seem to be a popular subject these days. Certainly the 787 has had it's share of trouble. Even a couple years ago, batteries in phones and laptops were catching fire, seemingly randomly. Mostly the fires have been harmless to people, but the equipment hasn't come out so well. During 2007 there were several laptops that spontaneously combusted Here is one (warning coarse language) Several manufacturers had recalls, and since then, there haven't been too many laptops that caught fire..

About the time everyone figures the trouble is over, people start getting burned with cell phones in their pocket. I have a couple batteries from my previous phone that are slightly bulged. The bulges signify something bad happening on the inside of the case. Bulging is a mechanical function, charging is normally a simple exchange of electrons.

I don't know the details of the 787 exactly, just what I have read, and the pictures I've seen. It seemed the original design had multiple cells packed together inside of the blue box. The new design has insulation between cells, and the cells are isolated. Lithium batteries are more likely to fail when heated. If one cell is misbehaving and getting warm, and touching another cell, the non-warm cell is more likely to do something bad, even though everything about it is normal. The misbehaving cell will inspire the adjacent cell to enjoy its company. Their friends may joint in, being neighborly, and things are getting quite hot now. The heat seems to multiply, especially being in a box, and suddenly there is smoke coming out.

There is a theory that all electronics run on smoke. When the smoke gets out, they quit working. Batteries tend to be the source of smoke for many electronics, so when the smoke gets out of them, things really don't work.

Why would anyone put something that dangerous in their airplane? As Collin Chapman used to say about building race cars, "Simplify, then add lightness", or Burt Rutan used to say, "Throw it up, if it comes down, it doesn't belong on your airplane". Basically what these people are saying, that making things light is the proper way to build airplanes, and cars. Why lithium? Look at this table:

      Fuel        |             Watts / Kilogram
    Lead Acid     |            0.05
    Lithium       |            0.224
    Gasoline      |           12.88

Lithium batteries are much lighter per unit of work (watts).

Wow, that does seem dangerous, or does it. Well, it is significantly more dense than a lead acid (the traditional airplane battery, although some airplanes are now ni-cad powered), and quite a bit less dense than gasoline. No one carries gasoline in their pocket.

How much battery does an airplane need. I built and airplane once, and was told I only need enough battery to start the engine. Once the engine is started, then the Alternator should take over powering all the accessories. All the radios, gear retracting motors, and lights all run off the alternator. A bigger battery might be handy in case the alternator quits, but then you are hauling around a battery for every flight that you may never need. A second "back-up" battery is just extra weight that you haul around.

Airplanes should be designed so failures are an inconvenience, not a catastrophe.  If your alternator fails, you know the battery will be dead eventually. Once it fails, you can continue without radios and such, or you need to land before it gets dark. With a backup battery, you may continue farther, but you will still need to land soon.

In a modern jet airliner, there are alternators on each engine. Two seems like a good idea. There are actually 3, since the aircraft actually has a third turbine engine called the auxiliary power unit (APU). The APU is hidden in the tail of the aircraft, and connected to a generator capable of starting both engines, and running the majority of everything electric in the aircraft.

The 787 is unique, in that there is no hydraulic system to help the pilots fly the airplane. The items normally controlled by hydraulic fluid are run by servo motors, including control surfaces and brakes. The electrical system is quite important.

The batteries will help start the airplane. On the ground, there will usually be a device called a ground power unit (GPU) that will allow the aircraft to be started. The GPU can also be used to charge the batteries.  If the aircraft is operated away from the GPU, the battery will usually start the APU since that is a smaller turbine. Once the APU is started, it will be used to start the other engines.

Will the new battery solution help prevent a catastrophic failure on the 787. Probably, since the misbehaving battery cell will be isolated from its neighbor. Will there be cell failures? Probably, but the new monitor system will alert the pilots, and isolate the bad cell when needed.

If someone wants to have a wanna be engineer to ride around on the 787 during test flights, I'll volunteer. Give me a call, we'll set something up.


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