787 Batteries Included – Why Li-Ion and aircraft shouldn’t mix

787 battery (over-cooked)

Poor Boeing – its 787 “Dream liner” fleet looks like it’s grounded for at least another month following fires in its Li-Ion battery. Many years ago I found myself researching and writing several articles on battery technology, and at the time I really didn’t like Li-Ion, even though it was being pushed as the latest thing. So I’m not that surprised that Boeing has had trouble. I’m only surprised that they used such risky technology in an aircraft, assuming it hadn’t been refined since I last looked at it. Given the problems they’ve had, it clearly hasn’t been refined.

Li-Ion batteries can actually be made from a very wide range of chemistries, all with different characteristics. The anode is normally carbon, but the cathode can be various metal oxides and the electrolyte a lithium salt – plenty of combinations to try. I understand that Boeing went for lithium cobalt oxide, which has one of the highest energy densities (better power-to-weight ratio) but is also considered one fo the most flaky. It’s the same chemistry as is commonly found in consumer devices with Li-Ion batteries. It’s the battery technology that the airlines felt so strongly was unsafe that they initially banned it from your luggage (only allowing later so business travellers could still use their laptops). It’s the type of cell that UPS won’t allow on international flights. And Boeing decides it’s a good idea to make a great big one and fit it in the heart of its new aircraft!

Apparently their plan is very much to mitigate the battery problems by encasing the cells in ceramic, put it in a strong metal box and venting it to the outside in case it starts smoking again. The FAA will be asked to sign this off as safe – potentially it could be considered unable to bring down the aircraft, although one has to wonder how well it will operate once the battery has self-destructed in a contained environment. If it’s not important to the operation of the aircraft, why’s it there at all?

Li-Ion does have an advantage over less exotic technologies in that you can store more power in a smaller, lighter package. But at a cost. Apart from the cells costing a lot more and needing fancy charge controllers to operate them safely(!), they’re also quite fragile in the short term; and in the long term they don’t survive for long.

Did you know, for example, that Li-Ion batteries decay badly when they’re fully charged? This means that if you keep your battery topped up it will lose capacity. If you leave it run down it will decay more slowly, but what’s the point of lugging a flat battery around? This characteristic makes it ideal for companies like Apple to fit into products like the iPhone. Whatever you do regarding charging the battery, your iPhone will die in a few years, forcing you to buy a new one (if you’re stupid enough).

Conventional battery technologies, like NiCd, are far more robust. You can discharge them, fast-charge them, trickle-charge them and generally abuse them. They last for years, with no need for fancy controlling electronics. Lead acid is even tougher, and has been used for decades in hundreds of millions of motor vehicles. Yes, it’s heavy but it’s cheap, there when you need it and has a very good record for not self-destructing.

Yet Boeing seems to be struggling on getting Lithium-Ion to work. They probably have a reason, but I can’t see what it is other than not wishing to back down on what’s looking like a bad decision.

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