September 24, 2010

Strike Witches 2 -- lithium batteries

The whole business of witches sometimes using up all their magic and recovering the next day, and sometimes really using up their magic and never being able to use it again? It occurred to me last night that it's like how lithiium batteries work.

Spoilers below the fold.


As long as you use a lithium battery and keep it within a certain discharge range, it's a very good rechargeable battery. But if you discharge it too low, it loses a considerable part of its capacity. And if you totally discharge it, it's destroyed. It can't be used that way again.

Commercial lithium batteries, such as the ones used in cell phones or laptop computers, have circuitry built into them that monitors how they're used and prevents them from being discharged enough to destroy them. That's why you don't usually hear about that kind of thing. But if you were to think about such a battery being used in a military situation, there might well be some sort of emergency override which you could throw that said, "I understand that this is going to destroy the battery but I still need the power right now."

Back when I was in college, OSU had inherited an Athena computer. It was the missile guidance computer for the Atlas class ICBM, fully transistorized but built before integrated circuits. Like all ballistic missiles of that era (late 1950's), once the engine stopped burning then there wasn't any more guidance to be done. It was going to go where it was going.

But during the critical period while the engines were burning, the missle was under direct control from the ground. And if the computer failed during that period, anything could happen. So on the main battle console there was a switch labeled "Battle Short". In case of a real launch (God forbid) the guys running the system would throw that switch, and it shorted across all the circuit breakers. At that point, the computer absolutely had to run for about ten minutes, and after that it no longer mattered because the civilized world had come to an end.

So when battle short was thrown, then if there had been a short circuit somewhere in the system, the power supply (which was way bigger than needed just for normal operation) would feed power into the short until it opened itself or until it melted into slag, and they just hoped that wouldn't prevent the computer from operating for the remaining ten minutes of its life.

In ep 4 of SW2, where Barkhorn was flying the Me-262, the problem was that there wasn't any over-discharge protection in place. Barkhorn was unconscious, but the jet striker was still running full blast, draining her magical power. If it had been allowed to keep going, it would have drained it completely, and Barkhorn would no longer have been a witch.

But with Miyafuji, she threw the battle-short switch. If she'd been fresh and rested, she might well have been able to do what she did without suffering permanent damage. But the air battle to cover the approach of the fleet was a tough one, and Miyafuji had been assigned to cover Yamato with her shield, which drew a hell of a lot of fire, and thus required a hell of a lot of power from her. (Plus she defended Sakamoto.)

All of them were exhausted after that fight, not necessarily all the way down to zero, but definitely low on magical fuel, and Miyafuji was probably more tired than the rest. She barely even had enough in the safe range to even fly well. On the other hand, it was an ato ga nai zo moment, and she made the conscious decision to drain her power completely in order to destroy the super-core.

On the other hand, maybe the analogy isn't exact. Maybe, maybe, in a few months or a year or two, her power will start to recover. Probably it won't ever be anything like as strong as it was, but maybe, maybe...

Posted by: Steven Den Beste in Engineer's Disease at 04:29 PM | Comments (3) | Add Comment
Post contains 680 words, total size 4 kb.

1

No battery I'm familiar with has a battleshort or equivalent, but I think most batteries for military radios are nickel-hydride.

Generators, on the other hand, all have a battleshort.  This overrides the overheat and low oil pressure interlocks.  It's for exactly the reason you talk about--sometimes, the lights just have to stay on.

What's fun is having a driver who doesn't understand what the battleshort is for and turning it on during training.  "But sir, the generator doesn't start without that switch on!"  *sigh*

Posted by: CatCube at September 24, 2010 05:58 PM (Te0W1)

2 Of course, if you misuse a lithium battery in just the right way... it catches fire.

Posted by: Mauser at September 24, 2010 09:15 PM (cZPoz)

3

Speaking of batteries and magic power and suchwhat, an obvious implication of something we saw in the last episode finally struck me (cuz I'm slow that way.)  Spoilage:

Posted by: Dave Young at September 25, 2010 03:37 PM (ZAk0Z)

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