I hope that Mac OS X turns off write caches on low battery.
I've never heard of such a thing. The best you can do is try to push the system into hibernation instead of going down hard. That *should* clear any disk caches as part of the graceful shutdown. But you're relying on a relatively fragile system now, once the battery is quite low who knows if that will even execute in the window of time you have left.
And at this point it is not unreasonable to expect the user to perform some sort of action. Shutdown before the power completely dies, or plug in the power.
