How to make a battery/capacitor/energy storage device that will still work in 10,000 years. Not hold charge for that long, but take it out of a box in the year 12000 and recharge it then, it would still work.

Hi, I'm trying to design a power storage device that could still be charged and used 10,000 years after it was made for some post apocolyptic fiction. Obviously you need magic to have something that actually held charge/generated power for that long, but that's not what I'm looking for. The idea is that you'd have a hand-cranked generator attached to the side of this thing made of stainless steel and archival materials, and that's where the power *comes* from. But I want to be able to store it for a few minutes at least once cranked. Everything I've seen seems to say that uncharged batteries and capacitors still only last a few decades at best; I was wondering if there is another solution/something I missed.

The energy storage device needs to be handheld (less than 40 cubic centimeters and 100 grams would be nice) and provide enough current to run a reasonably bright small LED flashlight (like maybe 0.3 watts, 100 milliamps at 3 volts.) It needs to hold enough charge to do so for, say, fifteen minutes (so ~75 milliwatt-hours). And it needs to be rechargable after sitting in storage (inert, not being used, in a sealed box protected from the elements) for a few thousand years.

Does such a thing exist, or will I have to invent science fiction tech/resort to using a larger long term storage solution like a vacuum flywheel that you plug things into but obviously can't be moved.