Nuclear batteries, powered by thermocouples that transform the heat of decay into a small amount of electricity. For long-running batteries, they've got pretty good energy density, although even the most advanced designs struggle to hit 15% efficiency.
The nice thing about these is that there's only a few kg of radioactive material, and an accident high enough to generate fallout will also burn up both the fuel and the shielding.
I'd love to get one of those for a laptop. Does the power, size, weight work out? Imagine running your laptop for 20 years without ever plugging it in!
The nice thing about these is that there's only a few kg of radioactive material, and an accident high enough to generate fallout will also burn up both the fuel and the shielding.