Voyager escapes the Solar System

When I was a young lieutenant at Fort Knox, 1977, NASA launched Voyager to explore the outer reaches of our solar system.  35 years later, it's still exploring.

With absolutely no attempt at hyperbole at all, it is fair to say that this is one of - if not the - biggest achievement of the human race. For, as we speak, an object conceived in the human mind, and built by our tools, and launched from our planet, is sailing out of the further depths of our solar system - and will be the first object made by man to sail out into interstellar space.

I've now seen the first tentative steps into space, and I've seen humans walk on the moon, and now I've seen a human-built robot leave the solar system.  This is very cool stuff and Voyager is predicted to have battery life until 2020.

So, why can't my cell-phone battery last longer than 12 hours?