A number of years ago I started toying with homemade VLF (Very Low Frequency) radio equipment. These radios covered a range of roughly 10Hz to around 10 KHz and to aid in reception with minimal noise, I needed to place myself and the equipment far away from manmade electrical interference. This meant going far away from the electrical grid for long periods of time.
As time marched on, it would seem that my need for portable power increased almost tenfold. I have many projects doing many different things, and I always seem to need power in places that scarcely offer it. (i.e. satellite/space station communication, VLF monitoring, geophone monitoring, watching satellite TV in the middle of nowhere, you get the picture).
To facilitate all of this I needed a luggable and robust power source at a minimum of cost.