Ultracapacitors built with a forest of carbon nanotubes hold promise of replacing conventional batteries according to a recent article at IEEE Spectrum which describes the current state of research. Ultracapacitors can be charged very quickly, discharge an incredible amount of current quickly, and can endure more charge/discharge cycles than today's batteries - and they weigh less too. All this adds up to massive changes for the future of portable devices such as cell phones, flashlights, and even electric cars and mobile robots. Current ultracapacitor technology can only store about 5% of the total energy as a typical lithium-ion battery of similar size, but the gap is narrowing as production methods and materials improve.