12.1 Introduction12.2 A Potted History of Eternal Solar Flight12.3 Why Has It Been So Difficult?12.4 Objectives of HALE UAV12.5 Worked Example – HALE UAV12.6 Cells, Batteries, and Real Life12.7 A Quick Aside on Regenerative Fuel Cells12.8 So What Do We Need from Our Battery Suppliers?12.9 The Challenges for Battery Developers12.10 The Answer to the Title12.11 SummaryAcknowledgmentsReferences