This thesis considers the drivetrain and battery system requirements of Hybrid Electric Vehicles. The data herein proves that a series hybrid electric drivetrain with Lithium-ion batteries and plug-in recharge promises to be viable and sustainable. However, for mass production of series HEVs comprehensive performance characteristics and prediction of ageing behaviour of Lithium-ion batteries is essential but currently not available. The main part of the thesis, following a graphical comparison of different energy storage solutions, is a detailed treatise on large Li-ion batteries. Construction and Li-ion working principles are summarised, together with several effects such as Peukert and memory effects, ageing of Li-ion cells, their temperature dependence and safety, and limits of charging/discharging. Preliminary performance tests on 50 and 100 Ah Li-ion cells showed the necessity for a careful investigation of suitable reference conditions in order to achieve reproducibly precise results from repeated discharge/charge cycles. Then the main tests result in detailed graphs and tables of the discharge and charge characteristics. These main tests include effects of rate of discharge, energy-efficiency, temperature, resting time between test-cycles, hysteresis, ageing, and degradation. A new testing method that is based on the step response technique is suggested and investigated to whether it gives a meaningful but rapid measure of open circuit voltage and equivalent circuit models of the battery. Statistically significant theoretical models, equations and graphs are included. The Appendix gives summaries of the author's seven main publications and presentations dealing with Systems Approach and five publications on Large Li-ion batteries, followed by most of these in full.