Automotive manufacturers have in the last few years put on the market new models of electric vehicles (EV) such as the Nissan Leaf and Tesla Roadster, demonstrating their awareness to the upcoming fossil/renewable energy transition. While the infrastructure around EVs is yet to be developed completely in most countries, potential buyers are being attracted by the idea of driving a vehicle which could save money by only using electricity as a source of energy. By extension, having a portable modular device in the form of a trailer attached to a vehicle and tted with extra energy storage would allow current EVs to be used beyond their usual range limits, as well as provide space for carrying extra gear. This paper focuses on the electrical modeling and design of a battery pack tted inside the trailer and a range estimation is calculated over a standardized driving cycle, using the high level programming language, Octave. A lithium based chemistry was chosen for numerical application, real data were used and case studies are presented to illustrate how the internal hardware of the trailer affects its performance. A value of 150 km of extra range was set as a target for the design, from which other trailer characteristics can be derived such as the weight distribution within the trailer and the maximum cornering speed. Overall, this work can be used as a preliminary design tool, able to accommodate different design options using the mathematical model employed.