Praveen Subramani
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6.UAP Final Report: Prismatic Cell Cycler
Student: Praveen Subramani, SB Candidate in Course 6-1: Electrical Science & Engineering
Faculty Supervisor: William J. Mitchell, Professor of Architecture and Media Arts & Sciences,MIT Media Lab/MIT Design Laboratory
Co-supervisor: Raul-David Poblano, Doctoral Candidate, Smart Cities Group, MIT Media Lab
Introduction & Prior Research
The Smart Cities Group at the MIT Media Lab is pioneering the future of Mobility on Demand systems to revolutionize urban transportation by transforming the way people move around cities. These systems consist of fleets of lightweight, energy-efficient electric vehicles that are strategically distributed throughout the city. Similar to the bicycle-sharing systems that are present in many European cities, users can walk up to any of the electrical charging stations, swipe a membership card, and pick up a vehicle. They can then drive or ride the vehicle to their desired destination and drop off the vehicle at a different station. This one-way rental system eliminates the need for wasteful return trips that are often unnecessary and allows for point-to-point movement that facilitates access to public transportation networks. These Mobility on Demand systems create a model for highly sustainable urban mobility by maximizing public access to a centrally maintained fleet of zero-emission vehicles.
Many challenges exist in designing, developing, and implementing this system, many of them related to the design of the electrical grid infrastructure and battery packs. One of the essential features of a vehicle in a shared-use system is the ability to rapidly recharge the battery pack (in 10-15 minutes) to enable quick turnover in vehicle rental and allow for a lower capacity pack onboard the vehicle itself. Rapidly recharging battery systems require a cell chemistry with sufficient energy density and low internal resistance to prevent overheating during charging, electricity sources that can supply large amounts of current, battery pack designs that can handle these high currents and heat, and high power chargers. Battery cells from A123 Systems, an MIT-spinoff developing nanophosphate-based lithium ion cells, are commercially available and can be rapidly recharged. Until very recently, the major cell produced by A123 Systems and utilized for high power applications was a small, cylindrical cell with a 2.3 Amp-hour capacity and 8 mΩ of internal resistance. These cells have been extensively tested and characterized by A123 Systems and the MIT Electric Vehicle Team (EVT), and battery packs made from the cylindrical cells have been constructed. However, a new cell known as the HD Prismatic (with a 20 Amp-hr capacity at 3.3V) will be available soon and presents an improved cell configuration for automotive applications due to its higher energy density and streamlined form factor.
