Tuesday, March 1, 2016

Building the battery pack, new sponsor (Li-ionWholesale.com)!

The Formula Hybrid team has been firing on all cylinders over the past few months, making lots of progress on the car's different subsystems. I thought I'd bring a little attention to just one of the cool projects in the works right now: the '15-'16 high voltage battery pack.

Tractive System Accumulator Container

The process of developing this year's high voltage battery pack (also termed "accumulator"), began with an evaluation of last year's methods and final product. The 2014-2015 season was the most recent time the team has attempted to build an accumulator completely from scratch. Due cockpit space constraints, we found it necessary to move away from the prepackaged A123 modules we'd used in the past and repackage the 26650 cells in a custom configuration. While the design proved effective, the development time associated with creating and validating scratch-built fusible links greatly set back the team's schedule. Additionally, the design time associated with creating all the new parts needed in the pack (bus bars, terminal plates, cell holders, insulation layers) took longer than expected.

With the team now more experienced with the challenges associated with accumulator construction, a new goal emerged: to use these developments to create a pack with greater capacity than our old modules had allowed. We began by investigating other potential cell chemistry currently on the market, as well as different cell form factors. After an extensive analysis of cell types, we settled on the Samsung INR18650-30Q. The 30Q provides excellent capacity (~3000mAh) and a continuous discharge current of up to 15A. The switch from LiFePo4 to Li-ion brought other changes as well. The cell count increased to 440 mainly due to the 18650 case size, and the total accumulator configuration changed as well in an attempt to maintain similar output voltages with the new chemistry. 

With our ideal cells selected, it was time for the ultimate challenge: finding a seller. Surprisingly though, this turned out to be quite easy. Our requirements for purchasing cells included competitive pricing, shipping from North America, and most importantly, verified legitimate cells. After searching the internet with mixed results, we came upon www.liionwholesale.com . Unlike many other sellers on the internet, they actually tested their cells to verify legitimacy and kept an active stock from their warehouse in Pennsylvania. Even better, they had upfront, highly competitive pricing for both small and large quantities on their website. Before committing to large quantities, we decided to purchase a couple samples to perform some independent testing,

The cells arrived promptly and well packaged. After a few charge and discharge tests, we confirmed that the cells were meeting their advertised specifications. Confident in the cell quality, we contacted Liionwholesale for a quote for a whole pack worth of cells. They got back to us almost immediately, and upon hearing about our team, offered a generous discount on the bulk quantity. They worked with us throughout the entire purchasing process and generally did everything they could to help the purchase go smoothly. After returning from winter break, the team was greeted with a brand-new box of cells!


Our configuration for the pack this year uses the cells in a vertical configuration, and attempts to maximize the cell density for our available space. The entire pack is made up of parallel groupings of 20 cells, each of which is then connected in series in a "snake" pattern to connect 22 of these groups in series. The isolation spacing between each group is also used to run temperature sensor and voltage sense lines. The cells are actively cooled with a CNC machined cooling plate mounted to the terminal plates through electrically isolating material. Cell groups are connected in parallel through nickel terminal plates located above and below. One of the special features of these terminal plates are the integrated fusible links waterjet cut into each one. Finally, the 22 strings are broken up into groups of 5,6,6, and 5 by segment maintenance switches to permit safer maintenance.


The accumulator interfaces with the rest of the car though the tractive system electronics compartment, located in the rear of the accumulator container. Custom bus bars and HV contactors route the tractive current out of the pack and to the motor controller.



Once we had the cells, it was time to do a test fit in our waterjet cut cell holding layers:


The battery system sub-team is currently gearing up for final pack assembly soon, so check back often for future updates on this and other ongoing projects at RPI formula hybrid. A special thanks to our new sponsor, www.Liionwholesale.com as well. They've helped make this awesome project possible! 


Team members Evan Engisch (left) and Nicholas Moy (right) show off the new cells after test fitting the pack

-Kyle Pollard



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