The Battery Pack

A typical bike battery runs between 24 and 48 V. We decided to push the envelope by designing our battery pack to be as close as possible to Cal Poly’s dictated maximum: 50 V. Each Panasonic NCR18650B cell has a nominal voltage of 3.7 V so our 14s battery pack has a nominal voltage of 51.8 V. Don’t worry, our advisor said it was okay.

 

Having satisfied our need for speed with a high voltage design, we turned our attention to the bike's range. The number of cells in parallel is proportional to the electric range, which can be determined by the battery pack’s Amp-hour (Ah) rating. Restricted by our budget and size considerations, we chose to place 6 cells in parallel. Each individual cell is rated at 3.4 Ah which results in a 20.4 Ah battery pack.

 

Our battery provides 1.057 kWh of energy (20.4 Ah x 51.8 V). For a typical load on the bike, each mile traveled using the battery will consume about 20 Wh of energy. Therefore, the range of our bike is approximately 50 miles.

 

In order to connect the cells in series and parallel, we spot welded pure nickel strips to ensure a safe electrical connection. Each nickel strip connecting the cells can pass about 5A of current. This allows the controller to safely draw 30 A of current from the battery. We doubled up on the nickel strip connections to enable the controller to draw up to 60 A of current from the battery.

 

In order to protect our battery and optimize its performance and lifespan, we installed a Battery Management System (BMS). The BMS ensures that battery charging and discharging loads are distributed equally among the cells by monitoring the voltage and current of each cell. In the event that any cell reaches its individual maximum load, the BMS will stop the charge or discharge to prevent any damage to the cells. For redundancy, we also purchased a top-of-the-line, real deal controller: the PhaseRunner.