Building Our New Battery Pack

Electrical System, Featured, General / Monday, April 18th, 2022

Heading into the 2022 Solar Car competition, the team is making big upgrades to the current car.  Among other improvements, we are adding a new suspension system, solar array, and battery pack. The battery pack, in particular, will allow the car to store more energy and last longer on the track. 

There are many steps involved in making a battery pack. Some of the things that are involved include selecting and ordering the batteries, acquiring materials for the battery pack, assembling individual cells into a battery with the appropriate voltage and current, and building a battery box that can safely store the batteries. 

The first step in creating a battery pack is battery selection.  During battery selection, it is imperative to consider several different factors:  capacity of each cell, voltage of each cell, the arrangements, and the number of cells required to fit our specifications. Our goal is to find a battery arrangement that gets as close to 5000 Watt-hours as possible, uses a minimal amount of cells, is lightweight, and is reliable.  To keep track and manage this information, we created a spreadsheet where we recorded information relevant to us, such as the nominal voltage, nominal capacity, weight, and maximum discharge current rate. One parameter that we took a long time to figure out was the capacity from a variety of cell arrangements.  

Along with the data on the spreadsheet, we also tried to simulate the performance of the motor with different batteries’ voltages. The simulator we used outputted the torque, efficiency, power, and load curves for our motors. We found that our selected battery resulted in a more efficient car which made it ideal for our upgraded 2022 car.  In the example graph below, you can see a comparison of two different batteries. Battery A results in a more efficient car because the motor has a higher efficiency curve and the motor consumes less power per speed (which is the x-axis)

Initially, we considered using lithium iron phosphate batteries for our upcoming race which is a low-density and high-safety battery chemistry. We thought it would be a good idea because, in a long-distance race, it’s all about minimizing power consumption at a constant velocity. Imagine your car reaches a constant velocity during a race, because of Newton’s first law of motion (object in motion stays in motion), mass would be irrelevant aside from tire losses as a result of a higher rolling resistance. However, realizing that we would have to deal with the need to accelerate frequently throughout the race, we quickly eliminated this option and looked at lighter and more energy dense cells.

During our selection, we compared the discharge graphs of the four cells. We usually only discharge the batteries at around 2 amps so we ended up using this graph for reference. Since, we would never discharge a battery past its voltage cut-off (the prescribed lower-limit voltage at which discharge is complete, beyond which further discharge can harm) we looked at the voltage of around 3 volts, and you can see that they differ only very slightly. We ended up deciding that the difference was negligible and went with the battery with the best predicted results based on the spreadsheet..

After selecting the battery, we must look for a good supplier. One member of the project, Thomas Kebede, notes that a good supplier “will help us make sure the batteries come at a reasonable cost, and without damage.” Sites such as Voltaplex and 18650 Battery Store are examples of what we consider to be reliable suppliers. 

In addition to selecting and ordering the batteries, we must obtain the materials required to build the battery pack:  

  • spacers are used to arrange the cells correctly; 
  • ribbons – copper or nickel strips – connect the cells; 
  • a way to attach the ribbons to the batteries. 

We will use a soldering iron which we have found to be the most reliable despite the small but real risk of damaging the cells. To combat this risk, we will cool the soldering area using compressed gas. 

Lastly, we must have a battery box to house the battery pack. This keeps the battery cool, protecting the battery from the outside, and protecting us from its high voltage. The battery box is made out of carbon fiber and fiberglass. 

Thomas Kebede (12) and Dylan Berger (11) gluing parts of the battery box together.

Having a good battery is important for solar car performance. Thomas Kebede notes that “a new battery pack will allow us to store more energy and increase motor performance,” resulting in a more efficient car which will propel us to more laps at the 2022 Solar Car competition! Our next steps in the process of making the battery pack is consolidating the battery management system (BMS) along with the mandatory intake/exhaust fan and ensuring the battery pack performs ideally during testing. 

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