While we initially decided to create a composite enclosure utilizing 0.125” FR4 Fiberglass, through a 3-point bend test we soon realized that it was no longer a viable option as the primary material of our accumulator structure. This ultimately brought us to our current design that employs 5052 Aluminum sheet metal as the main structure. FSAE requires this material to be 0.91” thick, thus we utilized a thicker set of sheet metal in order to be compliant with regulations. The design's primary method of jointing uses ¼-20 bolts spread evenly across each joint. The distance between each bolt aligns with general aerospace guidelines.

This design  incorporates 3 adjustable sections for batteries and other electrical components necessary for the accumulator.  This allows for these left most sections to change in geometry for testing various spacings around the batteries. This gives the team the opportunity to change geometries of the accumulator container to test which clearances around the batteries will provide the optimal sizing while still effectively enhancing temperature dispersion within the accumulator. Internal walls are regulated by FSAE as necessary components between each module of our accumulator. They also help with the integrity of the structure as well as internal supports for the batteries to be constrained within to minimize internal movement. The base is made of 0.125” 5052 aluminum and both the external and internal walls are 0.1” 5052 aluminum in order to comply with relevant FSAE regulations. 

For mounting the accumulator to the chassis, we have developed a system of tabs that will attach to the accumulator and eventually be bolted to their counterparts on the chassis. As per FSAE regulations, they have been designed to withstand a test load equivalent to 1/4 of the total accumulator mass accelerating at 40g in all directions.

As for the power source itself, each section contains a 7p-16s electrical configuration of battery units stacked together in a linear fashion. As battery cells all have an internal resistance, this pack will generate heat over the course of the prescribed driving cycles, and it is therefore prudent to ensure that the pack temperature does not exceed 60 degrees Celsius both to ensure rules compliance and to ensure thermal runaway safety of lithium ion cells. Under maximum allowed safe sustained current of 20A, a Sony 18650 Cell (as utilized in the accumulator) will dissipate approximately 5 Watts of thermal energy, correlating to a maximum sustained output of roughly 560 Watts per section.  

To combat this build up of heat, the accumulator features a forced convection air-cooling system using an array of 12V cooling fans. These are commonly available due their use in tower-style home computers, and we can use them to drive air in between each battery pack unit. Intake and exhaust for this forced convection system is displayed through the pattern of holes on the face of the accumulator. These holes are mirrored on the opposite plate as well to allow flow to pass through with ease. The amount of space left around the lithium cell sections is a key dimension to consider for this problem, but it is also driven by the amount of clearance required for other components such as busbars and bolt heads that will be extruded into each section of the accumulator. A high flow-volume 60mm 12V computer cooling fan is capable of outputting roughly 30 cubic feet per minute. With our current setup, CFD simulations show that this flow rate allows for the system to sustain roughly 50% of peak thermal output.