Our battery design has come a long way from last year’s competition, with main advancements relating to BMS data recording and case modularity.



The cell model we have chosen to use is the HG2 from LG. It boasts a high continuous discharge current of 20A, as well as a high capacity of 3Ah. In pursuit of a high efficiency, this year we have made the switch from a 48V system to 36V, changing our battery arrangement from 13S4P (13 cells in series, 4 in parallel) to 10S5P, allowing us to increase our overall available current draw without taking up more room within the vehicle.



The BMS we have selected to use is the TinyBMS by Energus, which supports between 4 and 16 series cells. This performs continuous monitoring of data, however the local storage only records the significant events, as opposed to the full discharge profile across an entire run.

 

At last year’s competition, power was repeatedly cut off from our motor by our BMS, and from looking at these event logs it became clear that this was due to large current spikes, peaking at 120A. We have since begun using the latest version of the firmware which has been updated to better deal with current spikes, however a full log of the data obtained by the BMS would be very useful.

TinyBMS

 

Thus, we have chosen to use a Raspberry Pi Zero which will be hooked up to the BMS in order to track and store its readings in real time. From this, we will be able to observe the full discharge profile across the entire run, rather than trying to deduce what is happening based on the recorded significant events. The Pi Zero will be connected to our CANbus system, sending data to our central motor controller where it will be logged.



We are very happy with how our battery design has progressed over the past year in terms of modularity and ease of assembly/disassembly. At the Shell Eco-Marathon last year, we experienced problems with some cell branches being faulty and discharging faster than others. However, our case design consisted of two large 3D printed blocks which slotted over each end, with bus bars being soldered over the printed blocks, essentially locking them onto the top and bottom of the battery. As you might imagine, taking the battery apart and putting it back together was a multi-person operation, and took a considerable amount of time.

 

CAD model of last year’s battery

 

As such, this year we have had a strong focus on the modularity of our design. Each parallel branch of our battery is its own self-contained module, composed of two 3D printed blocks which each have their own copper busbar and individual fuses for each cell. These 15A fuses were soldered onto two squares of nickel strip, one of which was spot welded to one side of the cell and the other was spot welded onto the copper busbar. The spot welding was largely done with help from Dukosi, one of our sponsors, who offered their facilities and expertise during the fabrication process. However, with the welds being so brittle, several did not survive very long, meaning we had to redo some of the welds in-house.

Spot welding at Dukosi

                                            Our battery module

 

The copper busbars are folded round to the shorter side of the module as shown above, and extra busbars are screwed on to make the connections between modules. This has been done in such a way that means that when the battery is assembled, all cross-module busbar connections and BMS wiring is contained to the upper face of the battery, making the battery a lot tidier and easier to work with.



Assembled battery with BMS wiring

 

In terms of the case design, we have opted for an inverted case idea using plexiglass and 3D printed plastic, with the plexiglass lid forming the top and all four sides and the 3D printed part  being only the bottom of the case. This, combined with the fact that all wiring is contained to the top face of the battery, means that work can be carried out on the battery without even removing it from its case. This case design also features a compartment underneath which neatly houses our BMS and a Raspberry Pi Zero for battery monitoring.



The plexiglass cover has been laser cut with a grid of leaf-shaped holes, which will help promote airflow once our cooling fans are attached. The plexiglass pieces have been assembled using acrylic weld cement. Our full battery and case is pictured below.

 

Battery and casing

 

If anyone has made anything similar we would love to hear from you! We also appreciate any comments or questions on our design.