For the Midnight Sun Solar Car Team (uwmidsun.com), I have been designing a building a prototype battery module. This module must fit in to the pack to be modular, safe, easy to manufacture, easy to assemble, and have low power loss.
This module will be a 24P 2S module, made up of a variety of materials and sections. This was made so that all of the access points for the terminals will be on the top of the modules and we will not need to reach any tools into the bottom in order to put the pack together.
Layer stackup:
- Gusset Plates (red and black) and Fuse Board
- Top Section:
- Acetal Cover Plate (Laser Cut)
- Electrical Grade Vulcanized Fiber (Fish Paper) Insulation (Laser Cut)
- EMS Sigma Clad 60 Busbar (Water Jet Cut)
- Nickel Strips (3D Printed Bending Dies)
- Acetal Cell Holder (CNC Machined)
- Cells and Aluminum Standoffs
- Mirror Top Section for Bottom
Mechanical Considerations:
The module has an acetal plate on the top and bottom with pockets to fit the cells and hold them in place. In order to avoid relying on the friction between the cells and the acetal plates holding the modules together, there are aluminum standoffs connecting the plates together. M3 bolts go into the top and the bottom of these standoffs to clamp the pieces together.
Above the acetal plates, we have a laser cut piece of fish paper that is used to cover the busbar and provide isolation. And above that fish paper, there will be another acetal plate to provide extra isolation protection and a more durable module. Since the fish paper is susceptible to water damage, this will also help to minimize the water that the fish paper comes in contact with during assembly/handling.
The red and black pieces on top of the module are gusset plates that will support the vertical busbar connections that will connect the modules in series.
Electrical Structure:
We wanted to minimize the total resistance of the high current path in order to minimize the power loss.
The material of choice for the high current carrying busbars (eliminating superconducting materials from the options) is copper, due to its low resistivity. Unfortunately, due to this property, copper also is difficult to attach to the cells. Resistance spot welding – the most common and cheapest method of battery assembly for hobbyists – requires the material to have a high(er) resistance in order to generate the heat to weld the material together when passing current through it. To solve this issue, we are using nickel and stainless steel clad copper from Engineered Material Solutions (EMS). This allowed us to use a resistance spot welder to connect the cells and the busbars, as the stainless steel provides a higher through-plane resistance – with the copper still providing very low across plane resistance. The other downside of having the copper in the material is having a high thermal conductivity, which wicks the heat of the weld away. With our spot welder, the K-Weld, we were not able to get consistent welds on the 0.3mm thick EMS material for more than 10 consecutive welds. Because of these issues, we switched gears and threw some nickel strips in the design. These connect the cells to the busbars, and allow a low weld energy to be used. The lower weld energy also increases the safety of the cells because there is less electrolyte that evaporates (causing internal impurities and thus eventually internal shorts) inside the cell – this info came from a post on the electricbike forums (https://www.electricbike.com/introduction-to-battery-pack-design-and-building-part-3/). The power loss from adding the nickel strips in was calculated to be negligible compared to the power loss due to the internal resistance of the cells.
The votage taps for measuring the voltage of each cell come off of tabs on the busbar. These connect to fuses and resistors on the voltage tap breakout board. The resistors will help to limit the current in the case of a short and will help to spread out the heat from the balancing resistors. The fuses will blow if anyhting bad happens. This breakout board on the top of the module connects to the voltage taps through a Molex MicroFit connector for easy disassembly in case the replacement of a fuse is required. These connectors are protected against offset installation and ‘scooping’ which could connect the wrong cells to the voltage taps – a very important feature. Having the fuses and the connector on the board enables us to switch to a distributed BMS if we choose to in the future, having the voltage measurements happening at the cells instead of on a centralized Analog Front End (AFE).
Manufacturing:
This was the fun part where I taught myself MasterCAM to program the 2.5D double sided tool path to machine the acetal plates the hold the cells in place. The Haas VF2 in our university’s machine shop made light work of the acetal.
Water jet cutting the busbars from EMS Sigma Clad 60 and the making a 3D printed die for bending the nickel strips into shape.
The top and bottom acetal insulation plates were laser cut using the Epilog Helix cutter at the University.
Assembly:
The whole assembly process was thought through and designed into the part, so that it all went together fairly smoothly, save for 1 part. The holes in the acetal capture plates were made so that the cells fit snugly when inserted individually. The problem comes when you are trying to put 48 cells in at the same time. It took quite a bit of force provided by c-clamps and distributed with pieces of wood. Eventually it went together, but the holes on future versions will definitely be increased in size. The burrs on the waterjet busbars made it difficult to slide the terminal connectors on them, so we ended up soldering them on for this rev and will order thicker terminal connectors for future revisions. All the red and brown wires you can see coming off the cells are thermistor wires that we attached for thermal characterization of the battery modules.
And now we have our first finished battery module prototype! Stay tuned for the results of thermal characterization and further testing.
