Battery Module Concept Rev 2

This Rev of the battery module builds from the comments on the Rev 1 module, but does not include any CAD for balance wires and connector mounting which will come in the next rev.

This module was designed by Micah Black and reviewed by Robin Pearce and myself.

Terminology

For terminology, see Battery Module Concept Rev 1

Balance Taps

The balance taps have not been included in the CAD of this design, but will located on the top of the top busbars and in the middle of the bottom busbars. The wires will run from these points to center of the module on the top where they will connect to an interface board containing connectors to the module balance taps and AFE. These connectors to the AFEs will be keyed to prevent backwards installation.

Busbar Design

Extra holes have been added in the busbar to provide isolation from the aluminum standoffs, and gusset plate mounting screws. All the cell connections have been redesigned to feature longer slits, forcing the current to go through the cell instead of through the busbar and thus achieve a much better weld point. Extra material has been left around the cells to help distribute heat generated in the copper and not break the copper due to what I expect was thermal shock as we noticed in spot weld testing.

A current density simulation and a prototype to test for hot spots when drawing 100A continuous will be completed before ordering the busbars.

Cell Holders

The protruding points on the top holder have been added to only allow the modules to be inserted one way. When installed, these match up with the divots on the other end of the top plate. I noticed that during assembly, these will catch on the gusset plates and not be able to be installed. These will be moved to the middle of the ends, decreased in length, and increased in width to allow easier installation and decreased chance of breaking during this installation.

All the radii have been increased to be able to be machined with an 1/8" endmill, and all (except M3 clearance holes) will be further increased to be able to be machined with a 1/4" endmill in the next revisions.

Material has been removed from the circles around the cells so that each group of 4 cells in completely enclosed instead of every single cell. Holes have been added in the middle of the cell groups at interstitial locations to reduce weight further.

Aluminum standoffs were added at 4 points central to the module to provide a secondary mechanism to keep the modules together (other than friction). These standoffs are completely isolated from the busbars and any other electrical part of the pack. These aluminum standoffs will also be covered with heat shrink to further reduce chances of shorting with damaged modules.

Gusset Plates

The gusset plates have been redesigned to feature isolated screws and captive nuts. We should no longer have to worry about tool shorting when installing the packs, and will only need to use one tool to install the nuts. Since the bolts are now isolated, the gusset plates are also the isolation covers that were previously on top of the modules. When installed, there is no exposed part of the pack that is not isolated.

The bolts and nuts clamp against the 3D printed plastic gusset plates. Robin was rightly concerned about the amount of clamping force that these bolts will be putting on the plastic, and the plastic will most likely crack and/or not allow the full clamping force, and creep over time to diminish the clamping force and thus increase the contact resistance. While the captive nuts are a great idea, it was also attempted in MSXII only to have the holes strip and no longer function as desired. We should look in to the required clamping force and calculations for the forces on the captive nuts, as well as make a prototype to validate these calculations and see if captive nuts are actually feasible. We will also look in to using other types of insulating washers (ceramic, hard fiber) to isolate the bolts while not clamping on the plastic.

Mounting for the gusset plates has been improved to include 3 screws through the top plate that screw into heat set inserts inserted from the bottom of the top plate before assembly. With this design, the gusset plates are much more secure and will not move if bumped during assembly. We will attempt to move the holes for the outer M3 screws that attach to the gusset plates further out so that a complete hexagonal hole around all the captive nuts can be achieved.

For ease of manufacturing, the supports beside the middle bolt hole will be made into full ribs so we don't have to deal with extra support material or suboptimal print overhangs.

When attempting to strip the holes for captive nuts on a loosely fitting FDM prototype, I could not break them without putting what seemed like an unreasonable amount of torque on them. This is all qualitative testing and proper calculations should be done, but I do not believe this will be a huge problem as long as we do not have to reassemble the pack 50 times (which is a possibility). With the captive nuts, it is also good to note that the force to hold the nut still is now transferred to the gusset plates and its connection to the modules instead of a wrench.

The holes around the M6 bolts will also be increased in size to accommodate a large tolerance when we assemble all the modules, as tolerance stackup becomes an issue.

Isolation Plates

We are still keeping a 2 layer approach to the insulation. The first layer will be fish paper (electrical grade vulcanized fiber) with an adhesive backing that is applied to the whole busbar immediately after spot welding to avoid potential short circuits when manufacturing. Fish paper is widely used in the electronics and telecommunications industries as insulation material, and seems like a logical choice. Better materials such as Formex are suitable but finding a distributor has proven difficult, and will likely be much more expensive.

The second isolation plate will be made of a rigid material, attached to the top and bottom of the pack likely with adhesive tape and the screws that will hold the aluminum standoffs in place.

Pack Layout

The pack layout has not changed from Rev 1.

Module Mounting

The modules will still be mounted with discs glued to the bottom of the enclosure at the corners of the modules. In the outer rows, these will be changed to also be the mounting points for cable management brackets.

Poron (polyurethane) foam will be added to the bottom of the modules to prevent the screws on the aluminum inserts from scraping during manufacturing when module are resting on tables. This will also provide cushioning and vibration dampening when installed in the pack.

Other Comments

Another good iteration, let's keep making it better!