Battery Module Concept Rev 1

This version of the battery module was designed by Eric Chiang. I (Micah Black) am attempting to document some of the features that I noticed required changes and features that were a good addition on this module, and have iterated on for the next concepts. Overall, this was a great starting point for the design, and can be improved upon greatly in the details.

Terminology

Busbars - The main current-carrying conductors within the pack. Connects the cells together in parallel and series arrangement of 24P2S per module. The busbars will be laser cut from EMS Sigma 60 material, which is nickel and stainless steel clad copper (60% copper, and spot-weldable due to nickel and stainless).

Gusset Plates - Orange parts seen above to provide structural support to the vertical mounting points of the busbars.

Insulation Covers - Black rectangular covers used to insulated the screws on busbar connections.

Cell Holders - These will be custom machined from acetal plastic to aid in mounting and busbar connections. Have circular holes for the cells and other mounting features for gusset plates, isolation plates, etc. These are the grey parts on the top and bottom of the modules.

Isolation Plates - Yellow plate seen on top of the modules, covering the entire horizontal surface of the busbars to isolate the busbars.

Balance Taps - Points where the balance wires connect to the busbars. The balance wires are used to measure the voltage of every cell in the pack.

Alignment Pins - Pins to align the isolation plates and gusset plates on top of the module. Can be seen along the top edge of the cell holders.

Balance Taps

The balance wires on this module are connected via screw terminals to the top corners of the busbars and to the middle of the bottom busbar. When removing modules from the pack, the screw terminals will have to be removed from the modules, as well as thermistor connections. The next design will feature an intermediate connector board to make it easier to remove modules from the pack, and simplify pack wiring. The wiring of balance taps and thermistors will be internal to one module and broken out to a single connector. The balance wires will be soldered to the busbar (unless the thermal and mechanical stresses could risk breaking the solder, in which case we will probably try to spot weld and crimp the balance wires).

Busbar Design

The busbar to cell connections seem a little thin to me. In the testing done while spot welding, the strips seemed to melt and break off from the heat, as the current was travelling through the copper instead of through the cell, and there was very little material to remove the heat. While we do want to create heat for spot welding, I believe that forcing the current path through the bottom layer will prove much more effective than a small area to heat up. Longer slits will be used to force current through the cell, and more testing will be done once we have prototypes in. The current flow out of the cells will be directed towards the busbar connections to avoid sharp turns in the current direction, lowering current density, local heat, and reducing the resistance path to the inter module connections. The design being evaluated only uses 1/2 of the total busbar cross section to deliver current effectively.

The other issue with this design for busbars is the negative cell can spot weld positions. It is common knowledge in the battery building communities that the center of the negative side of the cell should be avoided when spot welding to the cells. This is due to the internal construction of the 18650 cell, where too much heat or pressure on the center of the cell is more likely to damage the cell than at the sides, as the positive current collector flows down the center of the jelly roll inside the cell, and by geometry, the center of the cell is less supported. A small mistake putting too much pressure in the center could cause an internal short and thermal runaway. The picture below comes from this great article on the internals of an 18650 battery: https://www.electricbike.com/inside-18650-cell/

Cell Holders

Make these more machinable with less tools - the small radius of 1.5mm around some of the edges will require a separate tool change during machining. Also, fix the cell holes from 18.0mm to 18.3mm, the actual diameter of the cell. Adding a lead-in chamfer around the holes is also required so that we can lower the tolerance on the hole size and avoid stripping the PVC insulation from the cells when inserting them, and making the top plate easier to install when trying to push it on to 48 cells. This design also requires a good friction fit between the cells and the cell holders in order to hold the modules together without putting stress on the spot welds. More holes and cutouts will be added to significantly reduce weight. A standoff that goes through the center, screwed to both plates will be used to secure the pack without relying on the friction and spot welds. There are no features on the cell holders that do not allow modules to be installed backwards, a suitable locating feature will be added to the next concept. Mounting for the isolation materials and gusset plates will also be improved so that it cannot simply be pulled off the alignment pins. There is also lots of weight reduction that can be done for these parts.

Gusset Plates

With our busbar configuration, our contact resistance depends directly on how smooth we can keep the busbars. Minimizing this contact resistance is essential to limit power losses and heat generation. The gusset plates are required to keep the busbars flat and vertical in order to prevent any dings, bends, etc. during manufacturing, assembly, and maintenance.

Isolation Covers

The isolation covers seem to me like a redundant extra part. With the changes being made to the balance taps, and redesigning the gusset plates, we can merge these two parts into one, using captive nuts on the negative side of the busbars, and insulating the screws from the busbars.

Isolation Plates

My only concern with the isolation plates is that they will come off easily due to only being attached with alignment pins. They will serve their purpose just fine, even though they may be a little heavy. I will be looking in to other materials to use for insulation that are lighter, thinner, and easier to assemble (adhesive backing).

Pack Layout

The overall pack layout is very well done. Both positive and negative terminals are at the rear of the pack which allows for easier mounting of HV components and eliminates unnecessary long wiring runs. The current flow through the pack is easy to follow, and the assembly does not have any major foreseen issues. The pack layout allows all modules to be identical, including the end modules which makes manufacturing and maintenance much easier. The space between the modules at the rear can be used for AFEs and other battery management boards.

Module Mounting

In this design, the modules are mounted to the bottom of the pack using the black circles seen at the corners of the modules. These black circles will be glued to the bottom of the pack. A potential issue with this is the accuracy with which the circles must be glued to the bottom of the pack. A simple MDF jig can be used to place them accurately. Making the circles extend outwards and between the modules will also allow for a great surface area for bonding. Calculations should be done to determine the shear force that the circles and glue will see. If we can keep the parts as circles, that would make manufacturing a lot easier, as they can be cut from a rod (probably acetal).

Other Comments

First rev was great, revealed potential issues and areas to be streamlined, and provided an amazing design to work off of for improvements.