Current Sense, Contactor, Fuse, and Busbar Holder

Owned by Micah Black
Last updated: Oct 22, 2020
5 min read

Final Part Pictures, Location, and Use:

This part holds the busbar, contactor, fuse, and current sense module for the high voltage side of the battery box, arranged as shown in the images below. There are multiple features that are important to consider here including mounting the parts and providing enough torque resistance in order to properly tighten the bolts for low electrical resistance, assembly order of all the parts, strength to support the top side of the battery modules, still allowing airflow through the battery modules, and keeping the wiring neat.

Airflow

The batteries must still be cooled, so this arrangement of the parts must still allow openings for air to come through. Many extra holes were added to the part to allow this to happen.

Locating Features

All of the parts that fit in this part have a feature that will locate the parts over the holes so that they can be bolted in while not worrying about holding the part in place at the same time. These features are pretty much just a ridge around the outer edge of all the parts to keep them in place.

 

Current Sense Mount

A cutout was made in the bottom side of the current sense mount to allow the MX150 connector built in to the enclosure to come out. By installing it on an angle and then dropping it in, the part can be installed without interference (I’m pretty sure).
On the face towards the front of the image, I added an indent so that a cover can be located over the top and held in place.
The current sense has a specific direction of the current flow over its terminals outlined on the PCB and in firmware so we stuck with this orientation. I would have loved to flip it around, but the other thing to remember is that we need access to the circuit board during ASC scrutineering and BMS testing to simulate overcurrent faults, so the circuit board was left facing upwards.

 

Wiring Path

The red lines in this image shows the locations of the busbars. They go underneath the fuse and the current sense boards, but need to allow lots of room over the top of the connector on the current sense to allow room for the mating connector.

 

 

Busbar Cutout

The busbar that connects the highest voltage side of the battery module stack directly to the fuse. It also covers it in order to give some isolation.
The busbar goes along the 3 bolt holes, and then comes out in the slot near the fuse.
It will be made from 1.5mm thick water jet cut copper plate. It will sit underneath the fuse, and attach with an M8 bolt into the molded-in insert underneath.

Notice the cutouts in the busbar indent just below the two locating indents for the modules (between the 3 bolt holes). Here, these were created because of assembly order. The modules get installed by placing them in vertically, and we need to be able to remove the modules vertically as well. These cutouts were created to allow the busbar to bend around the locating features on the module so that this module can be installed vertically. Note that this method requires the busbar to be bent to shape to allow this to happen.

Another option would be to not allow this module to be removed vertically without first removing the 2nd module in the row. When the 2nd module in the row, then we can lift the end module and slide it back so that the bottom module locating features do not catch on the busbar, even if it is not bent.

 

 

Mounting Points for Parts and Electrical Connections

The bolts need to be able to be installed with a single tool to avoid the risk of electrical shock and using 2 hands inside the battery box (stick to the 1 hand rule). As such, the nuts (or threads) that the bolts hold to must be captive inside the parts, and allow the bolts to be torqued to provide low electrical resistance.

To achieve this goal, we use molded-in inserts. Molded in inserts have a greater torque-out resistance than heat-set inserts since they are used with thermoset plastics instead of thermoplastics. Thermoplastics (including 3D Printed materials such as PLA, PETG, Nylon) will re-melt under heat and deform under pressure. Thermoset plastics (Epoxies, 2-component adhesives, etc.) do not re-melt, and typically form cross-linked chains while curing. This makes them much harder and stronger. The other use of molded-in inserts over heat-set inserts is because inserts designed for molded applications have larger grooves that allow the plastic to flow into the grooves and have greater area to resist the force generated by the torque.

The stack-up for these electrical connections and mounts will be:

  • 3D Printed (Thermoplastic) outside part as a mold for the epoxy (Thermoset Plastic). This has features to resist torque on the epoxy to 3D printed part bond. These ‘spokes’ going in to the middle of the hole are there to align the molded-in insert in the proper position while the epoxy is curing.

  • Epoxy in between the molded in insert and the 3D printed part allow a thermoset plastic to make direct contact with the insert where the higher forces are required to resist the torque-out, and transfers the load to a larger diameter thermoplastic hole so that less force is required/exerted on the thermoplastic.

  • Molded-in insert.

  • Oversized stainless steel washer on top molded-in insert. This should be attached with some CA glue or other glue to the insert prior to molding in the printed part. This prevents any overflow epoxy from flowing in to the screw threads.

 

 

 

That was a long page… And I still haven’t gone over the manufacturing techniques yet.

 

To touch on that quickly, print using PETG for better heat resistance and use modifiers in Prusa Slicer when printing out the part to give 100% infill and 4 perimeters to the areas that will take large loads - the areas around the molded inserts especially.