Thursday, August 3, 2017

Support Frame for Two Tesla 5.3 kWh Battery Modules

This build will require 6 Tesla battery modules.   After the enlargement of the wheel well (described in this blog 11/21/13) the well provides for a battery box approximately 28.5" (left to right) by 20.5" (front to back).  Although 6 Tesla batteries would fit, to better distribute the weight, 4 will be placed in the wheel well and 2 will be initially be placed in the back seat and later moved to the motor compartment.  The 2 battery box will be made taller to accommodate an Orion BMS controller and it will be 14.125" (H) x 28.125" (L) x 12.625"(W).  The 4 battery box will be 17.625" (H) x 28.125" (L) x 12.625"(W).

Battery Module Mounting

Tesla 5.3 kWh battery modules each have a 3/8" wide "fin" as part of the Aluminum extrusions that serve as the side walls of the battery module.   These fins traverse the majority of the length of the module and we were able to utilize them to support the modules within the battery box. 

DSC05299 showing one of the "side fins" that is 0.125" thick and 22.8125" long.  The 3/8" wide fin runs nearly the length of the module as is shown in pictures DSC05331 and DSC05330.

DSC05531 showing the end point of the fin at the terminal end of the battery module.

DSC05530 showing the end point of the fin at the cooling inlets of the battery module.

DSC05290 showing the outline created with a Sharpie on the 0.5" leg of one 90 degree Aluminum angle used to support the side of the module.

DSC05291 showing the landing area on the 0.5" leg of the Aluminum angle.
The loop (center) extends 0.5" from the module, while the remainder of the fin only extends outward 5/16".  On the opposite side the fin extends a uniform 3/8" for the full length, and there are no loops.  (Picture DSC05287 bottom module).

Picture DSC05287 showing the difference in the fins on opposite sides of the modules.  The bottom fin is 3/8" wide, while the upper module fin is 5/16" with a pair of loops that extend 0.5" from the body.

The first battery box prototype uses a pair of 0.25" thick 6061 Aluminum plates (Alro Steel, , 888-888-2576 at $70.85 each) for side walls.  To each plate (14.125" x 28.125" x 0.25") was then mounted pairs of 6061 Aluminum 90 degree angle (1/2" x 1" x 0.125").  Each pair will function to both support the battery and prevent vertical movement (Picture DSC06046).

DSC06046 showing the Aluminum angles used to support the Tesla battery modules. Each pair was cut flush with the ends of the box and were installed using 1/4" x 20 bolts (17/64" drill).  Final assembly will use nylon insert lock nuts.

Picture DSC05453 showing one of the side panels with dimensions indicated for both the structural threaded rods and the holes for the pairs of Aluminum 90 degree angles used to support the Tesla battery modules.

DSC05355 showing close up details of the hole placement dimensions on one of the two side panels.

DSC05460 showing the pairs of Aluminum angle supports (0.5" x 1.0") placed on what will be the walls of the battery box. 

The internal width of the box is maintained using three 13.5" x 0.5" stainless steel threaded rods at the top, and three 13.25" x 3/8" SS rods at the bottom of the frame.  The top threaded rods will also function as lifting points when moving the box. 

DSC05438 of the rough structure that was used for fitment testing in the BMW prior to trimming the excess length of the threaded rods (final length 14.25").  A red fiberglass insulator sheet is shown at the bottom of the box.  To prevent the threaded rods from contacting the bottom of the battery module, the final design placed the sheet above the bottom three threaded rods instead of below it as is shown in this picture.

DSC05442 showing close up of the bottom of the box prior to relocating the fiberglass above the threaded rods.

DSC05444 of the corner construction detail.  The bottom 3/8" SS threaded rod passes through the 0.125" x 1" x 0.5" Aluminum angle.  The fiberglass was later replaced with black foam and relocated above the threaded rod to prevent any contact of the bottom of the battery module with the threaded rod.

DSC05561 Showing the initial design bottom corner details and how support was provided for the bottom plate.  Note that the red GPO3 is located between the threaded rod and the bottom of the battery module to prevent accidental electrical contact.   The Aluminum end plates ( 12.5" x 14.125", picture DSC05520 at end of blog below) were attached to the support frame with Aluminum angles ( 1.25" x 1.25" x 0.125", picture DSC05518 ) and 1/4"-20 SS bolts.  The bolts were secured with steel press-fit nuts (McMaster 95185A205, 25 at $12.35, picture DSC05474).

Picture 5474 of the zinc plated steel 1/4" 20 press fit nuts.  A 11/32" drill was used to prepare the guide hole and then the nut was forced into the opening using an arbor press or large vice.

Picture DSC05518 showing the press fit nut after installation in the 0.125" Aluminum end panel bracket shown above in DSC05561.  Since the threads are located on the back side of the Aluminum angle, the end plates remain flush with the face of the box frame and tightening of the bolts pulls the press nuts into the Aluminum bracket.

DSC05524 Close up of the installed mounting brackets.  The press fit nut is adjacent to the threaded rod in the middle of the picture.  The next generation battery box should have the corner angle bracket recessed 0.125" from the left edge so that a sheet of 0.125" GPO3 can be placed between the Aluminum end plate and the bracket itself to provide additional electrical insulation for the batteries.

DSC05465 with two Tesla battery modules installed (terminal end) and prior to trimming the bottom threaded rod and attaching the corner mounting brackets. 

Leaving a space for a top fiberglass insulation sheet, an additional bracket will later be added above the top battery module to support an Aluminum plate with the Orion battery management system, a pair of fuses, a pair of Zonka relays, and terminal connections.

DSC05525 end view of the of the battery box (cooling end) prior to the installation of the press nuts at the corner angle brackets and the installation of the end plate as described in this blog on 4/12/17.

Insulating the Modules and Protecting the 18650 Battery Fuses

Each 18650 battery used in the module is connected to the steel faces of the battery pack by 
individual fuses located at both the positive and negative terminals.  The fuses blow at about 25 amps.


The fuses are bonded using an ultrasonic metal-metal friction welding process that combines vertical force, ultrasonic power (60 kHz) and time (100ms). 

Tesla wire bonding patent: 

 During all the prototype work every effort was made to protect these fuses for fear of shorting one and damaging the module.  Careful inspection showed that the fuses are somewhat protected with individual plastic "cradles" that extend slightly beyond the surface of the battery module. 

DSC05273 showing a close up of the individual fuses that connect each battery to a common end plate.  Although the Tesla patent indicates a wire thickness of 0.38 mm, our calipers measured them to be about 0.3 mm in diameter.  The Tesla patent also indicates that each wire is "substantially Aluminum" and the wire bond to the battery is an Aluminum alloy that contains 50 ppm of Nickel and 0.5% Magnesium for additional strength. 

It is desired to place a protective fiberglass cover over each side of the Tesla modules, and I was concerned about the possible fragility of the fuse wires and wire bonds.  Close examination of the battery ends shows that each battery end resides in a clear plastic cradle which extends about 1.5 mm above the metal plate.  This plastic suggests that insulating foam and fiberglass placed against the face of the module would not be in direct contact with the individual fuses.

DSC05272 side view showing the plastic "cradles" that hold each battery and extend 1.5 mm beyond the face of the module.  These cradles will help prevent the fiberglass sheets from contacting and abrading the battery fuse wires.

DSC06047 Each Tesla battery module as supplied comes with a tightly fitting top and bottom protective plastic cover.

To further protect the wire fuses and the entire face of the battery module, it was decided to place individual sheets of electrical grade fiberglass between each module (DSC05281). 

DSC05281 of the 0.125" x 12" x 36" electrical grade fiberglass (also called GPO3) that was purchased to insulate each module from each other.  (McMaster Carr, part number 3345K51, at $18.48 per 12" x 36" sheet).  Foam tape was placed on the underside of the fiberglass to minimize vibration.

Picture DSC05487 bottom view of the box.  The clear plastic protective cover was maintained in the final design.  For clarity the layers were separated to show: (from the left) 0.125 bottom Aluminum sheet, black 3/4" polypropylene foam (0.5" would have been easier to install ! ),  fiberglass insulator sheet (GPO3), clear Tesla protective sheet, and finally, the battery module.

DSC05507 Picture showing all of the layers fully installed prior to adding the Aluminum bottom plate.  It was difficult to slide the Aluminum plate over both the foam and threaded rod, so the foam was cut away as shown adjacent to each threaded rod.

DSC05508 Showing the bottom after the 0.125" x 28.125" x 11.625" Aluminum bottom plate was installed.

Picture DSC05520 of one of the 0.125" x 12.625" x 14.125"end panels prior to cutting holes for the coolant lines (described in this blog 4/12/17)

Picture DSC0644  showing the final side view of the battery box.  Future improvements might include counter sinking the bolt heads or TIG welding the internal Aluminum support angles so that no support holes are required in the battery box walls.

initial posting 8/3/2017
blog updated 8/5/2017, 8/22/2017


  1. Where did you get that angle aluminum by chance? I'm not seeing those dimensions available most places.

    1. We sourced them at McMaster-Carr in Elmhurst Illinois (630-833-0300).

      The part number is 8982K91 at $15.44 per 8 feet.

      McMaster-Carr sells business to business. If you need help purchasing the material let me know and perhaps we can work with you to help you place an order. McMaster-Carr offers a variety of sizes that would work depending upon your design requirements.

    2. Is there a concern about galvanic corrosion? With the aluminum plates being in contact with steel hardware?

    3. Thanks for the offer on the angle aluminum by the way. I found something similar from Orange aluminum here in california.

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