Model 3 Battery Pack for Tesla Cobra EV

We decided to build a 3rd generation battery pack for our Tesla Cobra EV road race car. Our 1st pack was from a 2015 Kia Soul EV. It had very high power output (we were able to pull 375kW), but the capacity (27kwh) was a little short for our 20 minute races. We replaced it with 2/3 of a Chevrolet Bolt pack. Despite being water cooled, we found this pack ran hot. To keep it cool, we had to resort to building custom cooling plates between every cell pair. We did manage to keep the back cool, but were never able to get the capacity we wanted due to high voltage sag under load.

Despite the weight penalty, we decided that the Tesla Model 3 pack would give us the capacity we wanted. From our single cell test data, we measured the cell capacity at 4.5 Ah at 20 amps constant draw. This equates to 74 kwh for normal use. Under racing conditions, we estimate that we will have at least 48 kwh usable.

There is a fuse wire on each 21700 cell that will limit the maximum current that the pack can pull. We measured these to be .035” aluminum. We believe these are good up to about 30-35 amps peak. We have run the completed pack up to 1050 amps (25 amps per cell) for 15 second bursts with no problems.

One of the things that attracted us to the M3 modules was their water cooling arrangement. It is much improved over the model S. There is a parallel water header on each end of the module. We built a custom manifold for each end for connection to our cooling system. We are utilizing a Mezeire 70 lpm pump and a full size aluminum radiator. If you are buying individual modules, make sure that you get the water connections as they are not readily available!

The modules will be difficult to fit in many cars. The only way the pack would fit in our car was to stack all four modules on top of each other. Even at that, we had to modify our chassis to fit.
Short modules. 23 cell groups. 185 lbs. 70.5 x 11.5x 3.5” tall (4.25” including BMS). Length is measured from the outside-outside of the water outlets.
Long Modules. 25 cell groups. 220 lbs. 76”x11.5”x3.5”

Because there were no known solutions for communicating with the BMS boards when we started on the project, we elected to use an Orion 2 BMS system.
The cell tap wiring for the factory BMS is thin film aluminum tape bonded to the top case. The BMS boards attach to the aluminum tape via thin ultrasonic bonded wires. It is very difficult to solder to the aluminum pads, so even if you don’t use the BMS, leave it in place so you can connect your BMS wires to the conventional solder pads on the board.

The factory connection points for the bus bars/cable that connect the modules can be cut down and new holes drilled to shorten the modules - however beware that the material is laminated aluminum so you must be careful when cutting/drilling so you don’t separate the laminations.
The only structural part of the module casing are the sides where the factory hold downs are. The modules must only be supported from the sides and cannot be allowed to rest on the bottoms. The bottom is exposed foam insulating material that covers up the fusible links.

The only structural part of the module casing are the sides where the factory hold downs are. The modules must only be supported from the sides and cannot be allowed to rest on the bottoms. The bottom is exposed foam insulating material that covers up the fusible links.
The hold down points on each of the modules are in different positions and except for the small modules are difficult to attach to. To “simplify” matters, we elected to cut off the Tesla mounting points and bond aluminum angle down the sides. We do not recommend this as the plastic is difficult to bond to! We also learned that at elevated temperatures, the strength of most epoxies is shockingly low. We ended up having to use an expensive 3M structural epoxy, DP820.

We used Misumi 20 series extrusion to build a framework around the modules. The framework was sheeted in aluminum. The bottom is steel tube with a Carbon-Kevlar panel from penetration resistance.
Total weight of the finished pack and enclosure is 850 pounds. Exterior dimensions are 77” long x 13.5” wide x 18” high.