Tesla Model 3 Battery Build

[dougyip]

Background: We decided to build a 3rd battery pack for our Tesla Cobra EV road race car. The 1st 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. However, we 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 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.

[TheSilverBuick]

Wow

[JaniK]

Very cool writeup and project!

[ScythianNite]

does the pack need the aluminum spacers between each module? I assume that's more for the side mount stability than for pack spacing specifically

(disregard, i realized the BMS module needs spacing!)

This is a really exciting development, can't wait to see how the car performs with this!

[SRFirefox]

Thanks for doing this detailed write up! It should help quite a bit when I need a pack for my own conversion.

[jon volk]

How much did that Kia battery weigh?

[dougyip]

About 500 pounds with our stainless enclosure.

[jon volk]

Interesting. I'm thinking that might be an option to give me comparable power outoput but drop some weight. Id have to think both Volt batteries total 650-700lbs at minimum. Id be happy to give up 2kwh of capacity to drop 200 or more lbs.

[dougyip]

Yes, I think the Kia Soul battery is excellent. Despite being air-cooled, we actually never went over temperature (65 degC) during a race. We could easily pull 800+ amps all day long. We were also able to pull a full 27kW-hr out of the pack. The other bonus is that the factory BMS has been decoded and you can get voltages and temperatures from it.

[jon volk]

I think I’ve found a winter project perhaps. Where would I find the CAN documentation? DIYelectric or some Kia forum?

[dougyip]

Kia Soul EV Forum. The data is available via OBDII extended format. I have also gotten the onboard 6.6kW charger to work by broadcasting recorded CAN communications from a working vehicle. When you are ready, PM me and I can send over files.

[Ken_S]

Very cool. I am using a model 3 pack in my conversion... my box design is completely different, I will have to do a writeup when I am done. I am in a cold climate, so I decided to use a high strength fire retardant structural foam to provide most of the structural connection and double as insulation, with mechanical front/rear/top/bottom clamps on the ends and center.

Can you share more details about how you drilled the busbars? I accidentally de-laminated one of mine while experimenting with trying to bend it while I still thought it was a solid bar, and now I am working on an alternative solution. Are you sure they are laminated aluminum, the other busbars Tesla uses in the pack are laminated copper with some kind of silver surface finish.

[dougyip]

We actually just cut off the ends of the busbars with a cut-off wheel on a dremel tool, then re-used existing holes to attach our own connecting bus bars made from Eriflex flat bus bar material. I don't remember seeing any copper colored material at the cut ends.

[Ken_S]

We actually just cut off the ends of the busbars with a cut-off wheel on a dremel tool, then re-used existing holes to attach our own connecting bus bars made from Eriflex flat bus bar material. I don't remember seeing any copper colored material at the cut ends.

Very possible they used different materials for the module busbars, I was just curious. Any chance you have or can take closeup photos of the bolted connections using the existing holes? that was exactly my thought to begin with, but after cracking the laminations on my first attempt at bending it, I was hesitant to bend them up far enough to get a nut underneath.

[dougyip]

If you look at the 2nd photo, you can see one of my connections between two modules. . The 3rd photo shows two studs pre-installed ready for the external Eriflex connection to the HV fuse. I'm using existing holes. To decrease the amount I had to deform the bus bars, I made "backing nuts" out of 1x1x3/16" copper with 1/4 - 28 threads for studs. By using studs, you do not have to get a wrench underneath to back up the nut.

[Ken_S]

Good idea about fabricating a thin plate and threading in a stud. I'm going to try and replicate the technique.

[retrEVnoc]

Very cool. I am using a model 3 pack in my conversion... my box design is completely different, I will have to do a writeup when I am done. I am in a cold climate, so I decided to use a high strength fire retardant structural foam to provide most of the structural connection and double as insulation, with mechanical front/rear/top/bottom clamps on the ends and center.

Really interested in learning more about this. Insulating, fire retardant foam sounds like a winner.

[Johannes.l]

Hi!
Does the Orion BMS work well when you connect it like you did? I'm thinking of temperature sensors and so forth.
I have a model 3 battery pack on the way and I'm thinking of either go with the Orion BMS or wait for the open source board to be available. Both options have their pros and cons.

/Johannes

[FuTurs]

Very cool. I am using a model 3 pack in my conversion... my box design is completely different, I will have to do a writeup when I am done. I am in a cold climate, so I decided to use a high strength fire retardant structural foam to provide most of the structural connection and double as insulation, with mechanical front/rear/top/bottom clamps on the ends and center.

Can you share more details about how you drilled the busbars? I accidentally de-laminated one of mine while experimenting with trying to bend it while I still thought it was a solid bar, and now I am working on an alternative solution. Are you sure they are laminated aluminum, the other busbars Tesla uses in the pack are laminated copper with some kind of silver surface finish.

Ken, Did you ever finish this project? Did you finalize the development of your crimp type edge connection PCB for the interface between the Telsa Flex PCB Model 3 module to the 3rd Party BMS?

[Ken_S]

Ken, Did you ever finish this project? Did you finalize the development of your crimp type edge connection PCB for the interface between the Telsa Flex PCB Model 3 module to the 3rd Party BMS?

I somehow missed the notification on this.

The system isn't fully assembled yet (embarrassed to say, but life got in the way of progress ) However I have connected the Orion Cell taps to the battery modules. The connector I am using is: https://www.digikey.com/en/products/det ... LF/1002508

They crimped easily, I reinforced them with hot glue to provide some strain relief.