Asselin.Engineer

DIY Electric Longboard Part 3: Battery Pack

2018/09/14
ece star

This page is part of our home made eskate build.

Design

Design choices were discussed in depth in part 1.

As discussed in part 1, we chose to DIY our battery pack with 30x Sony VTC6 18650 cells. They are high current and power dense cells. We went with a 10S3P configuration to reach our goal of 1500W power.

The pack was designed to be able to reach 1500W of power and to be reusable for other applications.

Case

The 3D CAD was designed in SolidWorks and we 3D printed it on a CR-10. Aluminium plates were added to the top.

The plan

Copper Busbars

We wanted a pack that could be repaired easily so we decided to clamp the cells together instead of soldering. Our battery pack is almost solder free, except for the battery management system balancing probes. This is possible because we used thick copper for our busbars and pressed them to allow clamping the batteries together. Poron keeps pressure on the copper to clamp the copper busbars to the cells.

Nickel Plating Sidequest

More info on this sidequest: electroplating nickel onto copper. We decided to go bare copper.

Cut and Press

To link the batteries together, we needed busbars capable of handling 40A while still being smaller than the diameter of the cells. We calculated 0.6mm copper was enough. We bought sheets and cut them to the size we wanted busbars.

Pressing the copper was long. Michel designed a tool to make sure the pressure on all cells is the same. We pressed them on each side with a vise.

After, we had to solder the BMS wires on the busbars. Lots of heat was required and doing this in the case allowed the poron to stick to the copper.

Lots of heat required to solder BMS wires to copper busbars
Pressing tool

Cells

We carefully inserted the cells in 10S1P configuration to test the complete system. In case of failure we would only lose 10 cells instead of 30.

We were able to control the motor with the remote with this configuration! Success!!!

After, we added the remaining cells to reach our desired 30 cell 10S3P setup.

We lost 2 cells during the process. Clamping was harder than we thought after the first 10 cells. The pressure required to get the cells to their position on the center busbar made us pierce the green plastic and short circuit B- (the steel case on 18650 cells is B-). Using a wrench wrapped with electrical tape, we compressed the poron and were able to secure them properly.

There is no way the cells can move from their position. They are extremely tight and that is exactly what we had planned.

First complete eskate integration test was a success!
Carefully adding cells
Completed
Final Battery Pack! 1500W!

The board rides way too fast with this battery pack. It is too powerful and certainly dangerous. We will have to limit the power / change gears.

Annex: Poron

Poron == Rogers PORON® foams

To protect the cells and compress them, we used poron. Poron is a great material. Unfortunately, there isn't much information available online about the material. Here is what I found.

Poron does not conduct electricity and is insulating. Also, it's compression specifications supposedly do not vary much with age. These conditions are important for us and make poron a great material to compress busbars.

There are many types poron sheets available on the market. From what I found, they are exclusively sold by large engineering material suppliers. When you google "poron foam supplier", the first links are from Rogers corporation. They do not display any prices on their website.

We found a source with prices on McMaster-Carr and https://www.rogerscorp.com/ems/poron/industrial/index.aspx.

McMaster is a supplier of almost anything mechanical engineering (they also have cheap automotive wire 50A 100% copper wires. Nice thick wire for eskate). Their source of poron is Marian Inc.. There is almost no info on their foam page, except that they "We work with industry leading manufacturers of high performance foams like Rogers PORON® foams" (2018). So McMaster <- Marian <- Rogers. With some more research, I noticed even Alibaba sellers are resellers of Rogers.

So we tried buying poron sheets from Rogers. They did not want me to buy them but they sent samples. I am not sure anyone can get samples from Rogers corp. It's probably easier for you and them if you buy from McMaster. In our case, the battery pack is going to be reused as a prototype portable battery system for a coffee machine company. If that works, we'll need a ton of poron. That is possibly why we were able to get samples from rogers.

The first samples we got were way too soft. When I asked for rubber-like compressibility, that's when we received the dream stuff.

1) 4701-30-20125-04 (3.18MM) Too soft

2) 4790-92-15188-04 (4.78MM) Too soft

3) McMaster: Adhesive-Back Sheets, Soft 3/16: Too soft but used as vibration dampen.

4) 4701-50-20125-04 (3.18MM) Firm (Up to 12mm! Could have used this one)

5) 4701-60-20125-04 UR (3.18MM) Very firm

6) 4701-60-15188-04 UR (4.78MM) Very firm (BINGO!!!)

4701-50 I can't compress it more than 25% with my bare fingers.

4701-60 is the hardest they offer in this category. For harder or for impact protection, I would look into their Rogers PORON® XRD® foams. They even have a youtube channel displaying usage.

I put the first two soft samples in a vice for 24H at maximum pressure. As soon as I removed them, there was no way to tell which area of the sheet was pressed. On both samples. I don't know much about material science but that was pretty impressive.

Rogers Corporation seem to be the leaders in this kind of material. They were hard to find but their poron brand is very impressive. 100% worth it.