Tag: 18650

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Design and Build of Prototype Battery Module

For the Midnight Sun Solar Car Team (uwmidsun.com), I have been designing a building a prototype battery module. This module must fit in to the pack to be modular, safe, easy to manufacture, easy to assemble, and have low power loss.

This module will be a 24P 2S module, made up of a variety of materials and sections. This was made so that all of the access points for the terminals will be on the top of the modules and we will not need to reach any tools into the bottom in order to put the pack together.

Layer stackup:

  • Gusset Plates (red and black) and Fuse Board
  • Top Section:
    • Acetal Cover Plate (Laser Cut)
    • Electrical Grade Vulcanized Fiber (Fish Paper) Insulation (Laser Cut)
    • EMS Sigma Clad 60 Busbar (Water Jet Cut)
    • Nickel Strips (3D Printed Bending Dies)
    • Acetal Cell Holder (CNC Machined)
  • Cells and Aluminum Standoffs
  • Mirror Top Section for Bottom

Mechanical Considerations:

The module has an acetal plate on the top and bottom with pockets to fit the cells and hold them in place. In order to avoid relying on the friction between the cells and the acetal plates holding the modules together, there are aluminum standoffs connecting the plates together. M3 bolts go into the top and the bottom of these standoffs to clamp the pieces together.

Above the acetal plates, we have a laser cut piece of fish paper that is used to cover the busbar and provide isolation. And above that fish paper, there will be another acetal plate to provide extra isolation protection and a more durable module. Since the fish paper is susceptible to water damage, this will also help to minimize the water that the fish paper comes in contact with during assembly/handling.

The red and black pieces on top of the module are gusset plates that will support the vertical busbar connections that will connect the modules in series.

Electrical Structure:

We wanted to minimize the total resistance of the high current path in order to minimize the power loss.

The material of choice for the high current carrying busbars (eliminating superconducting materials from the options) is copper, due to its low resistivity. Unfortunately, due to this property, copper also is difficult to attach to the cells. Resistance spot welding – the most common and cheapest method of battery assembly for hobbyists – requires the material to have a high(er) resistance in order to generate the heat to weld the material together when passing current through it. To solve this issue, we are using nickel and stainless steel clad copper from Engineered Material Solutions (EMS). This allowed us to use a resistance spot welder to connect the cells and the busbars, as the stainless steel provides a higher through-plane resistance – with the copper still providing very low across plane resistance. The other downside of having the copper in the material is having a high thermal conductivity, which wicks the heat of the weld away. With our spot welder, the K-Weld, we were not able to get consistent welds on the 0.3mm thick EMS material for more than 10 consecutive welds. Because of these issues, we switched gears and threw some nickel strips in the design. These connect the cells to the busbars, and allow a low weld energy to be used. The lower weld energy also increases the safety of the cells because there is less electrolyte that evaporates (causing internal impurities and thus eventually internal shorts) inside the cell – this info came from a post on the electricbike forums (https://www.electricbike.com/introduction-to-battery-pack-design-and-building-part-3/). The power loss from adding the nickel strips in was calculated to be negligible compared to the power loss due to the internal resistance of the cells.

The votage taps for measuring the voltage of each cell come off of tabs on the busbar. These connect to fuses and resistors on the voltage tap breakout board. The resistors will help to limit the current in the case of a short and will help to spread out the heat from the balancing resistors. The fuses will blow if anyhting bad happens. This breakout board on the top of the module connects to the voltage taps through a Molex MicroFit connector for easy disassembly in case the replacement of a fuse is required. These connectors are protected against offset installation and ‘scooping’ which could connect the wrong cells to the voltage taps – a very important feature. Having the fuses and the connector on the board enables us to switch to a distributed BMS if we choose to in the future, having the voltage measurements happening at the cells instead of on a centralized Analog Front End (AFE).

Manufacturing:

This was the fun part where I taught myself MasterCAM to program the 2.5D double sided tool path to machine the acetal plates the hold the cells in place. The Haas VF2 in our university’s machine shop made light work of the acetal.

Water jet cutting the busbars from EMS Sigma Clad 60 and the making a 3D printed die for bending the nickel strips into shape.

The top and bottom acetal insulation plates were laser cut using the Epilog Helix cutter at the University.

Assembly:

The whole assembly process was thought through and designed into the part, so that it all went together fairly smoothly, save for 1 part. The holes in the acetal capture plates were made so that the cells fit snugly when inserted individually. The problem comes when you are trying to put 48 cells in at the same time. It took quite a bit of force provided by c-clamps and distributed with pieces of wood. Eventually it went together, but the holes on future versions will definitely be increased in size. The burrs on the waterjet busbars made it difficult to slide the terminal connectors on them, so we ended up soldering them on for this rev and will order thicker terminal connectors for future revisions. All the red and brown wires you can see coming off the cells are thermistor wires that we attached for thermal characterization of the battery modules.

And now we have our first finished battery module prototype! Stay tuned for the results of thermal characterization and further testing.

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Individual Cell Testing in Large Battery Packs

I have been very fortunate to be part of the Midnight Sun Solar Car Design Team at the University of Waterloo (uwmidsun.com). As the Battery Lead for our next solar car (MSXIV), I am responsible for designing, prototyping, testing, manufacturing, and integrating the battery.
This discussion/post/article/whatever you want to call it will go through some of the considerations when building a large battery pack for high performance applications including electric vehicles. This is not as critical for the performance of say a home-built powerwall from recycled cells, but from a safety perspective is probably more important in such an application as the cells may already be degraded (in invisible ways – pending internal shorts) from their first use.
Lets start off by posting a PDF of some research/literature review that I did a few months back to determine if individual cells should be tested for a solar car battery pack. The result of my evaluation is that basic individual cell testing should be conducted, but based on time frames, a full capacity measurement will not be done. If we had access to a highly parallel capacity testing rack (which I have had some ideas on building one – might come in the future) then capacity testing would be good to do.
Click the link below to download the PDF.
The testing was done with a Keysight B2902A SMU and a custom scale made with Phidgets hardware. Our goal was to be able to detect manufacturer defects that could cause cells to heat up or fail prematurely. To this end, we testing for DC and AC Internal Resistance, Self Discharge Current, and cell weight, and an estimate of capacity through differential capacity through the capacity ration.
A custom scrip tin Python was created to interface with both instruments and automatically collect the data into CSV files. The testing was completed in a short timeline (36 hours) and, we collected data for 1400 cells. The CSV files collected made up over 6GB of data.
To process the data, more python was used – if you can’t tell yet, I really like python, especially when tools like matplotlib and numpy make data processing super easy.
These will be following the guide I have outlined here:
Once we have processed the data, it will be used to identify outlier cells and match them in order to create and most balanced pack as outlined in the Individual Cell Testing Evaluation Guide linked previously.
With all of this data, we hope to create a battery pack that will power Midnight Sun XIV on its 3000km journey across the United States during the American Solar Challenge in the summer of 2020.
Written By: Micah Black
Project By: Micah Black
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Machining Acetal for 18650 Cell Holders

Please note that this is a quick writeup, and not intended to be an independent guide to machining or battery pack design.

I have been very fortunate to be part of the Midnight Sun Solar Car Design Team at the University of Waterloo (uwmidsun.com). As the Battery Lead for our next solar car (MSXIV), I am responsible for designing, prototyping, testing, manufacturing, and integrating the battery.

Our battery pack is made from 864 18650 style cells in a 24P 36S arrangement, for a nominal voltage of 131V and capacity of 10.8kWh. These 18650 cells are held in place at the top and bottom using acetal plates. In the image below, the acetal plates are the grey plates at the top and bottom of the cell supports.

The choice of acetal was made primarily because it is easy to machine, creating lots of chips and not melting too easily. After finishing the design in Solidworks, I imported it into MasterCAM 2020 where all of the toolpaths were created. This was my first real project in MasterCAM, but I am glad that I spent the time to learn the software as this opens up a whole new area of possibilities for manufacturing for future projects. Anyways, the part was designed so that no endmills smaller than 1/4″ were required, and could be milled using only 2.5D operations. The process was relatively straightforward, aside from getting used to all the different plane naming in MasterCAM. Because I had created this part to be machined, all the toolpaths were easy to pick out, though time optimizations could definitely be made. This round, I was only making 2 parts for the prototype module, but in a month or so that full production will be headed to the machine with much more optimized gcode.

The University of Waterloo has a Haas VF2 CNC Machine that students can use for design team projects, and is the machine that this part was created on. Again, it was a relatively simple process of setting tool offsets, setting the part zero, then loading the program.

After testing the sizes of the cell holes, we decided to go back over the holes and enlarge them to ensure a smooth fit into the slots and not destroy the PVC insulation while inserting or removing the cells.

From design to final product, this has been a huge learning experience for me, and I hope to continue to improve my machining skills in the future.

I’m looking forward to doing some lightweighting on this part for the final manufacturing run, but have learned a lot in the prototype process.

Written By: Micah Black

Project By: Micah Black

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LiitoKalaa Engineer Lii500 Reliability, Accuracy, and Repeatability Test

Accurately testing 18650 li-ion cells is a very important step in the process of re-using cells from laptop battery packs, or designing a new battery pack in order to create matched modules and maintain a minimum amount of balancing current to keep the modules balanced.
During my first term at the University of Waterloo, I joined Midnight Sun, the solar car student design team and am working on designing a new battery pack for the next car, MSXIV (Midnight Sun 14). During this process, we are looking to accurately test every single cells that we putting in to the car in order to determine their capacity and internal resistance. This will allow us to create perfectly matched packs if the testing is completed accurately.
New cells are being used in this car, and the tests must be able to distinguish cells that fall within the manufacturer’s specified tolerance ranges for the cells. That means that the testing method that is chosen must be accurate to 10mAh for the capacity and ideally less than 1mOhm for the internal resistance.
And so started my journey of figuring out how to test all the cells in a timely manner while not spending tens of thousands of dollars on proper commercial testing equipment.
Starting with, one of the cheapest and most popular cell testers on the market, the LiitoKalaa Engineer Lii500.
I have 9 such testers, part of my cell testing station, and used 9 different cells to test each one, one slot at a time.
Each module was tested with one cell in the same slot multiple times to determine the repeatability of a measurement in the same slot, then the cell was moved to a different slot to see how the measurements compared.
The results can be seen on this spreadsheet, and were somewhat surprising. For tests of the same cell in the same slot, the values did not vary too much, within a range of 20mAh. However, when the cell was moved to test different slots, results were changed to a spread of almost 100mAh for some testers, with the average spread between the 4 slots around 50mAh.
Given these results, these testers are unsuitable for determining small differences in capacity between a batch of new cells, but for testing cells from laptop pulls they are perfectly acceptable. Keep in mind though, that the results will be within a range of around 50mAh from the value that they show.

Test Conducted By: Micah Black
Written By: Micah Black