Battery Cycling on a budget (Part II): Tips to Get Started
Battery cycling is critical to understanding how a cell behaves over time and in different conditions. A well designed DIY cycler can be low cost, flexible, and give accurate measurements for critical battery analyses. A poorly designed cycler can be a time and money sink that does not produce any meaningful battery data.
You won't find a comprehensive, step-by-step guide here because each system should be customized (please reach out to us if you'd like hire a consultant for your DIY project!). What you will find are some tips and key considerations to getting started on designing and building a functional and cost-effective battery cycler.
General tips
Focus
Know what your needs are for a project and don't try to build a universal cycler. Don't build a system that can test both coin cells and automotive battery cells. If you set out to build a cycler that works for every battery, you'll build something that works well for nothing and poorly for most things.<br>For example, current sensors with a wide dynamic range often have poor resolution at low currents. If you need mA precision, don't build a system that can handle +/- 100A.
Safety
This is a multi-part blog post on its own (link). In short:
Stock up on safety tools - In-line circuit breakers and fuses are required. Remember to size the circuit breaker for the expected ac/dc current that you will be pushing. Invest in a class ABC fire extinguisher and place it nearby - make sure you read the instructions as part of safety training too.
Personal protective equipment (PPE) is essential. Get some safety goggles, insulating gloves, safety tape, etc
Insulate your tools - Many tools are long and conductive. Tape your tools with brightly colored, electrically insulating tape and save yourself from an unintended external short circuit event.
Build safety into your process - Build buffer time for proper setup and shutdown. Implement a daily safety check. Enforce a buddy system when turning on power for the first time. Diligence is required.
Education - Make sure everyone in the lab knows the general procedure for shutting down the cycler, and where the fire extinguishers are. The more trained you and your lab-mates are in how to safely handle batteries, the less incidents you will have.
Multi-layer HW and SW protection
Cycling Data
Batteries are inherently dangerous systems that can catch fire! If this is news to you, we highly recommend you reconsider a DIY cycler. Having both hardware and software layers of protection are mandatory for any battery system. Hardware protection includes external circuit breakers/fuses, as well as logic gate-controlled trip events. Software protection includes interrupt-type trips in a microcontroller program.
Hardware tips
Demo boards are your friends
Texas Instruments has developed amazing demo boards such as the LM5170EVM that helped us tremendously in being able to push bidirectional current quickly and without hassle. Companies like TI build these pre-made, pre-tested modules to help sell their chips. Use as many of these as possible to test basic functions.
Demo Boards
1) tested your current sensor, 2) tested 5A, 3) have a way to quickly verify correct/incorrect operation, and 4) have a built in fail-safe in case things do go wrong (see safety above). Testing each system separately will make integration of the final system much easier.
Test Separately, Measure Often
his is important. Don't try to push 10A current for the first time unless you've already 1) tested your current sensor, 2) tested 5A, 3) have a way to quickly verify correct/incorrect operation, and 4) have a build in fail-safe in case things do go wrong (see safety). Testing each system separately will make integration of the final system much easier.
Measure temperature
This is something we didn't do soon enough and came to bite us months later. Temperature is known to affect battery performance tremendously, and not having accurate, traceable temperature data put months of our battery data in question. The data was still usable, but imperfect. Better yet (if you have a bigger budget), control temperature in a thermal chamber.
Standardize connections and cables
Use the same exact part across your systems for things like: connector headers, crimps, cables, ring terminals, nuts and bolts, PCB standoffs, voltage supply ICs, analog sense ICs, busbars, etc. You'll be able to buy in bulk to save money, and you'll be able to easily swap things out without hassle. This will make your life SO MUCH EASIER.
Make room to grow
If designing a custom PCB for control/analog sensing, design with some extra GPIOs/circuits just in case, and make them easily accessible. Having an extra analog sense channel or an extra SPI comm line may be the difference between a 1 day patch and a 2 week redesign.
Data tips
Go wireless with data integration
A wireless communication interface between your hardware and software can make it much easier to add channels to your system. It also means less pesky wires to getting in the way and the ability to operate your cycler remotely. Raspberry Pi is the obvious choice here, but there are a host of other amazing tools such as the Onion Omega2 and BeagleBone Wireless.
Build a GUI
Making a GUI that is easy to use and user friendly is a difficult, but necessary, task. The GUI should simplify and streamline a lot of the complex logic that is going on underneath the hood while offering a certain degree of flexibility. The GUI empowers the technician to have control over a broad range of functions to perform the battery tests that they need. With web-based GUI frameworks such as an electron, we can develop cross-platform, desktop applications with a modern interface that is fairly flexible to meet our changing needs. The GUI for one of our BMS branches shows all salient measurements quickly, and allows us to send remote commands.
ReJoule's custom GUI
Implement a Database
The database records collected data and process information. Having a unified database is extremely helpful for accessing and querying relational data without loading irrelevant information into your machine. This makes for faster, more efficient, and more structured data analysis. We recommend using PostgreSQL database.
Conclusion
There's a lot that needs to go into designing and building a cycler. One last piece of advice is to take your time! If you rush through a design/build, you're not going to have a good time.
Hopefully these above tips help you along your DIY battery cycler journey! If you'd like more information or have any questions, don't hesitate to shoot us an email at info@rejouleenergy.com
Cheers,
The ReJouligans
