Senior Design Part 6: Protecting our Power!

2023-05-26 | By Will Siffer

License: Attribution Non-commercial Batteries Battery Charger / Management

Why Protect a Battery?

If you're like me, you've been avoiding adding lithium batteries to your projects to avoid the possibility of them blowing up or ruining your hard work. It's because of this that I have done quite a bit of research on all the possible problems that can happen to you and your circuit if the batteries aren't respected.

Fire! - Lithium batteries can catch on fire if over-discharged or overcharged.
Toxic Fumes can be released when the casing ruptures or deforms!
Explosions can even happen, spraying hot flaming gasses in every direction, possibly endangering you and your project.

Look at this picture below from the BBC article about lithium battery explosions.

battery

From: BBC Battery Article

It's not all bad!

Lithium battery packs store an immense amount of energy in a small package, which is part of the reason they are so dangerous. But they can also be incredibly useful for personal projects, allowing us to pack a lot of electrical energy into small sizes and packages and allow us to make our projects even smaller. For me, the battery needed to be small since we hope to be able to hang the RFID readers from poles. A lead acid battery would be massive and heavy, making our project too big to put around campus.

Now, to use these batteries effectively, we need to make circuits that can automatically cut the battery connections off from the rest of the circuit if the battery is in a dangerous situation. For my project, I wanted to protect against:

Over-Charge
Reverse polarity
Over-Discharge
Short Circuit

Each of those conditions were very important to me since they are the leading cause of lithium battery explosions. So, in order to make sure that I was being safe, I turned to DFRobot (linked below) for their solar charger. This PCB was far too big for my enclosure. In fact, the PCB for just this solar charger was almost 2/3 as long and half as wide as the PCB for my whole project! Using their design as a guide, I came up with the design in the thumbnail, enlarged below:

battery circuit

Battery Circuit featuring the DW06D

There are 2 main parts of this circuit, the first is the section in the box labeled "Anti-Reverse Battery" This circuit is something I personally found interesting since it is a pretty "dumb" design. The MOSFET switch AO3400 needs a positive voltage applied to the gate to allow current to flow through the bottom two pins, allowing current to pass in normal operation since it is connected to the positive wire on the battery. Whenever the battery is disconnected the switch closes but more importantly if the battery is plugged in reverse, the switch stays closed and the circuit lights up LED1, signaling the user that they have the battery wired wrong and protecting the circuit at the same time.

What about over-current and short-circuit conditions?

That's where the battery management chip comes in. The chip can protect against everything I listed above, using clever logic circuitry to determine the battery state and disconnecting the battery via the ground wire whenever it detects an issue. This includes overcurrent on both the charging and discharging cases as well as short circuits. There is a button that bridges the connection between the battery and the rest of the ground connection, this allows for the system to boot up when the battery is first connected in case the battery management IC cut the power to the rest of the system.

There are other features the battery management protects against, which makes it extremely worth it, including under and over-voltage protection that cuts off the battery from getting too charged or discharged. I was particularly excited about these features since it means the system will automatically save itself in the event it goes too long without maintenance.

This is all to say that I am finally confident with batteries and using them in projects safely, and I hope these explanations put you more on your way to practicing yourself as well!

This week was a Design Review week for EPICS, meaning I got the opportunity to share my design with industry professionals from FormLabs, Caterpillar, and ZF Automotive. While there isn't much to talk about in a blog post about presenting, it was something that I was very happy to complete since it gave me some much-needed feedback.

As always, I close with my EPICS reflection this week, this time reflecting on the social impact of my project partner and the work we do for Design Review. Until next week, my name is Will, and I hope you find something to be curious about today!

"This week was design review week, and I am extremely happy with how the team did overall with answering questions and presenting the work we have done so far. As we work further into the semester, there is a lot to reflect on, but I am most interested right now in how we focus our work toward our community partners and how these partners are changing. At the beginning of last semester, we focused on a single project partner with a single use case. As this project has grown, we have a fair amount of breadth now, and we even have expanded the project to be partnered with Purdue EPICS where we can support other teams who are working on projects that require transmitting data over long distances. This has been a great motivator in increasing the quality of our work since we now have more people that are going to use the project. Experiencing project growth and change has been an interesting challenge for me since it changes how we need to complete different tasks. As I move forward into my professional life, I look forward to seeing how the work I do is influenced by the end user and by those who will be working on and maintaining the product."

Check out the next week's post: Coming soon!

Be sure to take a look at the previous posts in this series:

Senior Design Capstone Part 1: What (or Who?) is LoRa?

Senior Design Part 2: Using Eagle to make a custom PCB

Senior Design Part 3: Designing an SMD PCB Full of Sensors

Senior Design Part 4: Assembling, Testing, and Debugging Circuit Boards!

Senior Design Part 5: Diagnosing PCB Issues