Created by Wong, Trevor, last modified on Oct 31, 2019

Objective

Our project is to design an efficient power bank for laptops. Although there are many products that can supply power to phones, our goal is to make such a project on a bigger scale in an inexpensive way so that it is viable for college students and provides multiple full recharges for laptops. While this is a rather broad objective, these are the specific features we would like to implement:

  • Safely charge an array of Lithium Polymer batteries
  • Display current charge
  • Display estimate of time until full charge
  • Display estimate of time until depletion based on current usage
  • Have a capacity of approximately 160 watt-hours (the legal limit for taking it on an aircraft)
  • Have a 19.5 volt dc jack for charging laptops
  • Have usb type a ports for charging phones and tablets
  • Take 19.5 volt dc input
  • Be reasonably portable and aesthetically pleasing

Once we have these features working, we intend to expand on our project by exploring additional features, including:

  • Ability to charge battery bank while it is in use
  • USB Type C charging / discharging
  • USB C port that both charges and discharges
  • Inverter for ac output
  • Battery monitoring system that can monitor temperature and capacity/health of cells

Background Research

Arguably the most important component of the battery bank is the battery itself. There are a number of different possible battery chemistries, each with their own pro's and cons. After comparing various battery chemistries, we settled on Lithium Ion batteries, because they have high energy density and can output a lot of power, which is necessary for powering and charging high end laptops. Additionally, this is the same type of battery most laptops use, so it makes sense that this battery chemistry is suitable for a laptop charger 1.

Lithium Ion batteries have a nominal voltage of 3.6 volts, meaning the capacity in watt hours for each cell can be calculated by 3.6 times the number of amp hours. If we choose 3Ah cells, it takes 15 to reach our target capacity of 160 watt hours. Because it would be inefficient to store the voltage at 3.6 and convert it to 19.5, we can put 3 sets of cells in series, creating a battery pack with nominal voltage 14.4 and max voltage 16.8. To charge the pack, we will need a battery management system that can ensure each set of cells in series has the same voltage (a process referred to as "balancing") and a power source that can supply constant current and constant voltage for different phases of the charging cycle 2.

It is possible to design a circuit that takes high input voltage and reduces it while regulating both voltage and current. Such a circuit is called a constant voltage constant current buck converter. Texas Instruments manufactures an IC specifically to help create this type of circuit (http://www.ti.com/lit/an/snva829/snva829.pdf). This can be used to take the laptop charger voltage of 19.5 and use it to charge the batteries to 16.8 volts without overcharging or charging too quickly. Our project also needs to increase the battery voltage back to 19.5 volts for the laptop to use. Texas interments also makes IC's that can be used in circuits that increase voltage, called boost converters (http://www.ti.com/lit/an/snva731/snva731.pdf). 

Finally, we need a way to monitor the amount of charge in the battery pack's voltage to display how much charge is remaining. We can implement a relatively simple micro-controller using an Atmega328p chip and the Arduino boot-loader. This can be used to monitor the voltage using a voltage divider and output to a display 3. The time remaining could be approximated using the change in voltage, also it would likely be more accurate to use a current sensor to measure the current going in and out of the battery pack. Knowing the total capacity of the pack, it would be fairly simple to know how long it will last or take to charge.

Design


Overview of Block Diagram:

  • Buck Converter: normalizes the voltage to a standard 5V which can be handled by the rest of the circuit.
  • LiPo BMS: Keeps the battery in its safe operating area, monitor the state of the battery, and overall control its environment.
  • LiPo batteries: stores potential difference.
  • USB-C port (optional, if time permits): a dual-functionality port which serves both as an input 19.5V as well as 19.5V out. Will require a lot of logic which is why it's optional.
  • Arduino for the USB-C logic and display: processes signals to give us the proper use of the usb-c depending upon whether the port is being used as a way to charge the battries or the laptop. Also monitors the batteries and displays power on the display screen.
  • Arduino-compatible display: displays amount of charge on batteries in conjunction with the arduino.
  • USB-A charging implementation OUT: connects to the BMS to output 5V for common phone charging.
  • Boost converter: increases the voltage from 5V that the circuit handles to a 19.5V that is best for charging laptops.



Parts


Possible Challenges

One of the hardest challenges in this project will be implementing a dual functionality  USB-C port that can function as an IN as well as an OUT. Since the logic behind data transfer in both directions is complicated, an alternative route to pursue would be to find an IC which already implements the logic and can be used in conjunction with the Arduino board. SInce these methods are not as easy as designing the rest of the circuit and since there is a time factor that comes into play for our product, we have marked the USB-C schematic with dashed lines to indicate the tentative nature of our design.

References

[1]K. Araujo, "Primary and Rechargeable Battery Chemistries with Energy Density", Epectec, 2019. [Online]. Available: https://www.epectec.com/batteries/chemistry/. [Accessed: 26- Sep- 2019].

[2]"Lithium-ion battery", Wikipedia, 2019. [Online]. Available: https://en.wikipedia.org/wiki/Lithium-ion_battery. [Accessed: 26- Sep- 2019].

[3]D. Mellis, C. Maw and R. Nugent, "Arduino - Setting up an Arduino on a breadboard", Arduino, 2019. [Online]. Available: https://www.arduino.cc/en/main/standalone. [Accessed: 26- Sep- 2019].


Attachments:

Untitled Diagram.jpg (image/jpeg)
Battery pack flow chart.jpg (image/jpeg)
Battery pack flow chart (1).jpg (image/jpeg)

Comments:

You guys are off to a great start! I would recommend trying to find a USB-C PD development board and ordering it as soon as possible to get some practice working with it (as USB-C will be the hardest software part of your project)
Posted by chorn4 at Oct 03, 2019 12:27

Looks good. However, I will strongly caution against USB-PD with USB-C. It is extremely difficult and complex.
Posted by fns2 at Oct 03, 2019 18:21