Prototype Project


Over the summer, our electrical team built a prototype project.

What is it? 

It is a scaled down electrical system of a solar car.

Why did we build it? 

To gain experience designing and building a working lower voltage (25.2V) system before we jump into a high voltage (100.8V) system.

Tell me more! 

Our prototype project consists of a solar array, a maximum power point tracker, a battery array, and a battery management system.

Solar Array
This array is made up of 22 Sunpower C60 cells. 
They are mounted on a contoured wooden frame to mimick the surface of a solar car and demonstrate the flexibility of the solar cells. 

Remember some time ago when we first tried to encapsulate one third of a solar cell?


After some further trials and errors, we scaled up our setup to encapsulate 22 solar cells at once.

and voila! The encapsulated solar array turned out as we expected!

 We excitedly measured the output of the solar array on the Quad Day when the Sun finally came out and....
the solar array, for a few seconds, before our spot was shaded by clouds again, gave out a higher than rated short circuit current! (Partly because the weather was chilly)


Maximum Power Point Tracker (MPPT)

Big thanks to Advanced Circuits for printing our MPPT PCB for us!

Our MPPT is essentially a boost converter. 
It boosts the voltage output of the solar array to match that of the battery array, while maximizing the current output (hence the name "maximum power point tracking").

Our rudimentary MPPT didn't have a very high efficiency, but we are working on improving the codes. We use perturb and observe algorithm to track the maximum power point of the solar array, where we increase / decrease the PWM of the MOSFET on the MPPT to reach the maximum power point on the I-V curve of the solar array. 



On the left of the MPPT is a Gigavac contactor. 
Thank you Gigavac for donating contactors to us!

These contactors has 2 coils, which let it to use a high powered coil during pull-in and a lower powered coil to hold the contacts in place once they have transferred. 
This allows the contactor to operate at an incredibly low coil power of 1W!


Batteries
This battery array is made up of 7 strings of 18650 lithium ion batteries, each string containing 3 batteries.

We tested all the batteries to get their capacities and internal resistance, and matched them so that all 3 batteries in the same string will have the closest characteristics.


We then spot welded the matched groups of batteries, and soldered a wire to the nickel tab so that the groups of batteries can be connected via connectors. Later, a wire leading to the battery management system is soldered to the tab too. 

Battery Management System

Thank you Texas Instruments for generously donating to us your battery management system (BMS) evaluation module, BQ76PLEVM-3 and microcontroller (MCU) target board MSP-TS430PN80USB!

During startup, the MCU, which is powered by a supplemental battery will first obtain data from the BMS to make sure the batteries are not undervoltage, overvoltage, overtemperature, or undertemperature. 

If no fault is detected, the MCU will switch to obtain power from the battery pack through a DC-DC converter, and start the pre charge sequence. Then, both the contactors will be closed and startup sequence will be completed.

The MCU will continue to obtain the batteries' voltage and temperature, and perform cell balancing when the system is running. 

In case any fault is detected, the contactors will be opened. 

...


Many thanks to our other sponsors, Engineering Design CouncilStudent Sustainability Committee, Physics Department, Computer Science Department, Geoff, our advisors, our team members, and everyone who helped and supported us, you made our first project a success! 


 Based on the experience we gained from designing and building this prototype project, we will start designing and building a high voltage system for our solar car. 
Wish us luck on that!


Thank you for reading!