Power System

The power system is divider into three components

1. 36V Battery - For powering the components and modules.
2. Battery Management System (BMS) - A module that monitors an even charging process throughout the battery cells in the battery pack. It protects cells from damage and failure, and detects unsafe conditions and responds
3. DC/DC Converter - To convert the 36V to lower values to power modules.
4. DC/DC Converter Current Divider - To limit the appropriate current to modules to ensure no risk of current suddenly rushing into the wrong component.

The power system was design and built by my team member Ionut who is an electrical engineer. He created the schematics and PCB layouts in KiCAD. The battery cell layout and BMS were done in AutoCAD.

36V Battery

The 36V battery is comprised of 3.6V lithium ion cells. We used recycled cells which were at 90% of their original capacity. They were taken from hospital equipment and modem battery packs. We purchased 100 at $1.10 each. The reasoning behind this was it was greener for the project and more cost effective. The 3.6V cells were rated at 2600mAh. The design consisted of 10 in series for 36V and 5 in parallel for 13000mAh total. They would be connected using nickle.

Battery Management System

The BMS was to ensure safe charging of the battery pack. It would make sure the cells charge evenly, and detects unsafe operating conditions and responds. This will allow prolonged life of the battery. We did not design this component, but we did obviously need to attach it to the pack.

DC/DC Converter

This module was designed to convert the 36V down to 5V 6A to power all the modules we have in our kit. We are using a DC/DC Flyback converter design. This was chosen due to the efficiency, complexity and number of components. Below is an image of the PCB that was designed.

Below is an image of our first assembly on a breadboard to test the circuit.

DC/DC Converter Current Divider

This design was something I worked on myself, but I made sure to consult Ionut since this is technically his part of the project. (I finished most of my designs by then, so I gave him a hand). We came across an issue that we read could happen when we connect our modules. Each would pull the required current they need to operate, but things are not perfect like on paper, so sometimes current may rush into one component and blow it out. To solve this, I created a PCB that will take the 5V/6A output of the DC/DC converter and regulate the current using resistors. Each output to their respective modules will use the appropriate connector based on specifications from Digikey. We also have USB ports to charge your smartphone that you will use with the bike. It uses a dedicated charging port IC controller, which auto detects the USB data lines and provides the correct voltages and current. In the first slide below, this is one half of the circuit. The second is the PCB.