|22||Electric Motor Scooter Battery System Upgrade and Custom Battery Management System
|Problem: Fagen Scooters in Champaign, IL ordered its first electric scooter a few years ago in hopes of capitalizing on the electric transportation market. Unfortunately, the model that they ordered was very poorly designed and built by a company called Numi. The battery system on the scooter has failed and even when operational, the scooter was not capable of monitoring its own battery health, discharge metrics or charging metrics. As such, the scooter is not operational and cannot be sold. The company that manufactured the scooter, Numi, has since gone out of business and Fagen Scooters has been unable to reach them for support or documentation of any kind.
Solution Overview: We will design and build a new and robust Battery Management System (BMS) for the Numi scooter. This Battery Management System will contain a diagnostics system so that it can warn the end user of specific problems. This will be very similar to a "check engine light" in a traditional vehicle. This way, the user will have a chance to service the scooter before it totally stops working like it did for Mr. Tom Dillavou, owner of Fagen Scooters. We hope to have warnings for unhealthy charging, unhealthy discharging, high battery temperature, and other things as we see fit. The BMS will also stop power going from the charger to the battery while charging if it detects a large power draw from the battery. This will mitigate the risk of battery damage during the charging process. In addition, we will also design and build a new lithium ion battery pack to give the scooter better all around performance. Mr. Dillavou would like the scooter to be lighter, faster, and for the battery charge to last longer. Additionally, in our process of documenting everything for ECE 445, we will distribute our documentation to Mr. Dillavou so that he will be equipped to support his customer after their future purchase of this scooter.
# New Lithium Ion Battery Pack
We will design and build a new 72 V battery pack from 18650 lithium ion cells. These 18650 cells are the standard in electric vehicles and most, if not all, EV manufacturers (Tesla, BMW, Nissan, etc.) use them in their own battery packs. We will research cell specifications and design the pack to come up with the most cost effective method using the highest quality cells possible. We will be using "name brand" cells (Sony, LG, Panasonic, or Samsung) to ensure that the battery pack is of the highest standard and performance. Cheaper unbranded Chinese lithium ion cells perform significantly worse than name brand cells and have a history of causing fires due to overheating. We will be designing this pack specifically around the Numi Quadhopper scooter owned by Mr. Dillavou, so space requirements will be one of the most important factors in the design process. Additionally, this pack will be designed in conjunction with our custom BMS so that we will be able to provide the most realtime data as possible to the BMS in order to diagnose problems as soon as they arise. This will prevent damage to the battery and will ensure that the scooter is behaving properly.
We will need to build the cell array in such a way that we can monitor each portion of the 72 V battery pack. For example, when our BMS throws a voltage "flag" we will need to diagnose the cells within the pack itself. Most of the time, rechargeable battery problems are caused by a few cells in the pack and can be fixed by replacing bad cells individually rather than buying a new pack all together (incredibly expensive). So, to do this, we will wire our pack to send data from each ~3.6 V (single cell) portion, so that we can monitor voltage from each one. If 1 of the 20 cell portions is reporting a lower voltage than the others, the end user will easily be able to open the pack up, find the problematic cell portion, and fix it rather than throwing the entire pack away and building a new one.
# Custom Battery Management System
We will also be designing and fabricating a custom BMS for the scooter as previously mentioned. This BMS will have the following sub-components:
## In-Line Current/Voltage Sensor Network:
The in-line voltage and current sensors will measure the voltages and currents of several subsystems within the electric bike. These subsystems are the following:
- load current from controller/display for the electric bike
- voltage of the battery
- voltage/current coming from the charger while it’s charging
- voltage across the electric motor
- load current of the electric motor
Due to the larger current draws coming from the systems on the bike, especially the motor, a high power in-line current sensor will be used to measure load currents. Look below for a link to the current sensor. High Voltage Transducers will be used to measure the voltage across the motor and the battery. These devices are capable of taking the current/voltage data and converting it to a lower voltage that can be measured by embedded hardware.
## Control Unit
The next subsystem is the embedded hardware that will be taking the voltages coming from the sensor network, converting it into digital data, analyzing the data, control the diagnostics output screen that the user will see, and will send a signal to cut charger flow to the battery if it detects a large power draw from the battery.
The data will be delivered to an STM32 ARM processer where the processor will be able to determine (from the sensor data) if there are problems with the battery or electric motor by throwing "flags" if the system is drawing an abnormal amount of current or if the system is reporting an abnormal voltage level. As a safety measure, there will also be a relay attached to the battery line to stop current flow from the battery to the motor / from the charger to the battery depending on if there is a problem when the battery is being charged or discharged. The voltages will be converted from analog to digital data using the onboard ADCs on the STM microcontroller.
These "flags" will be triggered when either current draw or battery pack voltage level is outside of a certain tolerance. For voltage, we will have the controller throw a flag whenever the voltage is 48 V or below. This number was calculated from a Samsung INR18650-25R cell (one of the cells we are considering to use) data sheet. We will be using 20 cells in series to make the 72 V required for the motor. The data sheet reports a discharged cell voltage of about 2.5 V. So, multiplying 2.5*20 gives us a discharged pack voltage of about 50 V. This means that the battery pack would report a voltage of about 50 V when it is completely "out of juice" and needs to recharge. Setting the threshold at 48 V or less gives us a good tolerance so that the system WON'T throw flags when the battery is experiencing normal discharge and WILL throw flags when there is an actual problem. A reported voltage below this voltage would indicate a problem with the battery and flag would be thrown by the BMS. Additionally, any reported voltage above ~72 (74 V or more to be exact) will also throw a flag. We decided on 74 V because of experience with multimeter/volt meter inaccuracy and component measurement fluctuation. Current will be handled in the same way, except with a max current of 40 A (the max current the existing motor controller can handle). We won't really need a low point cutoff, because the battery will be routinely providing less than 1 A of current when the motor is stopped (i.e. scooter is at a stop sign or red light).
## Relay System
A relay will be connected to a digital output pin on the microcontroller, and it will be used to cut flow from the charger to the battery if there is too much power going to the battery at any time.
## Diagnostics Output
This will be an array of LEDs that will communicate if there are any issues (flags) with the battery system, and what kind of issue it is.
The micro controller and sensors will be powered from a 5 V voltage regulator circuit made from stepping down the 12 V line available from the stock motor controller to the necessary 5 volts needed to power the microcontroller. The link to the regulator that will be used is below.
Link to Samsung 18650 cell data sheet: (https://vruzend.com/wp-content/uploads/2017/09/SAMSUNG-INR18650-25R.pdf)
Link to in-line current sensor: (https://www.digikey.com/product-detail/en/allegro-microsystems/ACS770LCB-050B-PFF-T/620-1541-5-ND/4473980?utm_adgroup=Sensors%2C%20Transducers&utm_source=google&utm_medium=cpc&utm_campaign=Shopping_Allegro%20Microsystems_0620_Co-op&utm_term=&utm_content=Sensors%2C%20Transducers&gclid=EAIaIQobChMI8bvnm9-r5wIVhcDACh3BtAHYEAQYAiABEgI5n_D_BwE)
Link to voltage regulator (stepping 12 V line down to 5 V for microcontroller): (https://www.digikey.com/product-detail/en/analog-devices/LT1086CT-5%23PBF/LT1086CT-5%23PBF-ND/891719?utm_adgroup=Integrated%20Circuits&utm_source=google&utm_medium=cpc&utm_campaign=Shopping_Linear%20Technology%2FAnalog%20Devices_0161_Co-op&utm_term=&utm_content=Integrated%20Circuits&gclid=EAIaIQobChMIi_Pg8-Cr5wIVUSCtBh2HYANHEAQYBSABEgKfIfD_BwE)
Criteria for Success:
- The scooter runs and drives from the new battery pack that we design and build.
- The new scooter battery pack weighs less than the depleted stock Sealed Lead-Acid battery pack.
- The new scooter battery pack lasts longer than the depleted stock Sealed Lead-Acid battery pack. (more charge cycles)
- The custom BMS can accurately report pack voltage levels and current draw.
- The custom BMS can correctly throw "flags" when voltage or current metrics are outside of thresholds.