Goals
Pack
- Design a matched battery pack that consists of balanced Li-ion cells
- Design a battery pack and enclosure that provides adequate cooling for the Li-ion cells
Battery Monitoring System (BMS)
- Design a BMS that is robust to electrical noise found from sources in and out of vehicle
- The BMS is robust and reliable to vibration, elements, and users
- The BMS provides test points for scrutineering
- The BMS is calibrated appropriately
- Provides at least the minimal active protection requirements for the Li-ion battery pack
- Over-voltage
- Under-voltage
- Over-temperature
- Over-current
Background and strategic fit
Battery Pack
The Battery Pack in MSXI consisted of Panasonic NCR18650 Li-ion cells that provided 120 V to our electrical systems. From a high-level perspective, the pack is charged by the solar array, which in turn, provides the power necessary for the car to move. These cells must be pre-approved by race officials.
In addition, we need to design an enclosure that allows the battery to be removed and impounded at the end of a race day. At a race, the box will be fitted with tamper-seals provided by the race officials, and should not allow access without removing these tamper-proof seals. The enclosure should also provide mounting points
Moreover, we should take into consideration what procedures we need to have in place, in the event of a battery incident (along with items that we need to keep in stock in our battery incident kits).
Battery Management System
The Battery Management System (BMS) is responsible for monitoring the state our Li-ion pack, ensuring that the battery is operating within its Safe Operating Area, calculating secondary data, reporting that data, and balancing it. It consists of AFE chips that allow the board to grab pack data, and then take appropriate actions.
Assumptions
Project Requirements
Rayce Regulations
These are requirements driven by the Rayce regulations.
World Solar Challenge 2017 Regulations V1.1: https://www.worldsolarchallenge.org/files/1504_2017_bwsc_regulations_final_release_version_11.pdf
| Section # | Title | User Story | Importance | Notes |
|---|---|---|---|---|
| 2.5.1 | Capacity | Cruiser class vehicles have no total mass limitations | High | |
| 2.5.3 | Specification | Teams must submit detailed specification of intended cells | High | |
| 2.5.8 | Operating Range | Cells must never be operated outside of voltage, current and temperature ranges supplied by the manufacturer | High | |
| 2.5.9 | Charging System | Any AC input charging system must meet following requirements ● the charger must be used with a residual current device ● the charger must be either permanently connected to the energy storage system, or connect to the energy storage system using an appropriate connector ● the output of the charger must be electrically isolated from the ac input ● the charger must stop charging automatically when the energy storage system is full or if a fault occurs. | High | |
| 2.5.13 | Mounting | Energy storage packs must remain restrained in a 20g acceleration | High | |
| 2.5.14 | Spill Protection | Spill-proof barrier must be present between energy storage system and occupants. If energy storage system is capable of spilling | High | |
| 2.5.15 | Ventilation | Ventilation must carry any gases to the exterior of the vehicle. If energy storage system is capable of emitting gases. | High | |
| 2.5.17 | Tamper-Evident | Energy storage packs must be constructed to be sealed using tamper-evident plastic seals similar to 3x100mm plastic cable ties. Said ties cannot be removed after scrutineer has been completed | High | |
| 2.28.5 | High Voltage Symbol | High-voltage energy storage packs must be marked with the high-voltage symbol | High | |
| 2.28.6 | HV Symbol Visibility | HV Symbol must be visible on any enclosure or barrier that can be access without tools, if HV parts are exposed | High | |
| 2.28.10 | Fuse | Energy storage packs must be protected by a fuse or circuit breaker to interrupt the short-circuit fault current of the pack Must be mounted in or on the energy pack | High | |
| 2.5.7 | Storage System | Maximum of 2 packs for entirety of energy storage system | Med | |
| 2.5.10 | Other Batteries 1 | Batteries used only to ● power a real time clock when the solar car is turned off; or ● retain data when the solar car is turned off; or ● power wireless tyre pressure monitors are not considered part of the energy storage system (max 2.0Wh) | Med | |
| 2.5.11 | Other Batteries 2 | Batteries used in handheld radios, cameras, mobile telephones or wristwatches carried by the driver are not considered part of the energy storage system | Med | |
| 2.5.12 | Capacitors | Capacitors are not considered part of the energy storage system if capacity is less than 10Wh. Such capacitors must discharge to less than 60V within 5 seconds | Med | |
| 2.5.16 | Removable Packs | ● Energy storage system must remain in a safe state when disconnected ● Team must provide a lockable box for storing the energy storage packs | Med | |
| 2.28.1 | Double Insulation | Double protection from high-voltage parts, such as enclosures or barriers | Med | Common Sense |
| 2.28.3 | HV Protection Against Direct Contact | HV parts outside of driver, passenger and luggage compartment must be designed to exclude fingers (Ingress Protection rating IPXXB). Test for IPXXD A probe is required. The probe consists of an insulated handle about 100 mm long and about 10 mm in diameter which ends in an insulated sphere (stop face) of 35 ± 0.2 mm diameter. On the opposite side of the sphere is a metal rod 100 mm long and 1 The tip of the metal rod is pressed gently against the opening. If the rod goes in there must be adequate clearance between the rod and the live parts i.e. the lamp should not light up. Tests must be made with the rod in all possible positions. The lamp must not light up. The full diameter of the insulated sphere (stop face) must not be able to pass through the opening. | Med | |
| 2.5.2 | Uncommon Cell | Teams must contact organizers if other cell chemistry are used | Low | We use Li-ion cells |
| 2.5.4 | Nominal Mass Endorsement | Specified and endorsed by manufacturer | Low | We have no mass capacity |
| 2.5.5 | Nominal Mass Conflicting Specs | Organizers will determine nominal mass if conflicting or unclear values are provided | Low | |
| 2.5.6 | Sum Nominal Mass | Sum off nominal masses must not be more than allowable mass | Low |
Internal Requirements
These are requirements driven by Midnight Sun.
- Must allow for reasonably easy installation that won't be very time consuming
| # | Title | User Story | Importance | Notes |
|---|---|---|---|---|
| 1 | Balanced battery pack | Build a balanced battery pack | Must Have | |
| 2 | Cooling | Designing a battery layout and enclosure that accounts for cooling requirements | Must Have | |
| 3 | Modular |
User interaction and design
Questions
Below is a list of questions to be addressed as a result of this requirements document:
| Question | Outcome |
|---|---|
| Are we going to run the pack at a higher voltage? Can we even run the pack at a higher voltage? | |
| Do we want to allow extra voltage to be released to balance the pack? | |
| Are we still planning on using the Panasonic NCR18650 cells? | Yes |
Not Doing
- Cell-balancing: MSXI had rudimentary cell balancing based on the voltage of a single battery string, which attempted to normalize all the cell voltages. After consideration, any effort we dedicate to doing cell balancing is better placed in building a matched pack (since the pack is replaced every race).