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
- Determine how the pack will be mounted to the car to minimize or dampen vibrations
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. This will consist of a pack built from the 18650 (18mm by 65mm) class of Lithium-ion cells (which are similar in size to AA batteries, which are also known as the 14500 class).
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 that should be shock-resistant.
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 Analog Front-End chips that allow the board to grab pack data, and then take appropriate actions.
Assumptions
Requirements
| # | Title | User Story | Importance | Notes |
|---|---|---|---|---|
| 1 | Matched battery pack | Build a pack with balanced cells | Must Have | |
| 2 | Cooling | Designing a battery layout and enclosure that accounts for cooling requirements | Must Have |
Selected Parts
| Part | Info | DigiKey Link | Price |
|---|---|---|---|
| LTC6804-1/LTC6804-2 |
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? | Yes. The goal will be to reach a maximum 160V. |
| Do we want to allow extra voltage to be released to balance the pack? If so, how are we going to accomplish this? | |
| Are we still planning on using 18650 cells? | Yes |
| Are we still going to use the Panasonic NCR 18650? | |
| How are we going to mount the pack in a way that provides shock-absorption? | |
| What will the dimensions of the battery-box be? |
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).