Update
The Aux BMS is planned to be combined with power distrubution and BMS Carrier in future revisions. BMS Carrier will have a comparator circuit for under voltage cutoff, and Power Distribution will handle all monitoring and charging.
Goals
- Actively protects the supplemental battery
- Isolates the supplemental battery if the killswitch is hit
- Allows for charging the supplemental battery
- Handle regulations on supplemental battery
Background and strategic fit
We use a supplemental battery to power a few critical LV systems before the main battery is connected and during BMS faults. See the Supplemental Power Strategy for more details.
By WSC regulations, this counts as part of the energy storage systems. Thus, we need to actively protect it. To reduce the number of boards we need to maintain, we'd like to make this a completely analog board without a microcontroller. The power distribution board can monitor the state of the supplemental battery through its powerpath. By ASC rules, if we plan on using a rechargeable chemistry, we need active protection, so we might as well support charging. In MSXI, we didn't monitor the supplemental battery properly or charge it, resulting in a deeply drained lead acid that would drop to 4V.
Assumptions
- We're using NiMH cells
- Our target LV voltage is 12V (~9 NiMH cells in series)
- We will be provided 12V through our DC-DCs
- The supplemental battery will live in the master battery box
Requirements
| # | Title | User Story | Importance | Notes |
|---|---|---|---|---|
| 1 | Protect the supplemental battery | We need to have active protection for the battery. | Must Have | Will probably use a fuse for current and comparators for voltage/temp |
| 2 | Isolation in safe state | By WSC regulations, none of the conductors exiting the energy storage system can provide more than 50mA. | Must Have | We plan on using some relay-based latch with a button to "prime" the battery connection. |
| 3 | Charge the battery | It would be nice if we could keep the battery charged during normal operation. | Medium | May need buck boost conversion if powered by 12V DC-DC - expensive |
Questions
Below is a list of questions to be addressed as a result of this requirements document:
| Question | Outcome |
|---|---|
| Do we need a microcontroller? | Ideally, no. We don't need to broadcast any CAN messages. |
| What kind of protection do NiMH battery packs need? | Under voltage over the entire pack only. We will charge the NiMH cells at a constant C/20 rate such that they will self-balance over a slow charge cycle. Ideally the auxiliary battery will not be run for a long time so discharge balancing is not a concern. Overvoltage in NiMH at C/20 is not a concern. Temperature protection is not a concern. |
| How do we charge the auxiliary battery pack? | The auxiliary pack will be charged from the LV (12V) DC power rail in the car. A charge controller IC will be needed that will provide a NiMH charge profile. |
| What is the NiMH charge profile? | |
| In what conditions will we use the auxiliary battery pack? How long will it be used? What is the current draw? | The auxiliary battery pack is used in cases when the main battery/12V DC-DC is not active or in a fault state. This includes vehicle startup (before HV battery is active), and battery isolation conditions (e-stop, BMS fault). Ideally the auxiliary pack will not be used for long, and will have on the order of 1-2A drawn from it at once. The auxiliary battery pack should have a capacity of >2000mAH. |
| What kind of communication does the auxiliary battery pack need with the system? How do we know when to switch in/out? How do we notify the system that the battery level is low? | The switching should be controlled directly by the power distribution. Refer to architecture for details. |