As of 2023-08-12, revision 1 of the aux battery is no longer sufficient to power the car over a race day of 9 hours due to updated driver fan loads and updated contactor loads.

Supplemental/Aux Battery - Revision 1 Notes

The purpose of this confluence page is so that electrical members may review my calculations.

Specifications:

The battery shall be able to energize protected LV systems until the main battery is energized

The battery shall be able to power protected and emergency systems in the case of a fault condition

The battery shall be mounted to the LV cutoff board and be easily swappable

The battery shall be able to power the average load until the end of the race day (8 hrs)

The battery cannot overheat

Voltage:

Nominally 12V, however voltage levels should not be altogether too significant as voltage is further downregulated to logic levels on each individual board.

Electrical Characteristics:

Energy Budget:

To characterize energy capacity, we will group loads into 3 sections:

Persistent Loads
1. These loads will be operating over the course of the entire race day. As of right now, the contactor power draw and horn power draw is based off the Systems Integration Meetings - Electrical - Confluence (atlassian.net) page point estimates, which may not be current?
Emergency Loads
1. These loads will be operating only in the case of a BPS emergency causing the main battery pack to be disconnected. In the case of a BPS fault, as of systems integration meeting 2023/07/05, the LV systems will be powered solely off of the aux battery. The LV systems must be able to hold up until car finds a safe place to park. These loads were based off of the WIP LV power budget as of 2023/08/12. We assume that the horn must run for 5 minutes straight, and the rest of the LV systems must run for 5 additional minutes (?) – MUST CONFIRM.

Load	Description	Energy Demand [Wh]

Load	Description	Energy Demand [Wh]
Persistent Loads:
Telemetry – Router	Assume 2.5W for a raspberry pi, 9 hr race day	22.5
Driver Fans	2 Noctua NF-F12 iPPC 3000, overvolted to 15V, 5.1W x 2 max tested power draw for 9 hrs - Forest Zhou, July 13, 2023	91.8
Contactors (BPS Switch)	3.5W, 9 hr race day	31.5
Total Persistent Loads:		145.8
Emergency Loads
Horn	Horn must be able to be run for 5 minutes in an emergency as per regs Assuming 7W horn (as per June 28, 2023 Sync)	0.583
All LV systems – Typical	91.0 W, 5 minutes	7.58
Total Emergency Loads		8.16300
Total		153.96

Required (nominal) capacity: 12.83025 Ah

Power Budget:

Load	Description	Power Demand – Typ [W]	Power Demand – Peak [W]

Load	Description	Power Demand – Typ [W]	Power Demand – Peak [W]
Telemetry	Raspberry Pi (Assuming switching regulator at 90% efficiency to downregulate 12V to 5V power)	2.5	2.5
Fans		10.2W – Point estimate	10.2W – Point estimate
Contactors	3 Contactors – 3-5A	3.5W – Typical	180W – Initial closing
Emergency LV Power	In case of BPS fault	54.111	130.630W
Horn	7W horn	7	7

Peak power necessary on startup	Telemetry, Contactors, Fans		192.7
Peak power necessary when phase 2 or 3, normal operation	Telemetry, Contactors, Fans	16.2
Power necessary on BPS fault (if no need to re-close contactors):	Fans, Contactors, Emergency LV Power (incl. telemetry), Horn	74.81100	151.33
Peak power necessary on BPS fault (if yes need to re-close contactors):	Telemetry, Fans, Contactors, Emergency LV Power (incl. telemetry) Horn	251.31100	327.83

Current – Typical Phase 2 or 3: 1.35 A

Current – Peak on startup: 16.06 A

Current – Peak on BPS fault: 12.61 A

Current – Typical on BPS fault & MPS must be re-opened: 20.9425

Current – Peak on BPS fault & MPS must be re-opened: 27.32 A

Things to Verify:

Contactor Loads
Horn Loads
1. Chances of being constant 7W is kind of low? What’s peak current demand?
Telemetry Loads
How long do we need emergency power for on BPS fault?
What’s an acceptable S.F?
Are we boosting the voltage coming out of the aux for fans or something? How are we getting 15V from 13.5-10V?

The point estimates I’ve found for these seem pretty imprecise, I’ve gotta ask around and get better numbers?

Will the MPS ever need to be opened again after it has been closed?

Pack Arrangements:

	10s1p	10s2p	10s3p

	10s1p	10s2p	10s3p
Watt-hours (rated)	120	240	360
Watts	120	240	360
Energy Loads
Persistent loads met?	No	1.65 S.F.	2.47 S.F.
Persistent + 5 mins emergency loads met?	No	1.56 S.F	2.34 S.F.
Power Loads
Typical Phase 2 or 3	7.41 S.F	14.81 S.F.	22.22 S.F.
Peak Startup	No	1.25 S.F.	1.87 S.F.
Typical BPS fault	1.6 S.F.	3.2 S.F.	4.8 S.F.
Peak BPS fault	No	1.59 S.F	2.38 S.F.
Typical BPS Fault & MPS Re-energization	No	No	1.43 S.F.
Peak BPS Fault & MPS Re-energization	No	No	1.1 S.F.

Runtime [hrs]	7.4	15	22
Mass (of cells) [kgs]	1.45	2.90	4.35
Resistance (of cells, nominal) [mOhm]	18	9	6
Minimum size (assuming square packing)	171mm x 68.4mm x 70mm	171mm x 136.8mm x 70mm	171 x 205.2mm x 70mm

Conclusions:

It is evident that 10s1p is insufficient to run all aux loads for one race day
10s2p contains enough energy for one race day and can supply enough power for all loads calculated (if loads are correct), however cannot re-energize the MPS if it is de-energized, must power the LV loads and must be re-energized. This is likely not necessary, as the car has to pull over and fix the BPS fault before continuing, as per regs. The safety factor for capacity is 1.65, which may or may not be sufficient, as thermal effects, parasitic resistances, and future alterations to loads may increase the required capacity.
If the loads are correct and safety factor is deemed sufficient, 10s2p will be used.

Mechanical Characteristics

Thermal Constraints

(TBD)

(Make sure the busbars don’t get so hot that the plastic melts, or that the module can withstand such heat)

Supplemental/Aux Battery - Revision 2