Formula SAE
Wiring Harness Design for EV
Milind Sharma, Trainee TeamKart
Abstract
Designing a robust and efficient wiring harness is crucial for these EVs to achieve optimal performance
and comply with competition regulations. This paper delves into the design and optimization of an FSAE
wiring harness for EVs, adhering to the specific guidelines outlined in the Formula Bharat rulebook.
The Basic Design
Shutdown circuit
For Clarification Normally Open/Closed refer to the state the relay is in when it is unpowered, in the
manufacturer’s box
LVMS
The low voltage master switch is a physical switch that allows current to flow through the shutdown
circuit and the entire low voltage system
BSPD
Break System Plausibility Device monitors faults in breaking system. Automatically shuts down the entire
system on any sign of problem in the breaking system
IMD
Insulation monitoring device continuously checks value insulation resistance. Shuts down the system if it
falls below a threshold level
AMS
Accumulator managements system checks the cells of accumulator container
AS and RES are required if the vehicle has autonomous capabilities
Inertia Switch
If the car decelerates at an abnormal amounts, usually in times of crashes, it shuts off the system
BOTS
Brake over travel switch gets activated when brake breaks. A switch behind the paddle which is normally
not accessible gets accessible when it breaks, allowing it to shut down the circuit.
TSMS
Tractive System Master Switch that needs to be turned on when starting the car to allow circuit to flow
through the rest of the circuit
AIR Coils
Accumulator isolation relay coils check if the current is flowing and accordingly activate High Voltage
Systems
Activation logic is basically the start button in the cockpit
This is designed to be failsafe in nature as they are series chained.
All contacts are closed for precharging. If any of the protective devices sense faultage, the contact is
opened, the chain is broken and the shutdown system de-energizes or opens the AIRS.
Accumulator Isolation Relays (AIR)
Composed of Contractors which are essentially Heavy Duty Relays with a Low Voltage Coil and Heavy
duty switching contacts.
To explain Relays, They are essentially an electrically operated switch, essential to control circuits which
use more power using low power signals. Used to reduce current through primary switch
EV 5.6 States both poles are switched on by AIR, Therefore two AIRS, one for negative and one for the
positive terminals. The second AIR should be able to cut off the connection of the tractive system to the
HV path.
It also states the AIRs need to be of mostly open type.
The AIR should have auxiliary monitoring contact to help with TSAL( Tractive System Active Light)
The TSAL indicates when the tractive system is active, ensuring safe operation around the vehicle. By
directly connecting an auxiliary contact of the AIR to the TSAL, you can ensure the light only illuminates
when the battery pack is actually connected to the system, providing a more accurate and reliable
indication.
The EV Rules state the Tractive System is pre charged to avoid harmful in rush current when AIR is
closed, To to this, the first AIR is Closed before the other, and the TS is charged by a capacitor that gets
charged.
EV 5.7.1 States that this is done till the voltage is about 95% of actual TS Accumulator Voltage.
Low Voltage Master Switch
Designed to toggle all electrical components not part of the Tractive System of the Vehicle.
These are 90 Degree rotary master switches which use removable keys and low switch contacts
Brake System Plausibility Device
This must be a standalone, non programmable circuit with simplistic design.
It toggles pertaining to Brake Pressure and Throttle Percentage, it also prevents the drivers from
breaking and accelerating at the same time to prevent throttle pedal getting stuck.
If >5kW power is delivered to the motor while hardbreaking occurs, the BSPD opens the shutdown
circuit and stop accumulator current flow.
The brake threshold is <30 bars and requires a brake system pressure sensor
If the condition of hard braking and throttle exist for 0.5 s, it triggers, resulting in cutting off power to
the motor.
After a set amount of time, usually 10s it resets.
This is the first part of the BSPD Device, which uses a comparator, seeing if voltage in the negative
terminal is lower than that of positive, before passing it onwards.
The Sensor 1 and 2 are Brake Pressure and Motor Power Supply Sensors.
Now seeing as the Truth Table of BSPD is supposed to be
Hard Braking
Power
BSPD Output
Yes
Yes
Shutdown the car
Yes
No
NIL
No
Yes
NIL
No
No
NIL
We add an and gate to get the output of said signals.
Then we need to ensure the 500ms delay, for which we use a capacitor, The capacitor gets charged, and
the voltage gets raised till the point its greater than negative terminal
Combining Both Said Components, then the signal opens the relay, which is routed in the Shutdown
Circuit. To actuate it without programs, we use a transistor as an electrically actuated switch.
There is also the idea of sending the initial signals before the AND Gate and verify them to be System
Critical Signals, and post that activate the relay on confirmation
Insulation Monitoring Devices
EV 6.3.5 These check the leakage current between high voltage supply and the chassis ground.
If it detects leakage, it indicates the resistance is failing below a threshold. This results in the IMD to cut
off power.
Similar to BSPD, IMD are simple non programmable circuits.
It essentially works like we supply HV+/HV- into one connector the other connector we supply the GLV
supply voltage/chassis grounds and pick up the status output.
If there is an insulation fault between HV and the chassis ground it will trigger the status output.
The recommended IMD by Formula Bharat
The IMD must be inside the accumulator box so as to even then act as a monitor
Accumulator Management Systems
These are responsible for monitoring the status of the individual cells within the accumulator package
and for balancing voltage across all cells, regulates power delivery to the motors and toggles
disconnection in case of overvoltage.
Composed of Cell Voltage Measurement Systems, Battery Management Units, Current and Temperature
Sensors.
Cell balancing by AMS is not allowed when AIRs are Open because
a) High Voltage Protection: When the AIRs are open, the high-voltage battery pack is physically
disconnected from the rest of the vehicle, isolating it and preventing any potential for accidental
contact or electrical hazards. Cell balancing, which involves actively manipulating individual cell
voltages, could pose a safety risk if performed while the pack is disconnected from the main
control systems and safety measures.
b) Unreliable Data and Imbalances: With the AIRs open, the Battery Management System (BMS)
may not be able to accurately measure and monitor individual cell voltages due to the
disconnected state. Any attempt at cell balancing based on this potentially inaccurate data could
lead to further imbalances and safety concerns.
BMS: Can be relatively simpler, mainly dealing with internal battery pack parameters.
AMS: May be more complex, requiring integration with various vehicle systems and managing diverse
energy sources and demands.
Shutdown Buttons
Then there are Three physical emergency shutdown buttons which can be pushed to shut down system
Inertia Switch
These are activated on sudden rapid deceleration, which mostly occurs in cases of collisions, mechanical
failures and so forth.
It has a gravity based trigger with high resistance to movement. During deceleration it gets thrown
forward triggering the switch
A small loose weight (called a proof mass) is trapped within a spring-loaded cage. A shock in any
direction will cause movement of the mass relative to the cage. If sufficiently shocked, the cage will
spring open which actuates an associated switch. The switch is reset by pressing the cage closed through
the flexible (red) top cover, retrapping the mass. These switches are also used to open a contactor (a
large relay) to disable the high power circuit of a battery electric vehicle upon collision.
Optimizations
Break Over Travel Switch
Triggers when break paddle over travels, usually in case of it breaking. It must not be resettable by the
driver.
The switches can be mechanical or electrical and of many variants given they are robust enough.
Results it cutting off power from the motor.
It can also result in alarms and specific lights
HVD
EV 4.5.10 states every TS Connector should have an interlock line which is a part of the shutdown circuit.
The HVD Interlock separates high voltage circuits from the rest of the vehicle.
HVD, swiftly disconnects the high-voltage circuits in various scenarios:
●
●
●
Opening of the battery compartment or access panels.
Loss of key connection between driver and vehicle (seatbelt unbuckled, door open).
Specific fault conditions detected by the BMS or other safety systems.
PreCharge Circuit
A circuit that ensures that the intermediate circuit is pre-charged to at least 95% of the actual TS
accumulator voltage before closing the second AIR must be implemented.
It Should also use a mechanical relay
When initially connecting a battery to a load with capacitive input, there is an inrush of current as the
load capacitance is charged up to the battery voltage. In our application using a large battery and
powerful load, this inrush current is very high.
The pre-charge circuit is required to charge the circuitry between the accumulators and the motor
controller to 95% of the maximum operating voltage before closing the second AIR. This must be done
to protect the motor controller and other components from the very large inrush current. Pre-charge
relay is located in the lower accumulator container.
The AIR exists as follows at both negative and positive terminals of the Accumulator. The DC Signal feeds
an inverter that then produces a controlled three phase supply to an AC Motor
The Inverter has inbuilt capacitance, so if we were to close the AIR together, a significant inrush current
would exist, stressing the components, especially the AIR.Hence we need to precharge it to avoid inrush
current
τ-Symbolizes the Time Constant which is essentially R x C
The Capacitor charges till 63% of its maximum capability in one Time constant,
And after 3 Time constants reach 95%, which is the required amount. The initial inrush current is
avoided through this.
The Idea is around 0.15 in this graph accordingly
Implementation of said idea is as follows. The Pre Charge Relay also is required to be in the Accumulator
Housing.
The Pre-charge circuit consists of:
● A pre-charge resistor, to limit the inrush current (R1)
● A contactor (high power relay) across the pre-charge resistor (K2) to bypass the resistor during
normal operation
The pre-charge circuit may have:
● A pre-charge relay (K1), to keep the load from being powered through the pre-charge resistor
when the system is off
● A contactor in line with the other end of the battery (K3) to isolate the load when the system is
off.
Operation works as follows
Off: All relays/contactors off
Precharge: K1 and K3 are turned on, precharge the load
On:K2 is turned on, K1 may be turned off
The Precharge circuit must be integrated with the Shutdown circuit while being monitored by the TSAL
The Data Loggers/ Energy meters when performing voltage measurement draws a very small current
from the circuit it is measuring.
If the pre charge is very gentle, the current drawn by the meter can equal the current flowing through
the pre charge, reaching an equilibrium, preventing the 95% Threshold
A very simplified circuit of the Pre Charge Circuit
At Steady State, no charge through the capacitor, Vc Dictated by RLog and RPC
Vc= Vin(RLog/RPC+RLOG)
For 95%
Vc/Vin= (RLog/RPC+RLOG)=0.95
0.95 RPC= 0.05 RLog
RPC<1/19 RLOG
Based on this, the energy meter's resistance is essential to be known.
Discharge Circuit
The Discharge Rules state that upon opening of the shutdown circuit, The tractive system voltage drop
below 60V DC and 25VARCMS in less than 5 seconds.
This is measured by TSAL and is fairly visually obvious.
If the components don't do that on their own, there should be a discharge circuit.
It should be able to handle the maximum TS Voltage. Should be connected always in the shutdown
circuit, be in Normally Closed Format.
The Discharge Circuit should also be on the inverter side of the HVD so as to not be removed when
accumulator is removed
The Basic Idea is of an RC Discharge
There should be adequate cooling and heating sinks and not be too close to other components.
Basic Diagram with both Pre Charge and Discharge Circuit
APPS(Accelerator Pedal Position
Sensor)
There are two APPS, mainly for Throttle and Brake, which then signals to the ECU which creates a
torque demand from the inverter.
These APPS must be actuated by the foot pedal.
These can be a software based function.
If an Implausibility occurs, Power to motor shutdown completely, not necessary to shutdown Tractive
System
The potentiometer is mapped to the pedal position with 0 volts at 0 pedal position.
To be noted it is not linearly mapped, as it is difficult for normal humans to control.
This results in a faulty diagnostic system, where it is hard to know why the car is not functioning.
To Go about this problem
In this scenario, in case of power failure or open circuit, if the ECU picks up Zero Volts, it can flag up this
as a problem in the APPS, as the lowest signal at 0 pedal is now supposed to be 0.5V
Able to diagnose and fix these faults in advance of the dynamic events
Having two APPS with different transfer functions. By comparing their outputs, the Electronic Control
Unit (ECU) can detect discrepancies and not see the others signal as some other sensors in case of
crossover of wires
Drivers often prefer a non-linear pedal feel, where small pedal movements at the beginning translate to
smaller torque changes for precise control, while larger movements later in the pedal travel provide
higher acceleration for power delivery. Different transfer functions in each APPS can be combined to
achieve this nonlinear characteristic.
This becomes the range in 0 to 5V V/S Mapping in 0.5 to 4.5V
Problem with this APPS Design is
a) Intersecting transfer functions
Optimizations
Active protection circuits (BSPD, IMD, AMS) are engineered so that they have a normally open output.
These are to be normally open, which means closed when healthy but open when
a)At Fault
b)No power(Normally Open)
All HV must be switched on by AIRS and the Pre Charge Relay.
Smart Switches: Analyze impact severity and adapt power cut-off level accordingly.
Smart Disconnects: Analyze potential hazards and adapt disconnect behavior accordingly.
Also the idea of them being checked with SCS Conditions According to Rulebook, i.e rechecking signal
failure
Realize conditions that are fed through the DQ System to further be able to predict Insulation issues
Integrating microcontrollers to analyze impact severity of car and prevent false alarms of Inertia Switch
Bibliography
Formula Bharat Rules 2024
https://www.youtube.com/watch?v=RYL2U7UWuZU&list=PLpHzkCiWVrS0ib4OUYNiA50polKLBGL2&index=1&pp=iAQB
https://www.youtube.com/watch?v=L6z1lT_QTXM&list=PLpHzkCiWVrS0ib4OUYNiA50polKLBGL2&index=2&pp=iAQB
https://www.youtube.com/watch?v=Yoe4o8ow41Y&list=PLpHzkCiWVrS0ib4OUYNiA50polKLBGL2&index=3&pp=iAQB
https://www.youtube.com/watch?v=p8N4gbJapzI&list=PLpHzkCiWVrS0ib4OUYNiA50polKLBGL2&index=4&pp=iAQB
https://www.youtube.com/watch?v=OGA6LWu8oDo&list=PLpHzkCiWVrS0ib4OUYNiA50polKLBGL2&index=5&pp=iAQB
https://www.wisconsinracing.org/wp-content/uploads/2020/10/2018_ESF_Submission.pdf
https://youtu.be/n594CkrP6xE
https://www.reddit.com/r/FSAE/
https://webthesis.biblio.polito.it/14504/1/tesi.pdf
https://www.irjet.net/archives/V7/i8/IRJET-V7I8842.pdf