Embedded Files
EV.5.6 Precharge and Discharge Circuits
EV.5.6.1 The Accumulator must contain a Precharge Circuit. The Precharge Circuit must:
a. Be able to charge the Intermediate Circuit to minimum 90% of the Accumulator voltage
before closing the second AIR
b. Be supplied from the Shutdown Circuit EV.7.1
c. Not be fused
A Precharge Circuit must be integrated into the Accumulator for a variety of reasons. Firstly, managing the inrush current that occurs when initially connecting the battery to a load or system helps to gradually charge capacitors and other components. By doing so, the circuit prevents sudden current surges that could damage the cells or connected devices. This controlled approach also reduces the risk of sparks or arcing during connection, and helps the overall system reliability and longevity.
The video shown to the right provides a great explanation as to why this process is needed, as well as contextualizes the electrical system using a water analogy (so it is very beginner-friendly)!
Video - Precharge Circuit (0:00-2:40)
Video - Precharge Circuit (0:00-2:40)
Schematic - Accumulator
Schematic - Accumulator
To prevent hazards associated with inrush current, EV.5.6.1.a requires that the Precharge and Precharge Controller supply to the Motor Controller (Intermediate Circuit) a substantial amount (at least 90%) of the Accumulator's Nominal Voltage before both of the AIR's close.
Our Precharge Circuit is highlighted on the left in green. As you can see, the High Voltage (HV) cables start at these components before entering the Tractive Load.
EV.5.6.1.b stipulates that the Precharge Circuit be supplied from the Shutdown Circuit, described further in EV.7.1.
Rules Reference - EV.7.1
Rules Reference - EV.7.1
EV.7.1 Shutdown Circuit
EV.7.1.1 The Shutdown Circuit consists of the following components, connected in series:
a. Accumulator Management System (AMS) EV.7.3
b. Insulation Monitoring Device (IMD) EV.7.6
c. Brake System Plausibility Device (BSPD) EV.7.7
d. Interlocks (as required) EV.7.8
e. Master Switches (GLVMS, TSMS) EV.7.9
f. Shutdown Buttons EV.7.10
g. Brake Over Travel Switch (BOTS) T.3.3
h. Inertia Switch T.9.4
EV.7.1.2 The Shutdown Circuit must directly carry the current driving the Accumulator Isolation Relays (AIRs) and the Precharge Circuit Relay.
EV.7.1.3 The AMS, IMD, and BSPD parts of the Shutdown Circuit must be Normally Open
EV.7.1.4 The AMS, IMD and BSPD must have completely independent circuits to Open the Shutdown Circuit.
The design of the respective circuits must ensure that a failure cannot result in electrical power being fed back into the Shutdown Circuit.
EV.7.1.5 The Shutdown Buttons, BOTS, TSMS, GLVMS and Interlocks must directly carry the Shutdown Circuit current
EV.7.1.6 The team must be able to demonstrate all features and functions of the Shutdown Circuit and components at Electrical Technical Inspection.
Stated by EV.7.1.1, the Shutdown Circuit consists of several critical components. Starting with the Accumulator Management System (AMS), this component regulates cell charging and discharging, monitors cell temperature, and triggers a relay which stops operation when in distress. Next, the Insulation Monitoring Device (IMD) validates isolation between the Accumulator poles and chassis ground.
The Brake System Plausibility Device (BSPD) ensures that the driver is not requesting torque while the brakes are depressed. Interlocks serve to disconnect electrical components on demand, and ensure that systems are not being supplied with power in an improper configuration.
Master Switches (GLVMS, TSMS) remain open unless actuated by a specific key, and also ensure that systems aren't accidentally energized. Shutdown Buttons are emergency stop (E-Stop) buttons to be actuated by the driver or passerby in case of emergency, and the Brake Over Travel Switch (BOTS) shuts down power if the brake pedal is actuated beyond normal operating limits (for example, if the assembly breaks or if the brake pressure is lost).
Finally, the Inertia Switch triggers when the lateral acceleration of the vehicle mimics strong decelerations as a result of a crash. These components are connected in series to ensure that if a single component fails, no power is supplied to the vehicle.
EV.7.1.2 stipulates that the circuit directly controls current to the Accumulator Isolation Relays (AIRs) and Precharge Circuit Relay, crucial for managing power distribution and system shutdowns.
EV.7.1.3 and EV.7.1.4 both regulate the AMS, IMD, and BSPD. Firstly, these components must be of the Normally Open Type which prevents electricity from flowing when the switch is not activated or compressed. When the switch is activated, it closes the circuit and allows electricity to flow. Additionally, they must have independent circuits to reliably open the Shutdown Circuit in case of failure, preventing electrical feedback from recycling power within the Shutdown System.
As per EV.7.1.5, the Shutdown Buttons, Brake Over Travel Switch (BOTS), Torque Sensor Monitoring System (TSMS), General Low Voltage Master Switch (GLVMS), and any required interlocks must directly carry current to the Shutdown Circuit. This is in place to prevent any delay that would result from these signals being passed through webs of other components.
And finally, EV.7.1.6 stipulates that teams must demonstrate full functionality of the Shutdown Circuit and its components during Electrical Technical Inspection to ensure operational readiness and safety compliance.
Pictured within the referenced rule is an example schematic for teams to inform their own Shutdown Circuits. There, you can see that the Precharge Circuit is powered within the Shutdown Circuit.
Rules Reference - EV.7.3 (Referenced in EV.7.1)
Rules Reference - EV.7.3 (Referenced in EV.7.1)
EV.7.3 Accumulator Management System - AMS
EV.7.3.1 An Accumulator Management System must monitor the Accumulator(s) Voltage EV.7.4 and Temperature EV.7.5 when the:
a. Tractive System is Active EV.11.5
b. Accumulator is connected to a Charger EV.8.3
EV.7.3.2 The AMS must have galvanic isolation at every segment to segment boundary, as approved in the ESF
EV.7.3.3 Cell balancing is not permitted when the Shutdown Circuit is Open ( EV.7.2, EV.8.4 ) The AMS must monitor for:
a. Voltage values outside the allowable range EV.7.4.2
b. Voltage sense Overcurrent Protection device(s) blown or tripped
c. Temperature values outside the allowable range EV.7.5.2
d. Missing or interrupted voltage or temperature measurements
e. A fault in the AMS
EV.7.3.5 If the AMS detects one or more of the conditions of EV.7.3.4 above, the AMS must:
a. Open the Shutdown Circuit EV.7.2.2
b. Turn on the AMS Indicator Light. The light must stay on until the AMS is reset EV.7.2.3
EV.7.3.6 The AMS Indicator Light must be:
a. Color: Red
b. Clearly visible to the seated driver in bright sunlight
c. Clearly marked with the lettering “AMS”
Rules Reference - EV.7.6 (Referenced in EV.7.1)
Rules Reference - EV.7.6 (Referenced in EV.7.1)
EV.7.6 Insulation Monitoring Device - IMD
EV.7.6.1 The vehicle must have an Insulation Monitoring Device (IMD) installed in the Tractive System
EV.7.6.2 The IMD must be a Bender ISOMETER® IR155-3203 or IR155-3204 (website) or an approved alternate equivalent IMD
Refer to the Rules FAQ on the FSAE Online website for approved equivalent IMD
EV.7.6.3 The response value of the IMD must be set to 500 Ohm / Volt or higher, related to the maximum Tractive System operation voltage.
EV.7.6.4 The IMD must monitor the Tractive System for:
a. An isolation failure
b. A failure in the IMD operation
This must be done without the influence of any programmable logic.
EV.7.6.5 If the IMD detects one or more of the conditions of EV.7.6.4 above the IMD must:
a. Open the Shutdown Circuit EV.7.2.2
b. Turn on the IMD Indicator Light. The light must stay on until the IMD is reset EV.7.2.3
EV.7.6.6 The IMD Indicator Light must be:
a. Color: Red
b. Clearly visible to the seated driver in bright sunlight
c. Clearly marked with the lettering “IMD”
Rules Reference - EV.7.7 (Referenced in EV.7.1)
Rules Reference - EV.7.7 (Referenced in EV.7.1)
EV.7.7 Brake System Plausibility Device - BSPD
EV.7.7.1 The vehicle must have a standalone nonprogrammable circuit to check for simultaneous braking and high power output
The BSPD must be provided in addition to the APPS / Brake Pedal Plausibility Check (EV.4.7)
EV.7.7.2 The BSPD must Open the Shutdown Circuit EV.7.2.2 when the two of these exist:
Demand for Hard Braking EV.4.6
Motor/Accumulator current is at a level where 5 kW of electrical power in the DC circuit is delivered to the Motor(s) at the nominal battery voltage
The BSPD may delay opening the shutdown circuit up to 0.5 sec to avoid false trips
EV.7.7.3 The BSPD must Open the Shutdown Circuit EV.7.2.2 when there is an open or short circuit in any sensor input
EV.7.7.4 The team must have a test to demonstrate BSPD operation at Electrical Technical Inspection.
a. Power must not be sent to the Motor(s) of the vehicle during the test
b. The test must prove the function of the complete BSPD in the vehicle, including the current sensor
The suggested test would introduce a current by a separate wire from an external power supply simulating the Tractive System current while pressing the brake pedal
Rules Reference - EV.7.8 (Referenced in EV.7.1)
Rules Reference - EV.7.8 (Referenced in EV.7.1)
EV.7.8 Interlocks
EV.7.8.1 Interlocks must be incorporated where specified (refer to EV.4.1.3, EV.5.5.2, EV.5.10 )
EV.7.8.2 Additional Interlocks may be included in the Tractive System or components
EV.7.8.3 The Interlock is a wire or connection that must:
a. Open the Shutdown Circuit EV.7.2.2 if the Interlock connection is broken or interrupted
b. Not be in the low (ground) connection to the AIR coils of the Shutdown Circuit
EV.7.8.4 Interlock circuits or connections do not require physical separation (EV.6.5) from Tractive System wiring or components
Rules Reference - EV.7.9 (Referenced in EV.7.1)
Rules Reference - EV.7.9 (Referenced in EV.7.1)
EV.7.9 Master Switches
EV.7.9.1 Each vehicle must have two Master Switches that must:
a. Meet T.9.3 for Configuration and Location
b. Be direct acting, not act through a relay or logic
EV.7.9.2 The Grounded Low Voltage Master Switch (GLVMS) must:
a. Completely stop all power to the GLV System EV.4.4
b. Be in the center of a completely red circular area of > 50 mm in diameter
c. Be labeled “LV”
EV.7.9.3 The Tractive System Master Switch (TSMS) must:
a. Open the Shutdown Circuit in the OFF position EV.7.2.2
b. Be the last switch before the AIRs except for Precharge circuitry and Interlocks.
c. Be in the center of a completely orange circular area of > 50 mm in diameter
d. Be labeled “TS” and the symbol specified in ISO 7010-W012 (triangle with black lightning bolt on yellow background).
e. Be fitted with a "lockout/tagout" capability in the OFF position EV.11.3.1
Rules Reference - EV.7.10 (Referenced in EV.7.1)
Rules Reference - EV.7.10 (Referenced in EV.7.1)
EV.7.10 Shutdown Buttons
EV.7.10.1 Three Shutdown Buttons must be installed on the vehicle.
EV.7.10.2 Each Shutdown Button must be a push-pull or push-rotate emergency switch
EV.7.10.3 One Shutdown Button must be on each side of the vehicle which:
a. Is located aft of the driver’s compartment at approximately the level of the driver’s head
b. Has a diameter of 40 mm minimum
c. Must not be easily removable or mounted onto removable body work
EV.7.10.4 One Shutdown Button must be mounted in the cockpit which:
a. Is located in easy reach of the belted in driver, adjacent to the steering wheel, and unobstructed by the steering wheel or any other part of the vehicle
b. Has diameter of 24 mm minimum
EV.7.10.5 The international electrical symbol (a red spark on a white edged blue triangle) must be near each Shutdown Button.
EV.7.10.6 Pressing any of the Shutdown Buttons must Open the Shutdown Circuit EV.7.2.2
Rules Reference - T.3.3 (Referenced in EV.7.1)
Rules Reference - T.3.3 (Referenced in EV.7.1)
T.3.3 Brake Over Travel Switch - BOTS
T.3.3.1 The vehicle must have a Brake Over Travel Switch (BOTS). Brake pedal travel exceeding the normal range will actuate the switch
T.3.3.2 The BOTS must be a mechanical single pole, single throw (commonly known as a two position) switch (push-pull or flip type).
T.3.3.3 Operation of the BOTS to the OFF position must Open the Shutdown Circuit IC.9.2.2 / EV.7.2.2
T.3.3.4 Repeated operation of the switch must not reset or restore power
T.3.3.5 The driver must not be able to reset the BOTS.
T.3.3.6 The BOTS must be implemented with analog components, and not using programmable logic controllers, engine control units, or similar functioning digital controllers.
Rules Reference - T.9.4 (Referenced in EV.7.1)
Rules Reference - T.9.4 (Referenced in EV.7.1)
T.9.4 Inertia Switch
T.9.4.1 Inertia Switch Requirement
(EV) Must have an Inertia Switch
(IC) Should have an Inertia Switch
T.9.4.2 The Inertia Switch must be:
a. A Sensata Resettable Crash Sensor or equivalent
b. Mechanically and rigidly attached to the vehicle
c. Removable to test functionality
T.9.4.3 Inertia Switch operation:
a. Must trigger due to a longitudinal impact load which decelerates the vehicle at between 8 g and 11 g depending on the duration of the deceleration (refer to spec sheet of the Sensata device)
b. Must Open the Shutdown Circuit IC.9.2.2 / EV.7.2.2 if triggered
c. Must latch until manually reset
d. May be reset by the driver from inside the driver's cell
Finally, EV.5.6.1.c states that the Precharge Circuit must not be fused. This is likely because fuses introduce an additional component that could potentially fail or require replacement, affecting the reliability of the circuit. Ultimately, minimizing additional points of failure is important for this system's reliability.
EV.5.6.2 The Intermediate Circuit must precharge before closing the second AIR. The end of precharge must be controlled by one of the following two options:
a. Feedback by monitoring the voltage in the Intermediate Circuit
b. A conservative time defined by the longer of:
Twice the time to charge to 90%
The time to charge to 90% plus 500ms
The Intermediate Circuit refers to the path between the Accumulator and Motor, inclusive of the Motor Controller. This circuit must complete at least 90% of its charging cycle before the second Accumulator Isolation Relay (AIR) is closed. If the other pole of the AIR Coil were to close before a full pre-charge, there would be potential for uncontrolled inrush current. Keeping one relay open until that time allows for the pre-charge to be completed safely. The completion of precharge can be determined either by monitoring the voltage in the Intermediate Circuit, or by using a predefined time based whichever of the following options takes longer:
Twice the time to charge to 90% capacity
The time to charge to 90% plus an additional half-second.
These measures ensure safe and controlled activation of the circuit, optimizing performance and reliability during operations.
EV.5.6.3 The Tractive System must contain a Discharge Circuit. The Discharge Circuit must be:
a. Wired in a way that it is always active when the Shutdown Circuit is open
b. Able to discharge the Intermediate Circuit capacitors if the HVD has been opened
c. Not be fused
d. Designed to handle the maximum Tractive System voltage for minimum 15 seconds
The Tractive System requires a Discharge Circuit, which ultimately allows the safe discharge of stored electrical energy from the Accumulator and Tractive Components when the car is turned off. This circuit incorporates protective devices to prevent damage from overcurrent conditions. These protections are critical to safeguarding the Accumulator Segments, motor controller(s), and other electrical components from potential faults or failures.
The video shown to the right provides a great explanation as to why this process is needed, as well as contextualizes the electrical system using a water analogy (so it is very beginner-friendly)!
Video - Discharge Circuit (2:40-3:49)
Video - Discharge Circuit (2:40-3:49)
EV.5.6.3.a requires that the Discharge Circuit be active when the Shutdown Circuit is open, so that the excess Tractive power has a means of exiting the Tractive Components during the power-down stage of our vehicle. Additionally, EV.5.6.3.b stipulates that as soon as the High Voltage Disconnect (HVD) is opened and disconnected, the Discharge Circuit must be able to dissipate the energy stored within the Intermediate Circuit elements which have a capacitive load, being the Motor Controllers. Keep in mind that even if the Motor Controller discharges on its own, additional discharge circuitry may need to be added. According to EV.5.6.3.c, These elements must not be fused, likely because fuses introduce an additional component that could potentially fail or require replacement, affecting the reliability of the circuit. Ultimately, minimizing additional points of failure is important for this system's reliability. Finally, this element must be designed to handle the Maximum Tractive Voltage for at least 15 seconds as stated in EV.5.6.3.d. This provides a large time buffer and factor of safety in the worrst-case scenario of discharge, and ensures that this critical safety system will not fail under normal or extenuating circumstances.
EV.5.6.4 Positive Temperature Coefficient (PTC) devices must not be used to limit current for the Precharge Circuit or Discharge Circuit
First of all, what is a Positive Temperature Coefficient (PTC)?
This term describes materials or devices that exhibit an increase in electrical resistance as temperature rises. This characteristic is opposite to those with a negative temperature coefficient (NTC), where resistance decreases with temperature. PTC materials are commonly used in applications such as resettable fuses (PTC thermistors) and self-regulating heating elements due to their ability to limit current flow and stabilize temperatures. They provide inherent thermal stability, making them valuable for applications requiring consistent performance across varying environmental conditions.
Photo - PTC Devices
Photo - PTC Devices
Using PTC devices to limit current in our application is likely barred because they typically have a slow response time to changes in temperature and current flow. Additionally, increasing ambient temperature may lead to inconsistent current limiting behavior as a result of increased resistance. In the case of our Precharge and Discharge Circuits, where rapid voltage stabilization is crucial, this inconsistent response could delay the process and cause inefficiencies in circuit operation.
EV.5.6.5 The precharge relay must be a mechanical type relay
Okay, so what is a mechanical type relay?
A mechanical relay is an electromechanical device that uses mechanical contacts to make or break electrical connections. It consists of a coil, an armature, and one or more sets of contacts. When an electrical current passes through the coil, it generates a magnetic field that attracts the armature, causing the contacts to close or open. Additionally, these relays provide electrical isolation between the control circuit (coil side) and the load circuit (contacts), enhancing safety and preventing electrical interference.
This is likely required as opposed to solid-state relays due to the fact that mechanical relays fail in the off-state, while solid state relays fail in the on-state.
Graphic - Parts of a Relay
Graphic - Parts of a Relay
Graphic - Mechanical Relay
Graphic - Mechanical Relay
The graphic shown to the left showcases an electromagnetic interaction between the coil, armature, and contacts within a Mechanical Relay. The spinning element connected to the battery is a representation of a motor!
Page updated
Report abuse