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EV.4.4 Grounded Low Voltage System
EV.4.4.1 The GLV System must be:
a. A Low Voltage system that is Grounded to the Chassis
b. Able to operate with Accumulator removed from the vehicle
EV.4.4.1.a dictates that, in accordance with T.9.1.2, the Grounded Low Voltage (GLV) system is a Low Voltage system. This stipulates that its voltage is within a range of predefined "low voltages" (60V DC or 25VRMS AC). Additionally, this system must be grounded to the chassis, which allows the current to flow back to a lower energy state. The chassis is "prime real-estate" for a common ground because of its low impedance and high conductance, and it significantly simplifies wiring.
Another justification for the system grounding to the chassis would be so that it can best support the Insulation Monitoring Device (IMD). This component powers down the tractive system off in the case that High and Low voltage systems intersect. If this were the case and the chassis weren't grounded, the car would keep working as usual but still connected to HV (which is potentially a very dangerous scenario). If grounded correctly, the IMD would be able to detect a connection between HV and GLV, and power down correctly.
T.9.1.2 Low Voltage - LV
Any voltage less than and including 60 V DC or 25 V AC RMS
EV.4.4.1.b stipulates that the grounded low voltage system should be able to operate independently of the Accumulator. For this to be the case, we need to incorporate an entirely separate battery with a voltage rated at or below the specified T.9.1.2 range. The issue now becomes once installed, how do we consistently access the battery to recharge it as needed (batteries don't last forever)?
Photo - Banana Jack Component
Photo - Banana Jack Component
One approach is to intentionally design GLV battery charge points into the vehicle at accessible locations. Other teams have integrated banana jacks located on the outside of the car to be used for GLV battery charging, which supposedly made it very easy to plug in a benchtop power supply whenever a charge was needed.
Alternatively, you can use a DC-to-DC converter which enables as soon as the Accumulator Isolation Relays (AIRS) close.
But what is a DC-to-DC Converter?
Graphic - DC-to-DC Converter
Graphic - DC-to-DC Converter
DC-to-DC converters are devices that temporarily store electrical energy for the purpose of converting direct current (DC) from one voltage level to another.
Now, using this component in our application is certainly possible, but to prevent overcharge, we need to take some precautions. Below are a few options that we have at our disposal:
Powerpath Circuit
Powerpath Circuit
You may have to design a power-path circuit which disconnects the battery when the DC-to-DC converter enables, and then construct a battery charging circuit to manage the current.
Diodes
Diodes
Another way to do this is to use diodes to control the current flow, such that the accumulator can't directly charge the GLV battery, but you can still discharge from whichever source.
SPDT Relay
SPDT Relay
Alternatively, switching between the two with an Single Pole Double Throw (SPDT) relay (with capacitance to ensure the GLV bus is kept high while the relay switches) when the DC-DC converter turns on is a viable option.
EV.4.4.2 The GLV System must include a Master Switch, see EV.7.9.1
Photo - GLVMS
Photo - GLVMS
Specifications for the Grounded Low Voltage Master Switch (GLVMS) are referenced in the following rules:
Rules Reference - EV.7.9.1
Rules Reference - EV.7.9.1
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
Rules Reference - T.9.3 (Referenced in EV.7.9.1)
Rules Reference - T.9.3 (Referenced in EV.7.9.1)
T.9.3 Master Switches
Each Master Switch ( IC.9.3 / EV.7.9 ) must meet the following:
T.9.3.1 Location
a. On the driver’s right hand side of the vehicle
b. In proximity to the Main Hoop
c. At the driver's shoulder height
d. Able to be easily actuated from outside the vehicle
T.9.3.2 Characteristics
a. Be of the rotary mechanical type
b. Be rigidly mounted to the vehicle and must not be removed during maintenance
c. Mounted where the rotary axis of the key is near horizontal and across the vehicle
d. The ON position must be in the horizontal position and must be marked accordingly
e. The OFF position must be clearly marked
f. (EV Only) Operated with a red removable key that must only be removable in the electrically open position
In rule EV.7.9.1, we are prompted to look at a Technical Aspects rule (T.9.3) which address points related to the Master Switch's configuration and location. Standardization of those two categories ensures the safety of drivers, spectators, and operators alike. This rule also requires that the switch act directly, so as not to incur delay through using relays or logic boards.
The specifications listed in T.9.3 provide guidelines for the location and characteristics of Master Switches at large. The instructions provided in the location section enforced to ensure that the switch is accessible to a driver, operator, or passerby for safety. For the characteristics rules, these are enforced to ensure that there are standards of what to look for in a system failure, and a driver, operator or passerby would quickly (and safely, without risk of electrocuting themselves) be able to enable the switch and cut power.
EV.4.4.3 A GLV Measuring Point (GLVMP) must be installed which is:
a. Connected to GLV System Ground
b. Next to the TSMP EV.5.8
c. 4 mm shrouded banana jack
d. Color: Black
e. Marked “GND”
Schematic - GLVMP to GND
Schematic - GLVMP to GND
The Grounded ow Voltage Measuring Point (GLVMP) is required by EV.4.4.3.a to be connected to the GLV System Ground. This, as stated in EV.4.4.1.a, must be grounded to the chassis. The location of this measuring point is dictated relative to the Tractive System Measuring Point (TSMP) in rule 4.4.3.b, whose specifications are defined in EV.5.8. This component also must be a Banana Jack (pictured in EV.4.4.1) colored black and labeled "GND", for ease of identification and access.
This component is shown in our 2022 ESF Master Schematic (pictured left) as being connected to 'X', which is a notation that we are using as our Earth Ground (Chassis).
EV.4.4.4 Low Voltage Batteries must meet T.9.2
T.9.2 focuses on the specifications for Low Voltage Batteries at large with guidelines regarding mounting, overcurrent protection, insulation, wet-batteries, lithium chemistry, and documentation. A synopsis of each section is as follows:
Rules Reference - T.9.2
Rules Reference - T.9.2
T.9.2 Low Voltage Batteries
T.9.2.1 All Low Voltage Batteries and onboard power supplies must be securely mounted inside the Chassis below the height of the Shoulder Belt Mount T.2.6
T.9.2.2 All Low Voltage batteries must have Overcurrent Protection that trips at or below the maximum specified discharge current of the cells
T.9.2.3 The hot (ungrounded) terminal must be insulated.
T.9.2.4 Any wet cell battery located in the driver compartment must be enclosed in a nonconductive marine type container or equivalent.
T.9.2.5 Batteries or battery packs based on lithium chemistry must meet one of the two:
a. Have a rigid, sturdy casing made from Nonflammable Material
b. A commercially available battery designed as an OEM style replacement
T.9.2.6 All batteries using chemistries other than lead acid must be presented at Technical Inspection with markings identifying it for comparison to a datasheet or other documentation proving the pack and supporting electronics meet all rules requirements
Graphic - Shoulder Harness Bar
Graphic - Shoulder Harness Bar
T.9.2.1 requires that the Low Voltage Batteries (and extraneous power sources) be mounted below the Shoulder Belt Mount. As specified in T.2.6, this must wrap around the Shoulder Harness Mounting Bar (pictured left in teal).
For context, the Shoulder Harness Mounting Bar (SHMB) is the bar that goes across the Main Hoop (in our case) which is used as the upper mounting point for the shoulder harness.
Rules Reference - T.2.6 (Referenced in T.9.2.1)
Rules Reference - T.2.6 (Referenced in T.9.2.1)
T.2.6 Shoulder Harness
T.2.6.1 From the driver’s shoulders rearwards to the mounting point or structural guide, the Shoulder Belt Side View Angle must be between 10° above the horizontal and 20° below the horizontal.
T.2.6.2 The Shoulder Belt Mount Spacing must be between 175 mm and 235 mm, center to center
T.2.6.3 The Shoulder Belts must attach by one of the four:
a. Wrap around the Shoulder Harness Mounting bar
b. Bolt through a welded tube insert or tested monocoque attachment F.7.9
c. Bolt or clip to a tab or bracket ( T.2.4.3 ) on the Shoulder Harness Mounting bar
d. Wrap around physically tested hardware attached to a monocoque
T.2.6.4 Any bolt used to attach a Shoulder Belt, directly to the chassis or to an intermediate bracket, is a Critical Fasteners, see T.8.2, with a minimum diameter that is the smaller of:
The bolt diameter specified by the manufacturer
10 mm or 3/8”
Schematic - GLV Fuse
Schematic - GLV Fuse
T.9.2.2 stipulates that all Low Voltage Batteries must have some form of overcurrent protection, put in place for safety. In our case, we opted to use a Grounded Low Voltage (GLV) Fuse to fulfill this role.
But what is a fuse?
A fuse is an overcurrent protection device with a fusible link that melts and opens a circuit when an overload condition or short occurs. The fusible link melts because the fuse is made of a metal that has a lower melting point than the copper of the conductor.
Positioning our fuse just after the GLVMS maintained compliance for both rules in this section.
As for T.9.2.3, this rule requires that the hot terminal of our battery be insulated so as to prevent shorting the positive and negative terminals. In our case, the wires attached to the positive and negative terminals of each cell are insulated.
T.9.2.4 refers specifically to wet batteries, which, also known as a flooded battery, are a type of battery that contain liquid electrolytes. The electrolyte is a mixture of water and acid, and the battery also has lead plates and separators. When the battery is connected, the acid bonds to the lead plates, triggering a chemical reaction. This rule specifies that the casing on these batteries must contained in a non-conductive marine type container.
But why marine type?
For context, batteries in boats are housed in waterproof plastic boxes so that water spills into the hull and cannot short the battery terminals, which could otherwise potentially start a fire or render the battery inoperative.
For our purposes, this is helpful because it also means the battery acid cannot spill out if a vehicle flips over, which prevents injury to the driver.
T.9.2.5.a and T.9.2.5.b are rules specific to Lithium-type batteries. Firstly, the housing for cells of this type must be designated Non-Flammable as defined in F.1.18.
Rules Reference - F.1.18
Rules Reference - F.1.18
F.1.18 Nonflammable Material
Metal or a Non Metallic material which meets UL94-V0, FAR25 or approved equivalent
This rule specifies the flammability characteristics of metallic and non-metallic (often thermoplastic) materials to ensure that materials used extinguish potential fires in a timely and safe manner.
Table - UL94 Ratings
Table - UL94 Ratings
UL94, the Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances testing, is a plastics flammability standard which determines the material's tendency to either extinguish or spread the flame once the specimen has been ignited.
Table - FAR25 Testing
Table - FAR25 Testing
FAR 25.253 is a standard of the Federal Aviation Administration (FAA) for determining the flammability characteristics of materials & components used in the aircraft. The purpose is to establish repeatable, reproducible, easy test methods to assess potential fire risks that may be happened in the aircraft.
And as stated for T.9.2.5.b, batteries must be commercially available and designed as an Original Equipment Manufacturer (OEM) style replacement. This rule prevents teams from designing potentially unreliable battery chemistries, or from sourcing batteries from Non-OEM sources. These are made by a different company that reverse-engineers an original battery and starts producing copies which may have functional discrepancies from the original design.
File - HeadWay Datasheet
File - HeadWay Datasheet
HeadWay LiFePO4 38120__ Specifications1.pdfFinally, T.9.2.6 requires that batteries with chemistries that are not lead-acid should be marked to identify them for comparison to a datasheet. In our case, we had to bring a printed datasheet for our HeadWay LiFePO4 38120(38105) cells.
This is in place to ensure that these batteries are rules compliant, and in our case, the markings (or specifications) of our HeadWay cells are printed on the battery skin for validation.
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