Embedded Files
EV.5.1 Accumulator
EV.5.1.1 All cells or super capacitors which store the Tractive System energy are built into Accumulator Segments and must be enclosed in (an) Accumulator Container(s).
Video - Super-Capacitors vs. Batteries?
Video - Super-Capacitors vs. Batteries?
A supercapacitor is a high-power density energy storage system which acts as a short-term power supply and can store from ten to a hundred times more energy than conventional capacitors. They require a much shorter time to charge and discharge, as well as a significantly larger power density than batteries.
In an automotive context, supercapacitors are often used in regenerative braking systems. When an electric vehicle decelerates, the energy created from the kinetic movement of the vehicles is stored using supercapacitors to be used later in quick acceleration. Supercapacitors assist the battery in times when the electric vehicle requires a sudden peak in energy output, improving overall operational energy efficiency and product lifecycle of both the electric vehicle and battery.
In this rule, it is established that any battery cell or supercapacitors which store Tractive (High-Voltage) Energy are required to be housed within the Accumulator Container. This is because there are a variety of general requirements, external and internal structure requirements, requirements for holes and openings, and requirements for the attachment specific to the High-Voltage housings.
EV.5.1.2 Each Accumulator Segment must contain:
Maximum static voltage of less than 120 V DC
Maximum energy of 6 MJ
The contained energy of a stack is calculated by multiplying the maximum stack voltage with the nominal capacity of the used cell(s).
File - HeadWay Datasheet
File - HeadWay Datasheet
HeadWay LiFePO4 38120__ Specifications1.pdfShown to the right is the datasheet for cells which we used in 2022-2023 (HeadWay LiFePO4 38120(38105) cells). Each cell has a maximum voltage of 3.65V and nominal capacitance of 8Ah (Amp-Hours)
Our 2022-2023 team defines a segment as having four cells in parallel, twice in series. This is denoted as 4P2S.
To calculate the static voltage of our stack, we use Nodal Analysis (shown below). To calculate the contained energy of our stack, we multiply the findings of our static voltage with a Nominal Capacity calculated using an Equivalent Capacitance Analysis (shown below), and converting from Amp-Hours to Joules.
Graphic - Nodal Analysis
Graphic - Nodal Analysis
Starting from a reference node (GND), there are two nodes where our voltage steps up by the Maximum Cell Voltage, 3.65V.
Therefore, the Maximum Static Segment Voltage is 7.3V.
Graphic - Equivalent Capacitance
Graphic - Equivalent Capacitance
Combining our capacitors in their respective four parallel components brings us 8Ah x 4, for two totals of 32 Ah. Combined in series, this brings us to 1/32Ah + 1/32 Ah.
Therefore, the Nominal Capacity is 32 Ah.
The Maximum Contained Energy is then calculated as 7.3 V x 32 Ah x 0.0036 J/h = 0.84096 MJ
EV.5.1.3 No further energy storage except for reasonably sized intermediate circuit capacitors are allowed after the Energy Meter EV.3.1
Manual - Energy Meter Wiring
Manual - Energy Meter Wiring
As shown in the diagram above, the energy meter must be placed in series with the negative terminal(s) of the Accumulator(s) such that all energy passes through the current shunt. Additionally, as shown to the left, it must be connected to the positive battery terminal with a fused sense lead in order to measure voltage. All of these connections must be on the motor controller side of the AIR coils.
The goal of the Energy Meter is to track the High Voltage (HV) lines to ensure that the rules regarding maximum power and voltage within the Tractive System are followed. That said, EV.5.1.2 serves to prevent teams from bypassing this compliance-check with a second Accumulator beyond the meter.
Rules Reference - EV.3.1
Rules Reference - EV.3.1
EV.3.1 Operation
EV.3.1.1 Supplying power to the motor to drive the vehicle in reverse is prohibited
EV.3.1.2 Drive by wire control of wheel torque is permitted
EV.3.1.3 Any algorithm or electronic control unit that can adjust the requested wheel torque may only decrease the total driver requested torque and must not increase it
For full transparency, I am almost certain that referencing this rule instead of EV.3.2 was a typo. These operation-based rules state that having a reverse gear is not allowed, we are allowed to use an analog-potentiometer to request torque, and that torque vectoring cannot be used to amplify power requested by the driver.
For more information about the Energy Meter, see EV.3.2.
EV.5.1.4 All Accumulator Segments and/or Accumulator Containers (including spares and replacement parts) must be identical to the design documented in the ESF and SES
This rule specifies that each of the Segments and Containers used at competition must not be different than what is documented in the Electrical Systems Form (ESF) or Structural Equivalency Spreadsheet (SES). Ultimately, the ESF and SES serve as first-passes at determining whether or not the designs of these critical components are rules compliant. If a team passes the review of these forms, but comes into competition with a completely different build, then there is no way of determining whether or not the components are actually compliant on-site.
That said, the segments do not need to be identical. Every segment design must be properly documented in the ESF and SES, but you can absolutely have multiple variations of segment designs within your Accumulator Container. The rule of thumb here is just to not go into competition with undocumented Accumulator Segment/Container(s).
Page updated
Report abuse