INTRODUCTION:

Lithium-ion (Li-ion) batteries are the choice of batteries used in Midnight Sun (MS) 15 and provide multiple benefits over other batteries, yielding efficient results when used to power solar electric vehicles. The installation of the batteries includes parallel and series arrangements to achieve an optimal voltage and current to power the vehicle's load: the electric vehicle's motors. One of the main benefits of lithium-ion batteries is that the design of the cells eliminates the burden of battery memory effect compared to Nickel-Cadmium (Ni-Cd) and Nickel-Metal Hydride (Ni-MH) batteries. A battery cell can undergo the memory effect when it repeatedly does not discharge entirely; therefore, it "remembers" the specific value it discharges to. For example, a Ni-Cd or Ni-MH battery can discharge to 20% constantly, and the battery will "remember" this state of charge as 0%, limiting battery operating time. Another property of lithium-ion batteries is the cells' high energy density. Li-ion batteries can store more energy than Ni-Cd or Ni-MH batteries and could emit charge throughout longer durations of time. Furthermore, lithium-ion batteries also provide a higher amount of current for high-power applications because the cells can contribute up to a maximum of 3.6 Volts (3 times more than Ni-Cd or Ni-MH batteries). The explanation of the previous statement concludes that current and voltage have a proportional relationship due to Ohms Law; nevertheless, an increase in voltage will allow for an increased current. In addition to li-ion cells being suitable for high power applications, the efficiency of Lead Acid batteries is 80-85% compared to lithium-ion batteries, which have a 95% efficiency. The percent efficiency defines the amount of usable energy stored within a battery. It is also essential to consider a battery's depth of discharge (DOD) which is the amount of energy that can be drained from the lithium-ion or Lead Acid battery to avoid complications. A lithium-ion battery's DOD is 85% compared to a Lead Acid battery that discharges up to 50% of its available energy. Regardless of the advantages that come from the use of li-ion cells, one must install a battery management system. A battery management system (BMS) is an electronic system that oversees and manages a rechargeable battery pack installed in an electric vehicle. The BMS ensures reliable battery operations, continuous battery health, and monitoring to reduce the risk of explosions and increase the life span of the batteries.

COMPONENTS:

Eight key components make up a battery management system:

1. Electrical Fuse: Electrical fuses are devices integrated within circuits to protect them from large amounts of current: avoiding damage to the circuit components and hazardous conditions. Fuses work by having a metal strip or wire that melts when it encounters a large amount of current, and therefore there will be an opening in the circuit causing the current to stop.

2. Power Relay: Power relays are electromagnetic switches that control circuits and amplify electric signals. A power relay includes four main components: coil, armature, primary circuit, and secondary circuits. The primary circuit is the circuit that controls the secondary circuits (controlled circuits that contain specified loads) and incorporates the coil of the power relay. As the current enters the coil, it becomes an electromagnet and attracts the armature, allowing the current to pass through to the desired secondary circuit. Furthermore, power relays play a significant role in electric vehicles because the design of power relays is appropriate for high current, power, and voltage applications. Therefore, power relays would generate less heat and avoid eroding due to the high-power consumption compared to regular relays.

3. Precharge Circuit: A precharge circuit controls the inrush current. The inrush current is the instantaneous current supplied by a power source to a load with a capacitor when the power source initially turns on. Furthermore, the inrush current charges the capacitor to a set voltage close to the voltage of the power source. Therefore, limiting the inrush current to a set current value that still allows the components to operate is essential since the inrush current causes a high peak current damaging the circuit components.

4. External Power Cut-Off Switch: An external power cut-off switch is placed on the outside exterior of Midnight Sun 15 to immediately stop the car's operation in the case of a dangerous event.

5. Real-Time Clock (RTC): The primary purpose of the real-time clock is that it acts as a logbook alongside the memory component of the battery management system to record the data of the battery pack and the real-time clock records the time stamps.

   Open

   

6. Cut Off FETs: The Cut Off FETs are metal–oxide–semiconductor field-effect transistors (MOSFETS) that act as electric switches to control circuits and amplify signals. The primary use of a MOSFET in the Battery Management System is to manage the connection of the battery pack with the load or the charger during certain conditions. The behaviour of a MOSFET is determined by analyzing the data from the voltages of the battery cells, current measurements, and using real-time detection circuitry.

7. Electrical Isolation: Electrical isolation in electric vehicles insulates wires and prevents safety hazards such as electrical leaks or fraying in wires. Leaks in electrical wires imply the issue of the defective insulation of electrical energy. On the other hand, frayed wires are electrical wires with the internal live wire exposed. If frayed wires are present in an electrical system, this can damage wires and cause other cables to also fray leading to an increase in the occurrence of a short-circuit when the wires come into contact. Electrical insulation is composed of electrical insulators: materials that decrease the flow of electricity. Isolation also proves its importance when utilized to isolate live wires with a high voltage potential from human and component contact. If isolation is not present or has faults, electrical conductors can make contact and transfer electrons more readily and cause damage to the car's electrical systems or circuit components. Additionally, touching an electrical conductor that may be exposed or if the exposed live wires connect with the vehicle's chassis, the electricity can migrate throughout the chassis and cause severe injuries and burns to drivers. Therefore, it is necessary to have an active isolation detection circuit as part of the battery management system because the active monitoring from the isolation detection circuit notifies drivers of minor faults in the insulation, inhibiting the safety hazards and defects from worsening. More information on electrical isolation can be accessed through this link.

8. Battery Cell Monitor: The battery cell monitor optimizes the performance of the batteries and ensures that the batteries are in proper condition; the monitor collects the observed data from the battery cells to display to the driver. A battery cell monitor can measure the voltage and current of the installed battery pack to determine the battery pack's state of charge (SOC). The cell balancing section interprets an explanation of the state of charge. The two types of battery cell monitors are voltage and shunt types. Voltage battery cell monitors provide the instantaneous voltage of the batteries by using an integrated voltmeter to measure voltage. During the charging of the cells, the voltage-based battery cell monitor also tracks and compares the voltage of the battery pack to a set voltage defined as a full charge and trips the charging process when the battery pack reaches this specific voltage. However, this is an issue because the weakest cells limit the charging process of the overall battery pack. When the weak cells have reached maximum charge, the voltage-based battery cell monitor will identify this as a full charge and trip the charging process. Simultaneously, other battery cells inside the battery pack or module will then have an imbalance in charge. Moreover, a voltage-based battery cell monitor has logic installed to convert the voltage reading from the voltmeter to determine the state of charge. However, voltage battery cell monitoring is not the most accurate method as the voltage of the battery changes depending on different conditions at the time of measurement. Figure 6 is a discharge voltage graph of a lithium-ion battery cell that displays the cell's voltage as the dependent variable and its state of charge as the independent variable. Discharge graphs consider certain conditions of the battery cell to accurately measure a battery's state of charge: discharging rate, number of cycles operated, and battery type. Furthermore, the graph outlines that the voltage remains approximately constant for 20% to 80% SOC but has a steep curve after 20% and 100% SOC. The voltage-based monitor proves helpful when calculating the SOC of an almost entirely full or empty battery. Therefore, the system that is best suitable to yield more accurate estimated results is the shunt-type battery cell monitor. The shunt-type battery cell monitor is connected to the battery cell's negative output terminal to allow the discharging current from the battery to pass through the low-resistive shunt with a known resistance value to the battery cell monitor. The monitor measures the voltage drop between the two points of the shunt. Through Ohms Law, the equation V=IR is rearranged to I=V/R: determining the current passing through the shunt. The determination of current is then applied to perform coulomb counting: the shunt-type battery cell monitor is calibrated to conduct the coulomb counting method by integrating the current over time, approximating the SOC of the battery pack.

SAFE OPERATING AREA (SOA):

The safe operating area (SOA) are predefined measurements where a lithium-ion battery cell operates in optimal conditions, and its performance and battery life are not affected. The main variables to regard are voltage and temperature:

Open

The suitable operating range for a lithium-ion battery is between -20°C to 55°C and 2.5 V to 4.2 V. Each bullet includes an internal link to a specific section with further information, except under-temperature:

• If the battery has a voltage of 2.5 V, it enters the under-voltage area.

• If the battery has a temperature of -20°C, it enters the under-temperature section.

• If the battery has a voltage of 4.2 V, it enters the over-voltage area.

• If the battery has a temperature of 55°C, it enters the over-temperature section.

UNDER-VOLTAGE AREA:

Description: Suppose the cell begins to have a voltage below 2.5 V. In that case, the battery can over-discharge up to its total capacity inducing lithium dendrite growth: metallic microstructures that form on the negative electrode of the battery cell. During more elevated temperatures, the dendrites react with the electrolytes of the battery cell, and one of the chemical reaction products is gas. An amplified amount of gas molecules increases the internal pressure within the battery cell and causes safety problems such as battery explosions and electrolyte leakage. Similarly, lithium plating is another issue a li-ion battery can encounter when entering the under-voltage area. Additional elaboration of lithium plating entails metallic lithium forming on the cell's anode and causing the battery to malfunction during future operation.

Safety Mechanism: On Midnight Sun 15, the under-voltage protection board interrupts the connection of the battery pack to the loads and includes:

• a fuse to limit short-circuit current.

• inrush-current limiting (circuit components that limit inrush current).

• reverse polarity protection (internal protection circuit utilized to protect components in the case that the power supply has a reversed polarity).

OVER-VOLTAGE AREA:

Description: On the other hand, if the battery overcharges and has an over-voltage above 4.2 V, it can enter thermal runaway because it is unsafely charging beyond the permitted charge capacity. Thermal runaway is explained more elaborately in this section.

Safety Mechanism:

• Charging Control: The charging voltage and current of the vehicle's charger are regulated to meet the charging limits of the batteries found on the battery manufacturer's data sheets. The design of the vehicle's charger can control the charging process by incorporating two charging stages: Constant Current (CC) and Constant Voltage (CV). Constant Current is the first stage of charging that involves charging the battery pack with a constant current. Constant Voltage is the second charging stage that allows the charger to charge the batteries at a constant voltage with a low current. Therefore, this two-stage charging process limits the charging voltage and current to prevent battery damage and irregular charging.

• Safe Charging: Safe charging implies that the charger receives input from voltage and temperature sensors that monitor the batteries to disable the charging process through protection circuitry if the batteries exceed the safe operating area or when the batteries have a SOC of 100%.

BATTERY THERMAL MANAGEMENT SYSTEM:

A high increase in energy throughout a battery cell will result in further spiraling internal temperatures; this also is defined as the battery entering thermal runaway. Throughout the process of overheating, the chemical reactions that occur in the battery gain the required activation energy to continuously take place, yielding a further increase in temperature and pressure. The battery can then melt or combust, placing the driver and other batteries at a safety risk. Along with thermal runaway, thermal propagation is also a concern with batteries. Thermal runaway propagation occurs when one battery cell or module enters thermal runway, causing the adjacent batteries to heat up and enter thermal runaway as well. Nonetheless, the whole battery system enters thermal runway, and the batteries become out of control. The BMS’ thermal management system controls the battery pack's temperature and ensures that the batteries operate within a safe range. It is governed by the vehicle controller integrated within the BMS and receives signals based on monitoring the battery pack. Passive and active thermal management systems are the two different types of systems that manage the temperature of the batteries. Passive thermal management systems are systems that don’t utilize energy to cool down the batteries, and active thermal management systems need a power source to operate. Below are examples of passive and active thermal management systems:

Active Cooling Systems:

Liquid Cooling - Liquid cooling is an active cooling system that uses a pump to distribute the coolant liquid (a specific type of fluid used in cell cooling) into metal plates. Metal plates are placed under, on top, or between the batteries to allow the coolant liquid to regulate the temperature of the batteries and prevent overheating. For clarity, there are two types of liquid cooling: dielectric liquid cooling (direct-contact liquid) and conducting liquid (indirect-contact liquid). Dielectric liquid cooling involves a cooling system where the batteries are in direct contact with the coolant liquid, and the coolant liquid is mineral oil. Furthermore, the conducting liquid method involves the coolant liquid and the batteries having in-direct contact. This method can be applied, and the solution used is a mixture of ethylene glycol and water. In a repetitive cycle, the liquid coolant absorbs the heat and is redirected to a radiator with fans that blow air onto the radiator to cool down the liquid coolant.

Forced Air-Cooling - Forced air-cooling could also act as a thermal management system and is mostly the chosen method of battery cooling for Midnight Sun 15. Fans force and blow the air onto the batteries to ensure the batteries are at a safe operating temperature. The orientation and spacing between the batteries are also critical to meeting thermal cooling demands. Identifying the best orientation to place the batteries is determined by the data specifications provided by the manufacturer and through experimental testing with thermistors.

Figure 14 displays a constructed forced air-cooling system.

Passive Cooling Systems:

Phase Change Material (PCM) Method - The phase change material (PCM) method is a passive cooling system that uses phase-changing material to absorb or release the heat produced from the batteries. During the thermal management of the batteries, the material will change phases from solid to liquid and enter the melting process as it absorbs heat. On the other hand, when the material releases the heat, it will undergo the freezing process and change phases from liquid to solid. The implementation of the PCM method is alongside the liquid cooling or air-cooling systems as it only transfers the heat away from the cells due to the small size of the phase-changing materials and the ability of the materials to absorb or release heat. Alongside the application of the PCM method, expanded graphite and metal foam can be added to the thermal management system due to the thermal conduction material properties of expanded graphite and metal foam. Similarly, one can coordinate the phase-changing material method with heat fins to control the temperature of the batteries: heat fins aid in increasing the heat dispersion rate and transferring the heat away from the batteries.

Heat Sinks - Heat sinks are devices that use materials with high thermal conductivity, like aluminum and copper, transferring the heat away from the batteries through the process of conduction. Thermal conduction involves heat transfer by direct contact of the batteries and the heat sink, where the heat diffuses from the batteries to the heat sink's fins. Additionally, heat sinks only transfer heat and are used in conjunction with another thermal management system to dissipate the heat into the environment.

Natural Air-Cooling - Natural air-cooling systems use air to redirect the heat away from the battery pack as air is a thermal medium. This thermal management method is applicable when a high volume of air can be blown over the batteries to manage the temperature. The source of air could be from the cabin or natural air from the environment, with the exception of proper ventilation slots.

OVERCURRENT CONTROL:

The controller of the battery management system controls the flow of overcurrent. The overcurrent protection system of the BMS has three current protection values set for safety detection. When the output current (supplied current to load) reaches the first protection current value, the controller of the battery management system limits the output current in the system. If the output current is not interrupted after reaching the first value, the current will increase to the second or third current protection value. After attaining the third protection value, the battery management system will block the charging or discharging path.

SHORT-CIRCUIT CONTROL:

Equivalently, the function of MOSFETS also significantly plays a role in disconnecting the battery pack and further disrupting the power supply to the vehicle in the case of under-voltage and over-voltage conditions. In the state of an external short-circuit, the MOSFETS disengage the battery system's discharging path to the load; accordingly, this permits one to detect the root cause of the short-circuit through inspection.

CELL BALANCING:

Description: Cell balancing is a vital component of the battery management system because the individual battery cells within battery packs must have an equal charge distribution to optimize the battery pack's energy. Apart from this, cell balancing involves distributing the charge of the battery cells equally; thus, all the individual cells' state of charge (SOC) is equal. The SOC of the batteries is a percentage measurement that indicates the amount of charge a battery cell has. For example, the state of charge of a battery cell can be 100%, meaning that the battery has a full charge. However, irregular discharging and charging of battery cells in a battery pack or module tremendously damages battery cells. Irregular discharging and charging stems from the issue that the capacity of the weakest cells (cells with the smallest capacity) limits the charging and discharging of the whole battery pack. Individual battery cells differ in charging and discharging rates due to multiple factors. In terms of charging, the cell with the smaller capacity will charge at a faster pace and may enter the over-voltage area, in contrast, to the adjacent cells that may have not fully charged. Similarly, the cells with a smaller capacity will discharge faster than neighbouring cells and may enter the under-voltage charge area.

• Active Cell Balancing: Active cell balancing or non-dissipative balancing is a cell balancing method that can redistribute the charge from cell to cell, cell to a battery pack, or battery pack to cell. Further, maintaining an equal state of charge throughout the battery cells in the battery pack by methods of charge shuttling or an energy converter. Correspondingly during the charging phase, the battery cells that obtain a maximum charge faster than others reassign the energy to individual batteries with a larger capacity. Simultaneously with the discharging stage, the weaker cells that discharge quicker receive the excess energy from other cells to ensure that the battery system is balanced. Active cell balancing poses as a suitable cell balancing system because it requires less time to balance the charge of the cells and generates less heat than passive cell balancing. However, it is not the most cost-efficient method and is more complicated to integrate; therefore, one may lean towards passive cell balancing.

• Passive Cell Balancing: Passive cell balancing, also known as dissipative balancing, is accomplished by placing resistors across the weakest battery cells and is the preferred practice of cell balancing for Midnight Sun 15. The resistor would drain the battery cell's energy; thus, the state of charge of the batteries is equivalent in the module or pack. Passive cell balancing is the more popular balancing system as it is easier to integrate and manage than active cell balancing. Consequently, passive cell balancing generates more heat than active cell balancing because the energy dissipation yields in the production of heat within the cell. Unlike non-dissipative balancing, the excess energy in the cells is "bled out" rather than redistributed. Additionally, in the case of passive balancing, it is crucial to have a thermal management system to manage the heat dissipation and eliminate the risk of the batteries entering thermal runaway.

CONTROLLER AREA NETWORK:

The CAN Bus, more formally known as the Controller Area Network, is responsible for the communication between the electrical control units (ECUs) of the vehicle. Electrical control units are units set to control certain electrical vehicle operations and interconnected via the CAN Bus to communicate with other ECUs minus the complex wiring. Through the CAN Bus, ECUs process messages, ignore information from different electrical control units, or broadcast data to alternative parts of the CAN network.

MS BMS CARRIER BOARD:

• Receives signals via CAN Bus communication and sends information to the vehicle controller to employ battery or protection circuitry applications.

• Broadcasts batteries state of charge data to car display along Controller Area Network.

• Establishes control over fans and relays.

• Records voltage, current, temperature, and relay states.

• Identifies overcharge, overcurrent, overtemperature, and undercharge, implementing circuity protection.

• When the car turns on, the carrier board examines the status of the battery pack. It determines if the battery system's current, voltage and temperature meet safety requirements and if it is suitable to connect the relays.

• Through CAN communication, if the battery system installed discharges until it reaches a low charge or proves to operate outside the applicable safety limits, the carrier board will disconnect the battery pack from the power button.

• The power distribution system is fundamental in the electrical subsystems of Midnight Sun 15 and distributes power safely to the car's internal systems. The power distribution unit oversees messages, sends signals concerning the power distribution system's state to the carrier board if an error occurs in the distribution of power, and enables changes to the system by switching relays.

MS BMS CURRENT SENSE BOARD:

The current sense board plays an integral role in the electrical systems of Midnight Sun 15 and is real-time current measuring circuitry that senses large abnormal amounts of current. Providing overcurrent protection to the batteries in the case of an electrical fault and ensuring the current is inside safe operating limits. Apart from this, current sensing is also essential to determine a more accurate state of charge measurement for the batteries using shunt-based and magnetic-based approaches. Shunt-based and magnetic-based approaches are described in more detail here.

MS BMS ANALOG FRONT END BOARD:

The analog front end (AFE) board measures the voltage of the battery cells in the specific battery pack by use of its voltage sensing channel. It also has thermistor temperature sensors that conduct temperature readings on the lithium-ion batteries to detect if the cells are operating alongside the appropriate temperatures amid charging/discharging cycles. However, regarding cell balancing, the AFE board is additionally programmed to initiate the implementation of the battery balancing circuits if the batteries in the battery pack have an imbalanced charge.