Productivity estimation of battery trolley mining truck fleets

ABSTRACT The mining sector is facing an immediate challenge to reduce its carbon emissions. The movement of material with mining truck fleets is a critical factor in reducing emissions. As the industry gradually replaces diesel trucks with more environmentally friendly transportation systems, it is important to analyse new solutions. This paper presents a new type of mining truck fleet called the Battery Trolley system, which is a combination of advanced technologies, including a battery-electric drivetrain, autonomy, trolley assist, and energy recovery systems. Depending on the site-specific differences and technology adoptions, decision-makers have three Battery Trolley system configurations to choose from: dynamic charging, stationary charging, and dual trolley Battery Trolley systems, each of which has its own advantages and disadvantages. One of the challenges of using Battery Trolley systems is their capacity and productivity. This study uses open-accessible project parameters and databases, with engineering assumptions, to create a mining scenario for both the dynamic charging and stationary charging perspectives. The study provides an equation to evaluate the Battery Trolley system’s mining productivity. Results show an incremental analysis of the Battery Trolley productivity as the trolley power increases from 8 MW to 32 MW in various Battery Trolley configurations. The study compares the Battery Trolley productivity for several open-pit mining applications, including copper, iron, and overburden waste. The results indicate that: (i) trolley power limitations significantly affect the capacity of Battery Trolley systems; (ii) a stationary charging option can achieve higher capacity and productivity than dynamic charging; (iii) Battery Trolley productivity varies in different applications under the given simulation assumptions.


Background
The effects of climate change have sparked worldwide consensus on the need for decarbonisation [1].In order to meet the emissions reduction targets set by various governments, innovative technological solutions will need to be implemented for significant decarbonisation.The mining industry, being an energy-intensive sector, is heavily reliant on fossil fuel energy.To combat climate change, the mining sector must adopt clean and renewable energy technologies as well as costeffective low-emission alternatives [2].It is feasible to maintain competitiveness while replacing high-emission systems by prioritising the electrification of mining equipment [3].With advancements in electrification and battery technologies, diesel-powered mining equipment and transportation can be gradually replaced with a combination of electricity-powered and energy storage solutions.The switch to an 'all-electric' mine will likely lead to greater emphasis on electricity generation and battery storage [2,4].Battery Trolley systems are poised to drive an electrification revolution and create the first zero-emission truck fleet in the mining industry.Battery-electric drivetrains, autonomy, Trolley Assist, and energy recovery systems technologies are crucial to realise the Battery Trolley systems concept.

Research aims
This research introduces the concept of Battery Trolley systems and assesses their technical viability.Three Battery Trolley system configurations: dynamic charging; stationary charging; and dual trolley Battery Trolley systems are available for decision-makers to choose from, each with its own advantages and disadvantages.As a mining haulage system, the capacity and productivity of Battery Trolley systems are crucial for mine planning and scheduling.This study creates several mining material movement scenarios based on the same haul route from dynamic charging and stationary charging perspectives, using openly-accessible project parameters and a database with engineering assumptions.Consequently, equations are provided to evaluate the mining productivity of Battery Trolley systems.A sensitive analysis of Battery Trolley productivity is calculated as the trolley power increases from 8 MW to 32 MW in various Battery Trolley configurations for considering the installation of substations when various alternatives are large-deployment.It examines different Battery Trolley configurations that offer mines the most optimal integrated haulage solution from a decarbonisation, capacity, and productivity standpoint [5].

Research scope
This paper focuses on the use of battery-electric trucks in open-pit mining operations, excluding underground applications.It does not address the detailed technical details of these systems from mechanical and electrical engineering perspectives.An ultra-capacity truck requires a much larger battery package to power its wheel motors when compared to a battery-electric passenger vehicle.A battery capacity of 2,000 kWh has been selected for the simulation study.This simulation assumes that mining trucks operate on the same haulage route irrespective of the type of material hauled in order to better compare the influence that Battery Trolley configurations.
In this study, capacity equals the number of trucks connected simultaneously to a trolley segment, while productivity means the mining haulage production rate (tonnes/hour).In capacity research, only the dynamic charging and stationary charging Battery Trolley systems are analysed and compared because the dual trolley alternative does not significantly affect haulage capacity or productivity when compared to single trolley system.In addition, the calculation of productivity does not take into account power distribution losses in the mine.However, as the losses are small, the actual productivity is slightly lower than the calculated results.Furthermore, this paper only analyses the technical feasibility of Battery Trolley capacity without considering any economic or cost comparisons.
To date, the Battery Trolley has been a conceptual haulage system.Configurations and parameter values will change with technological advancements and mine site requirements.Nonetheless, this paper aims to provide a conceptual theory for analysing Battery Trolley configurations and production capability from a mining system perspective.

Opportunity
Given the significant scale of diesel mining truck emissions, the mining industry is continuously exploring green alternatives to decouple its energy sources from fossil fuels.The industry is seeking innovative solutions for this purpose, with many articles publicly available that discuss the batteryelectric vehicle as a potential solution for mining haulage systems.In an overview of the status of mine electrification, Ertugrul [6] aimed to help shape the transition to an 'all-electric mine'.This paper highlights the large-scale deployment of battery-electric vehicles as the future direction of research.A pilot study of battery-electric vehicles was deployed at the Kittilä Mine in Finland [7], which describes the improvement in working conditions seen during the trial and identifies some unresolved issues that need to be addressed before BEVs can replace diesel-powered equipment.Zuliani [4] introduced the latest technological advancements in renewable generation and energy storage, and identified the key factors that would allow a mine to use 100% renewable energy for its trucks and electricity.The Global Mining Guidelines Group published version 3 (2022) of the 'recommended practices for battery electric vehicles in underground mining' document to guide the future development of BEVs in underground mining [8].In addition, committees such as the International Energy Agency (IEA), the government department of energy, and the international mining association have expressed the practical advantages of BEV adoption.

Battery trolley technical feasibility
Trolley Assist systems are proven technology in the mining industry, and are being utilised by several OEMs such as Komatsu, Caterpillar, Hitachi, and ABB.Freeman [9] investigated the economic feasibility of implementing a Trolley Assist system in an Australian surface mine and evaluated its expected performance, providing site-specific requirements for cost-competitiveness. Mazumdar [10] studied the performance improvement of mining haul trucks operating on Trolley systems.Since only one or two trucks rely on trolley lines, he identified availability as one of the main issues affecting productivity.The study showed the results of one or two haul trucks utilising trolley lines, along with their impact on parameters such as DC voltage and current.The transition from diesel-electric trolley operation to battery-electric trolley haulage is proposed as being technically feasible.Research on Battery Trolley practice at the Aitik mine in Sweden in 2022 [11] investigated the feasibility and economics of operating large haul trucks with battery power and overhead trolley line charging.The results of this study showed that battery-electric operation was much cheaper than diesel-electric operation under reasonable assumptions.
A simulation study by Lindgren et al [11] analysed dynamic charging Battery Trolley systems (also known as in-cycle charging systems) using a retrofitted battery-electric truck at Aitik mine in Sweden.The study provided precise parameter values for the simulation test, including truck specifications, truck output power, elevation profile, speed profile, haul cycle energy consumption, and others.This paper aims to create a similar mining scenario to the Aitik Mine study to analyse the productivity of Battery Trolley systems, with three hypothetical operation scenarios in copper, iron, and waste conditions.Engineering assumptions will be made for other parameters, such as equipment utilisation rate, battery swapping time, and timing of battery swapping, based on the unique mining conditions.

Methodology
This study is based on the application of mining system theory.The objective is to facilitate the large-scale deployment and application of the state-of-the-art Battery Trolley technology in surface mining haulage systems.The study first identifies the deployment requirements, operational process, and power source for various Battery Trolley configurations.Secondly, to accommodate the unique conditions of each mine site, mining decision-makers must assess the suitability of different Battery Trolley configurations compared to conventional diesel powered Truck-Shovel adoption.
The shovel and truck selection and matching method [12] is a calculation-based approach for scheduling trucks and production tasks.It is a methodical approach to selecting shovel capacity, establishing the truck fleet, considering boundary limitations, and calculating productivity outcomes.Hypothetical mining scenarios with the same haul route are constructed to calculate the capacity and productivity of battery trolley systems for the movement of copper, iron ore, and waste materials.This methodology is widely employed in designing and planning both greenfield and brownfield mining projects.In addition, a comparative analysis of parameter increments and productivity for various materials is crucial for future design and deployment of Battery Trolley systems.

Theory of battery trolley systems
The mining industry is working towards achieving zero-emissions fleet requirements and the deployment of Battery Trolley systems is one option to achieve this goal.Several OEMs such as Komatsu, Caterpillar, Hitachi, and ABB are likely to adopt battery-trolley operation on top of their existing Trolley Assist systems that are already being used in the industry.Battery Trolley is designed to offer an emission-free haulage mining system that utilises the full source of electrical power through autonomous high-intensity battery-electric trucks, Trolley Assist systems, and energy recovery systems.Figure 1 illustrates the conceptual operation of Battery Trolley systems on the trolley ramp.The theory and system descriptions in this paper are based on the previous publication by Bao et al. (2023) with additional technical explanations [14].

Technology uptake
It is the development of advanced technology that provides the opportunity for the Battery Trolley to become a reality.Full automation, deployment of energy storage technology, and electrification of end-use services are the critical drivers for the Battery Trolley to achieve its decarbonisation pathway and are core components in future plans for deeper phases [15].
• Battery-electric drivetrain technology Electromobility, the development and use of electric-powered vehicles, is a widespread technical trend in the industry [15].BEVs are one of the options available to achieve ambitious decarbonisation goals.Firstly, the industry is rapidly designing new batteries with improved performance and lower costs, which will enhance the competitiveness of BEVs in the mining sector.Secondly, battery electric trucks have simpler mechanical systems and control logic compared to conventional hybrid vehicles, leading to lower failure rates and easier maintenance [16].
• Autonomy technology Autonomous haulage trucks have become popular in recent years due to their ability to improve safety, equipment availability, and overall productivity on a mine site without requiring a human operator in the cab [14].For the Battery Trolley, utilising autonomous technology to determine the optimal time to raise or lower the pantograph is the best approach.Battery Trolley systems can benefit from the use of autonomous trucks both in terms of safety and productivity.
• Trolley Assist technology BEV is one option for mining trucks, but to address its limitations in battery size and energy density, Trolley Assist technology is necessary.The Trolley Assist provides energy to the BEV trucks, especially during uphill travel where the most energy is consumed, thus enabling long hauling capabilities for the battery trucks.

• Energy recovery system
Battery Trolley can utilise an energy recovery system to recover the energy generated during braking, which is then used to partially recharge the onboard battery during downhill travel [16].The varying depths involved in mining operations result in significant differences in haul cycles and the potential energy that can be recovered per cycle [17].

Battery trolley's advantages and disadvantages
To date, Battery Trolley is a conceptual mining system that has been retrofitted and tested by original equipment manufacturers and several mining companies.Without extensive practical experience with the Battery Trolley, it is difficult to accurately determine its pros and cons.However, conclusions can be drawn about its advantages and disadvantages by comparing it with conventional truck-shovel systems, In-Pit Crushing and Conveying (IPCC), and Trolley Assist systems.
The Battery Trolley offers a green solution to achieve the first zero-emissions truck fleet in mining haulage systems by using battery-electric power instead of relying on fossil fuels.An analysis of the energy consumption profile of mining haul trucks indicates that about 70% to 80% of energy [14] is consumed on the uphill ramp, where the Battery Trolley system provides grid power to the battery-electric trucks.In addition, the Battery Trolley addresses the challenge of battery size faced by the mining industry compared to existing battery-electric vehicles.The relatively long-standing trolley services provide opportunistic charging when in contact with the trolleys over selected sections of the mine with permanent roadways, eliminating or delaying battery swaps during normal shifts.The Battery Trolley also leads to lower maintenance and energy costs for a single truck as it does not have a diesel engine and electricity is relatively cheaper.Over the entire mine operating life, the Battery Trolley incurs lower operating costs compared to conventional diesel truck fleets due to its use of electricity as end-use energy, similar to IPCC.
Despite the benefits of the Battery Trolley, there are still several technical challenges associated with its use.Transitioning from diesel-electric to battery-electric power can significantly increase electricity costs and demand for the mine, as well as the capital expenditures for power infrastructure and battery stations.To operate a battery-electric load and haul fleet, mine design must be altered to allow for battery stations and related equipment, and some haul routes may need to be modified compared to a conventional diesel fleet [18].Another limitation is reduced flexibility, as current fleets do not require semi-or permanent infrastructure, making them difficult to move as mining operations progress [19].Mine scheduling and planning must accommodate downtime for electrical lines and support extensions and relocations.In addition, the road surface must be level for the pantograph to stay in contact with the overhead wires, resulting in higher maintenance for the trolley ramps [7].The Battery Trolley fleet also faces challenges such as battery size and performance, high upfront capital costs, system capacity and availability, truck fleet dispatching, restrictions in mine design, and maintenance schedule arrangements for ancillary equipment.Other forms of charging, such as dynamic charging, inductive, or other advancing technologies, may become more prevalent, in addition to battery swapping or charging [8].

Battery trolley systems configurations
There are three configurations of Battery Trolley systems based on charging methods and unique subsystem technologies: dynamic charging, stationary charging, and dual trolley Battery Trolley systems.Each configuration has its own advantages and disadvantages and can be utilised in different mining scenarios.It is understood that research is underway to develop side-mounted trolley configurations [20].The following discussion only relates to conventional overhead trolley configurations.

Dynamic charging battery trolley configuration
Dynamic charging technology allows grid power to be used to power the electric drive motors and simultaneously charge the onboard vehicle battery.In dynamic charging Battery Trolley systems, the onboard battery can receive enough charging electricity from the uphill grid charging and the downhill energy recovery to balance energy consumption in one haul cycle.The dynamic charging Battery Trolley consists of the battery-electric truck, the Trolley Assist system, and dynamic charging technology.
Figure 2 illustrates the operational process and power source of the dynamic charging Battery Trolley systems.The battery-electric trucks load and haul using battery power, and switch to trolley mode when they reach the trolley ramp.During this time, energy is consumed at a lower rate for cooling and idling while the grid power is used to simultaneously charge the onboard battery and power the wheel motors.Once the battery-electric truck reaches an ex-pit flat road, it returns to battery power mode to complete hauling, queueing, dumping, and returning manoeuvres.On the downhill ramp, the energy recovery system converts the truck's braking power into electric energy that can be stored on the battery, allowing the truck to reuse battery power for the return journey to the loading point.
Compared to other Battery Trolley configurations, dynamic charging adoption has both advantages and disadvantages.
The advantages are: (1) No battery charging or swapping station is required, reducing the need for infrastructure and capital expenditure.(2) Dynamic charging leads to higher truck productivity as it allows for completing a haul cycle in a shorter time, without the need for battery charging or swapping (which can take at least 20 minutes at present technology levels).This technology charges the battery during productive work, leading to nearly a 100% operational duty cycle [21].(3) The frequent charging opportunities offered by dynamic charging provide shallow charging cycles and a high energy throughput of the battery over its lifetime, leading to a lower risk of battery degradation and missed charging opportunities [11].
The disadvantages are: (1) Less capacity.While trolley charging typically provides only enough energy to power the vehicle while it is connected, dynamic charging requires higher voltages and currents to recharge the batteries and necessitates limiting up-ramp speeds in order to ensure sufficient charging time [21].This results in fewer trucks being able to use the trolley lines simultaneously, leading to decreased truck productivity.(2) Low flexibility.If a trolley system breaks down or needs to be relocated, potential productivity may be lost as battery-electric trucks cannot operate on-ramp without a trolley.In addition, dynamic charging trucks cannot complete production tasks that are located far from the trolley line.(3) Charging time restrictions.When considering dynamic charging, it's important to weigh the power from the trolley ramp against charging time.Higher power from trolley roads requires more substation installation and fast battery charging which puts more strain on batteries and other components.Lower power from trolley roads may extend charging time or require slower driving when connected to the trolley line, decreasing truck productivity [11].

Stationary charging battery trolley configuration
Stationary charging requires a battery station for battery charging or battery swapping.The choice between charging and swapping methods depends on the charging C-rate (a measure of the rate at which a battery is discharged relative to its maximum capacity) [22] and swapping time.The battery station is located on the crest of the pit to provide ample permanent space for infrastructure and truck parking.The stationary charging Battery Trolley system consists of a battery-electric truck, Trolley Assist systems, and the battery station.The decision between battery charging and swapping in the battery station is influenced by multiple factors, including battery management strategy, state of charge, charging rate, capacity, and dispatching.Another technology under consideration is the battery-swap system, where the battery pack is removed and replaced with a full one instead of being charged in place.Although the battery swap is quicker than charging, it introduces additional mechanical complexity, particularly in harsh mining environments [21].The specific steps of the swapping process include making safe parking, installing wheel chocks, isolating the electrical system, removing the battery, and replacing the battery pack.From a charging rate perspective, if the quick charging time is less than the swapping time, which is assumed to be 20 minutes, and the battery charging current rate reaches 3C (as shown in Table 1), the quick charging method will be the preferred option for the mining truck battery packs.
In regards to battery station location, the mine designer should choose the charging spot to maximise the use of the battery operating range in a suitable location.There are three options: at the bottom of the ramp; on the hauling uphill ramp and; on the crest of the pit.Firstly, as a relatively permanent infrastructure, the battery station must be kept away from the working faces and blasting area, making the bottom of the ramp an unsuitable location for installation.Secondly, the uphill ramp is also not ideal for a battery station because ultra-capacity trucks require ample parking and charging space, and the station must have sufficient room to store large battery packages, creating significant infrastructure requirements.Currently, the crest of the pit is considered the optimal location to instal the battery station.However, in order to take advantage of regenerating energy downhill, a reasonable distance should be considered between the battery station and the downhill ramp to ensure there is enough battery capacity available for regenerating.Another approach is to replace or charge a battery that is not fully charged (for example, 90% capacity), but this requires further evaluation and decision-making through site practices.
Figure 3 depicts the operational process and power source of the stationary charging Battery Trolley systems.Battery-electric trucks operate using battery power for loading and hauling, switching to trolley mode when they reach the trolley ramp.The battery's energy consumption decreases during cooling and idling.Meanwhile, the grid power can provide maximum power to the wheel motors, enabling the truck to operate at a faster speed on the trolley ramp.Upon reaching the ex-pit flat road, the truck switches back to battery power mode to complete hauling, queueing, dumping, and returning manoeuvres.The battery-electric truck requires battery charging or swapping at battery station within each cycle or every two or three cycles, depending on the onboard battery size and energy consumption.On the downhill ramp, the truck enters energy recovery mode, converting its braking power into electricity that is stored in the battery.The truck then uses the stored energy to return to the loading point.
Compared to other Battery Trolley configurations, stationary charging has several advantages and disadvantages.The advantages are: (1) Higher flexibility.A common belief in the mining industry is that high flexibility makes conventional Truck-Shovel (TS) the dominant surface mining haulage system.Flexibility is a crucial factor in evaluating a haulage system and deciding whether to use it.Although Battery Trolley systems have less flexibility than traditional TS, the stationary charging method can partially make up for this limitation.Even without the power supply of trolley lines on the ramp, battery-electric trucks can still fulfil specific production requirements by swapping or charging their on-board batteries.In additional, stationary charging trucks, which could be a pure battery-electric truck fleet, can adapt to different orebody shapes and geological conditions, making them capable of dealing with unexpected changes in an open pit mine.(2) Higher capacity.This study will examine the capacity of dynamic and stationary charging methods in a later section.The stationary charging alternative has a higher capacity and productivity than the dynamic method, due to power limitations, which means more trucks can be used at the same time.(3) Higher utilisation.From a system utilisation and availability perspective, the dynamic charging alternative must stop work during trolley relocation and breakdown periods, while the stationary charging method can continue to meet haulage mining requirements through reliance on a battery station.
The disadvantages are: (1) Capital intensive.The stationary charging alternative requires a large battery station infrastructure, leading to a higher upfront capital investment.In addition, the increased truck cycle time due to charging or swapping requires a larger truck fleet to offset the operating time loss.(2) Battery station risk.Safety concerns regarding the battery station infrastructure pose a risk to production.Site operators must monitor the battery's health and fire risk to ensure safety.(3) Frequent charging or swapping.The low energy density of the battery requires frequent charging or swapping, making large-scale Battery Trolley deployment challenging.The stationary charging alternative also has a lower operational duty cycle from a single truck perspective.Unlike commercial and personal vehicles, mine equipment must operate continuously with minimal downtime [21].

Dual trolley battery trolley configuration
Research indicates that for downhill hauls, a bidirectional substation facilitates energy feedback to the grid [10].It is practical to instal a dual trolley system for improved energy capture performance in a Battery Trolley system.The uphill ramp trolley allows for grid power to be utilised to power the electric drive motors, while the downhill ramp trolley captures the braking energy and returns it to the grid.The dual trolley Battery Trolley comprises batteryelectric trucks, double trolley systems, and a battery station.An alternative to a dual trolley system is to apply an onboard Energy Recovery System (ERS) to feed energy back into the battery [17].This is discussed a little later.Figure 4 depicts the operational process and power source of the dual trolley Battery Trolley system.The battery-electric trucks load and haul using battery power, and switch to trolley mode when they reach the trolley ramp.During this time, the energy is consumed at a much lower rate for cooling and idling.Meanwhile, the grid power can provide maximum output power to the wheel motors, allowing the truck to operate at a faster speed on the trolley ramp.Upon reaching the ex-pit flat road, the truck returns to battery power mode to complete hauling, queuing, and dumping manoeuvres.The battery-electric truck requires battery charging or swapping at battery station within each cycle or every two or three cycles, depending on the on-board battery size and energy consumption.The truck then enters energy recovery mode downhill by engaging the trolley line, which captures braking energy and returns it to the grid.The truck then reuses battery power to return to the loading point.
Compared to other Battery Trolley configurations, dual trolley adoption has several advantages and disadvantages.
The advantages are: (1) Improved energy recovery performance.Conventional battery-electric truck energy recovery systems are capable of fully utilising recovery potential in shallow pits, but with increasing depth of surface mining, long-distance downhill hauls become a challenge.The dual trolley system improves energy capture performance, leading to significant energy savings and improved recovery characteristics.(2) Higher payload.The addition of any weight to a mine haul truck results in a corresponding loss of payload.Choosing the technology and size of the ERS for battery-electric trucks involves balancing the effective payload with the power and storage capacity of the ERS [17].
External energy recovery through overhead power lines can increase the effective payload of the trucks by reducing the weight of the ERS.(3) Reduced energy waste.Without the weight of the ERS, there is less energy waste during a single truck haul cycle.However, the battery station location on the crest of the pit requires special measures to effectively capture regenerated energy downhill and avoid excessive retarding energy waste.The dual trolley Battery Trolley system resolves this issue by capturing braking energy and returning it directly to the grid.
The disadvantages are: (1) High capital expenditure for trolley systems.Installing two parallel interconnected trolley lines for energy recovery from downhill truck drives will double the already substantial cost of the overhead trolley lines.Dual trolley configurations are more suitable for sites where the trolley infrastructure already exists and conventional ERS performance is lower than expected [17].(2) High maintenance requirements.Maintaining smooth haul routes and close tolerances between the haul road surface and overhead lines makes dual trolley adoption the strictest maintenance requirement among various Battery Trolley configurations due to the need for two trolley lines.(3) Lower flexibility.Battery-electric trucks will become more reliant on the trolley lines, with one line providing hauling energy and the other harvesting regeneration energy.This will reduce the haulage system's flexibility and availability in the event of trolley system breakdown or relocation.
All comparisons are made among different configurations of the Battery Trolley system.For instance, the stationary charging option has more flexibility than dynamic charging, but it less flexible when compared to conventional diesel trucks.Thus, all advantages and disadvantages are relative.

Battery trolley systems capacity
A major issue with adopting a Battery Trolley system is its capacity, which immediately affects productivity, since one or two trucks rely on trolley lines [10].Despite the performance improvements offered by Battery Trolley systems, they can be offset by trolley power limitations and the time needed for charging or swapping batteries.To determine if Battery Trolley systems can meet mining production requirements, it is recommended to study the system's overall productivity from both dynamic charging and stationary charging perspectives.This will help decision-makers in the mining industry tailor proper production scheduling.

Input parameters
The Battery Trolley system is capable of hauling various materials, including copper ore, iron ore, and surface waste.To ensure its ability to meet the demands of different materials, it is crucial to assess its productive performance under different configurations.The ongoing study of Battery Trolley pilots involves simulating a hypothetical mining scenario with reasonable assumptions, as shown in Table 2.In addition, the bank material density (D bm ), swell factor (F s ), and fill factor (F f ) may vary in different applications, as shown in Table 3.

Dynamic charging adoption in a copper mine
Table 2 and Table 3 present the hypothetical operating conditions in a copper mine.A retrofitted CAT 795F AC battery-electric truck was used to complete the overall haul cycle simulation in the Aitik mine operation [11].Table 4 lists the truck specifications, including dynamic charging and haul cycle parameters.The on-board battery was able to receive enough charging electricity from uphill grid charging and downhill energy recovery to balance energy consumption during one haul cycle.The next step is to use the mining system theory of shovel and truck selection to choose an appropriate shovel capacity for a 290-tonnes payload truck.Given the well-blasted rock and medium hard digging conditions of the material, the bucket fill factor is estimated at 0.85.A shovel is typically matched with a haul truck that can be loaded in 3 to 6 passes.As the CAT 795F is an ultra-capacity truck that requires a high volume bucket for a match, this simulation chooses a value of 4 for the number of passes.In order to meet the requirement of loading one mining truck in 4 passes, the shovel capacity must be selected accordingly.The shovel with a 55 m 3 dipper capacity is suitable to be deployed at the loading point in a copper mine, under the specific operating conditions.Table 5 is the shovel parameters: The shovel productivity (Q s ) and truck productivity (Q t ) calculation are: The calculation outcomes of the shovel and truck productivities are 6,762 t/h and 522 t/h respectively.Therefore, the number of trucks in a fleet (the combination of one shovel and several trucks) is 13: The truck operation follows a discrete event and the truck scheduling follows a uniform distribution during one haul road time under the optimum truck dispatching scenario.The dynamic charging adoption results in the distribution of the trolley power (8 MW) between the electric wheel motors (3,500 kW) and the charging on-board battery packages (4,500 kW), allowing only one truck to enter the trolley ramp at a time.The trolley ramp speed during dynamic charging is limited to 20 km/h, lower than the maximum speed of 27 km/h, to allow for more time to charge the batteryelectric truck by engaging the trolley line.This slows the truck speed, but increases the charging of electricity for the battery packages to meet the energy needs for the entire cycle.Overall, the trolley system cannot accommodate two trucks at the same time.Hence, trolley power is the most critical factor that limits the productivity of the Battery Trolley.To study the maximum productivity of the working face bench, it is assumed that multiple fleets carry out material transportation along the same haul route.The equation for calculating the number of fleets in the dynamic charging condition is presented below: The limiting value of 1 is obtained via the truck capacity limit of each trolley segment.The result of the calculation is 1.5, the lower limit result is 1.Therefore, 1 unit truck fleet (1 shovel and 13 trucks) can be deployed in the dynamic charging Battery Trolley system in this specific mining scenario.According to the equation, maximum Battery Trolley productivity is 6,762 t/h.

Stationary charging adoption in a copper mine
Compared to the dynamic charging Battery Trolley system, the trolley ramp in the stationary charging alternative has no speed limit.The electric wheel motors can fully utilise the ramp's acceleration capability.However, the major drawback of the stationary charging method is that trucks have to recharge or replace their battery packages at a battery station located at the top of the pit.
The average power consumption of each truck in the Aitik mine's battery-electric operation is 519 kWh per cycle.The battery capacity of 2,000 kWh provides enough energy for nearly four cycles.However, this study assumes that battery-electric trucks need to charge or swap batteries every two cycles due to depth of charge and battery health consideration.Thus, the actual capacity of the stationary charging Battery Trolley system may be higher than the calculated results.Despite the increased cycle time from battery charging or swapping operations, the battery-electric truck can still arrive at maximum speed on ramp, which is an advantage over dynamic charging operations.The time saving calculation shows: The stationary charging Battery Trolley system saves 1.55 minutes and has an average haul cycle time of 38.45 minutes.Table 6 displays the parameters of mining trucks using the stationary charging method.
The calculation equation mentioned above results in a truck productivity of 407 t/h in the stationary charging method, with a fleet of 17 trucks.This is larger than the dynamic charging scenario of 13 trucks per fleet.It is primarily as a result of the increase in cycle time.
Due to the adoption of stationary charging, the trolley power of 8 MW can support two trucks, allowing for a maximum output power of 4,000 kW for the electric wheel motors.As a result, two trucks can operate simultaneously on the same trolley ramp.The equation for calculating the number of fleets in the stationary charging condition is presented below: The result of the calculation is 4.1, with the lower limit being 4. A fleet of 4 units (4 shovels and 68 trucks) can be deployed in the stationary charging Battery Trolley system in this specific mining scenario, yielding a Battery Trolley productivity of 27,048 t/h.

Trolley power incremental analysis
Due to the current limitations of the trolley power (8 MW), only one truck at a time can be engaged on the trolley line in the dynamic charging system, while only two trucks can be engaged simultaneously in the stationary charging system.ABB has supplied the haul truck trolley assist infrastructure with a rectifier substation providing more than 12 MW of DC power.It is important to analyse the impact of increasing the trolley power when analysing the Battery Trolley capacity.By considering rectifier substations providing 12 [26], 16, 20, 24, 28, and 32 MW of power, the variations in Battery Trolley capacity can be analysed in the aforementioned mining scenario.
In the dynamic charging system, each battery-electric truck requires 8 MW of trolley power for haul operation on the trolley ramp.Therefore, an increase to 12 MW of trolley power will not allow for more trucks to be engaged simultaneously on the trolley line.However, with 16 MW of trolley power, the capacity can be increased by one truck.In the stationary charging system, each battery-electric truck requires 4 MW of trolley power.Therefore, compared to 8 MW of trolley power, 12 MW of trolley power will allow for one more truck to be engaged simultaneously on the trolley line, while 16 MW will allow for two additional trucks.
According to the calculation equation for the number of fleets, Table 7 and Figure 5 demonstrate the relationship between trolley power and productivity in copper ore material operations using dynamic and stationary charging configurations.

Battery trolley system productivity equation
As previously explained, the trolley power limits the number of trucks that can be engaged on the trolley line simultaneously.However, the electric motor output power on the trolley is another parameter that influences the Battery Trolley capacity.The equation for calculating the maximum number of trolley trucks is: Where: N mt : Maximum number of trolley trucks, is the maximum number of trucks capable of engaging the trolley grid simultaneously due to trolley power limitations.P t : Trolley power, is the power which the rectifier substation is able to provide for the trolley grid.It is the maximum power capacity for a Battery Trolley system to engage battery-electric trucks, kW.
P emo : Electric motor output power on Trolley, is the power that battery-electric truck's electric motor output on the trolley ramp.The electric motor output power is the direct factor that influences truck speed.In this study case, P emo is 3,500 kW and the corresponding ramp speed is 20 km/h in the dynamic charging adoption while P emo is 4,000 kW and the corresponding ramp speed is 27 km/h in the stationary charging counterpart, kW.
B: Boolean variable, represents 0 or 1 value.In the dynamic charging Battery Trolley system, B equals 1 while B equals 0 in the stationary charging counterpart.
P dc : Dynamic charging power on the trolley, is the power the trolley line provides to recharge the on-board battery when the battery-electric truck engages the trolley line.
The number of trucks per fleet calculation equation:

Applications in iron ore and waste materials
Battery Trolley systems can be used for several open-pit mining applications, including copper, iron, coal, and overburden waste.However, as coal material has a lower bank density, smaller payload trucks with larger trays are usually deployed.The 290-t payload mining truck deployed in the study scenarios is not suitable for coal mining production.

Iron ore mine
The hypothetical operating conditions in an iron mine are shown in Tables 2 and 3.As determined by the calculation method mentioned previously, a 35 m 3 capacity shovel was selected to perform 4 passes for loading a single mining truck.The parameters of the 35 m 3 shovel can be found in Table 8.
According to the previously mentioned calculation, Q s is 7,069 t/h.The Q t remains the same as in the copper scenario (522 t/h and 407 t/h).As a result, N ft is 14 in the dynamic charging alternative and 18 in the stationary charging alternative.The productivity of the dynamic charging and stationary charging Battery Trolley systems are 7,069 t/h and 21,207 t/h, respectively, according to the Battery Trolley system productivity calculation equation for an 8 MW trolley power condition.Table 9 and Figure 6 display the relationship between trolley power and productivity in the iron ore material operation with dynamic and stationary charging configurations.

Waste material
Table 2 and Table 3 display the hypothetical operating conditions for surface waste material.As per the previously mentioned calculation method, the study selects a 35 m 3 shovel capacity to satisfy the requirement of 4 passes for loading one mining truck.The parameters for the 35 m 3 shovel can be found in Table 10.
According to aforementioned calculation, The Q s is 6,413 t/h.The Q t is same with the copper scenario (522 t/h and 407 t/h).Therefore, the N ft is 13 in dynamic charging and 16 in stationary charging alternatives respectively.The dynamic charging and stationary charging Battery Trolley productivities are 6,413 t/h and 25,652 t/h respectively according to Battery Trolley system productivity calculation equation in 8 MW trolley power condition.Table 11 and Figure 7 show the trolley power and productivity in the waste material operation with dynamic and stationary charging configurations.

Battery trolley productivity in current power limitation
The conventional configuration of the rectifier substation at the mine site offers 8 MW DC power to the trolley wire line.ABB can offer 12 MW with its proven trolley assist technology, making it the highest capacity rectifier substation for mine sites at present.This section presents the Battery Trolley productivity in current power limitations for copper, iron, and waste material in the aforementioned mining scenario.Table 12 and Figure 8 show the trolley power and productivity in the waste material operation under dynamic and stationary charging configurations.

Results
The simulations indicate that, given the specified mining scenarios and assumptions, the following results are observed: (1) The limitations in trolley power have a significant impact on the capacity of battery trolley systems.Consider the copper scenario as an example: With dynamic charging alternatives, only 1 shovel and 13 trucks can be deployed, whereas stationary charging alternatives allow for 4 shovels and 68 trucks to be deployed, all within an 8 MW trolley power limitation in a hypothetical copper haulage system.However, even when the trolley power reaches 32 MW, the maximum permitted number of fleets remains at six for dynamic charging and 16 for stationary charging.(2) Irrespective of the trolley power level and the materials that the battery trolley system needs to haul, the stationary charging option consistently exhibits higher capacity and productivity compared to dynamic charging.Let's take the copper scenario as an example: With dynamic charging alternatives, only 13 trucks can be deployed, resulting in a productivity of 6,762 t/h.Conversely, stationary charging alternatives allow for the deployment of 68 trucks, achieving a productivity of 27,048 t/h, all within an 8 MW trolley power limitation in a hypothetical copper haulage system.It's worth noting that as the trolley power increases from 8 MW to 32 MW, the gaps in productivity between dynamic and stationary charging widen significantly.The productivity of dynamic charging alternatives is 40,572 t/h, whereas it reaches 108,192 t/h in stationary charging options when the trolley power reaches 32 MW.(3) Battery Trolley can be used in various material applications, but its performance varies among different haulage materials.The productivities for hauling copper, iron ore, and waste materials in the dynamic charging alternatives under the 8 MW trolley power limitation are 6,762 t/h, 7,069 t/ h, and 6,413 t/h, respectively.Subsequently, in the stationary charging alternatives under the same 8 MW trolley power limitation, the productivities increase to 27,048 t/h for copper 21,207 t/h for iron ore, and 25,652 t/h for waste materials.Furthermore, when the trolley power limitation is increased to 12 MW in the stationary charging alternatives, the productivities further improve to 40,572 t/h for copper 35,345 t/h for iron ore, and 38,478 t/h for waste materials.
Although this study presents valuable insights and findings, there are also limitations to consider.Firstly, the Battery Trolley is still in the conceptual design stage and has not been widely implemented in actual mining sites.At the time of writing this paper, to the best of the authors' knowledge, a limited number of OEMs are conducting research on Battery Trolley systems with dynamic charging adoption.No practical applications have been reported for stationary charging and dual trolley configurations.As a result, the calculation model cannot be validated against actual measurements.Secondly, this research relies on a number of assumptions and the parameters values from the Aitik copper mine.Each mine has its own unique mining parameters and characteristics, so the simulation results may differ significantly from actual practice due to different parameter values.Finally, as new technologies continue to emerge, they may impact the Battery Trolley systems.For instance, advancements in battery charging and swapping methods, such as fast-charging or autoswapping battery technology, could enhance the competitiveness of Battery Trolley systems.The development of side-mounted trolley systems may address some of the deficiencies of overhead trolley lines that have been identified in this paper.On the other hand, the emergence of inductive charging and other charging methods could provide new opportunities for Battery Trolley systems.

Conclusions
This paper introduced the theory and configurations of Battery Trolley systems.It analysed the systems' capacity limits and proposed an equation to calculate the various materials' productivity

Figure 1 .
Figure 1.The diagram of battery Trolley systems on the trolley ramp [13].

Figure 2 .
Figure 2. A schematic of dynamic charging battery Trolley systems operational process and power source [14].

Figure 4 .
Figure 4.A schematic of dual trolley battery Trolley systems operational process and power source [14].

Figure 6 .
Figure 6.Trolley power and productivity in the iron ore material operation with dynamic/stationary charging configurations.

Figure 7 .
Figure 7. Trolley power and productivity table in waste material operation with dynamic/stationary charging battery Trolley systems.

Figure 8 .
Figure 8. Trolley power and productivity in various material operation with dynamic/stationary charging battery Trolley systems.
Figure 3.A schematic of stationary charging battery Trolley systems operational process and power source [14].

Table 5 .
Shovel parameters in the copper mine operation.

Table 7 .
Trolley power and productivity in copper ore material operation with dynamic/stationary charging configurations.Trolley power and productivity in copper ore material operation with dynamic/stationary charging configurations.

Table 9 .
Trolley power and productivity in iron ore material operation with dynamic/stationary charging configurations.

Table 10 .
Shovel parameters in the surface waste material.

Table 11 .
Trolley power and productivity table in waste material operation with dynamic/stationary charging battery Trolley systems.

Table 12 .
Trolley power and productivity in various material operation with dynamic/stationary charging battery Trolley systems.