🎓 Lesson 6
D4
Sizing DC Fast-Charging Hubs for Multi-Shift Fleets
Sizing a DC fast-charging hub means figuring out how many chargers, how much power, and what infrastructure you need so that all your battery-electric mining trucks can recharge quickly enough to keep working across multiple shifts without downtime.
🎯 Learning Objectives
- ✓ Calculate required total DC charging capacity (kW) based on fleet duty cycle, battery size, and shift overlap
- ✓ Design charger allocation per bay using charge time, dwell time, and charger utilization rate
- ✓ Analyze grid connection impact by estimating peak demand, transformer sizing, and demand charge implications
- ✓ Explain the trade-offs between charger power level (e.g., 150 kW vs. 350 kW), charging speed, and infrastructure cost
- ✓ Apply IEEE 1547 and CSA C22.3 No. 6 standards to evaluate interconnection feasibility
📖 Why This Matters
In underground and open-pit mines converting to battery-electric haul trucks, unplanned charging delays can reduce fleet productivity by 20–40%. Unlike diesel refueling, battery recharging takes minutes to hours—and if the charging hub is undersized, trucks queue like at a single gas pump during shift change. Proper sizing isn’t just about ‘enough chargers’—it’s about matching power delivery to operational rhythm: shift handovers, maintenance windows, and battery thermal management. Get it wrong, and you risk stranded assets, missed production targets, or costly grid upgrades.
📘 Core Principles
Charging hub design rests on three interdependent pillars: (1) Fleet duty cycle modeling—capturing truck uptime, haul cycle time, battery depletion per cycle, and rest/cooling periods; (2) Battery recovery dynamics—including SOC-dependent charging curves, thermal derating above 60°C, and minimum safe recharge thresholds (e.g., 20–80% SOC for longevity); and (3) Infrastructure scalability—where charger count, power rating, and grid interface must align with utility demand charges, transformer thermal limits, and future fleet expansion. Critically, multi-shift operation introduces overlapping demand peaks: e.g., late-night maintenance crews may charge while day-shift trucks return—creating simultaneous load spikes that exceed nominal 'average' demand. Thus, sizing must be based on worst-case 15-minute demand windows—not daily kWh totals.
📐 Required Total Charging Capacity
This formula estimates the minimum aggregate DC charging power needed to sustain multi-shift operations without cumulative SOC deficit. It accounts for total energy deficit per shift, available charging window duration, and charger utilization efficiency due to thermal throttling and queuing.
Total DC Charging Capacity (kW)
P_total = (E_deficit × N_shifts) / (t_window × U_util)Minimum aggregate DC power required to replenish fleet energy deficit across all shifts within available charging time and realistic utilization.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| P_total | Total required DC charging capacity | kW | Sum of rated power of all DC chargers needed |
| E_deficit | Energy deficit per truck per shift | kWh | Usable energy consumed per shift (accounting for regen braking efficiency) |
| N_shifts | Number of concurrent shifts requiring charging | dimensionless | Typically 2–3 for 24/7 operations; reflects overlapping demand |
| t_window | Average effective charging window per truck | hr | Time available per truck for charging (includes queuing, cooling, connection) |
| U_util | Charger utilization rate | decimal (0–1) | Fraction of rated power actually delivered over time (thermal, operational, and system losses) |
Typical Ranges:
Open-pit LFP haul trucks: 0.75 - 0.85
Underground NMC shuttle cars: 0.65 - 0.75
💡 Worked Example
Problem: A mine operates 3 shifts (24/7) with 12 × 90 kWh rigid-frame haul trucks. Each truck consumes 75 kWh per 8-hr shift. Average dwell time at charger = 45 min. Chargers operate at 85% average utilization (due to cooling, queuing, voltage derating). Required charging time per truck to recover 75 kWh is 60 min at 125 kW (75 kWh ÷ 1.25 hr = 60 kW avg—but must deliver in ≤45 min).
1.
Step 1: Calculate total energy deficit per shift = 12 trucks × 75 kWh = 900 kWh
2.
Step 2: Determine usable charging window per shift = 45 min = 0.75 hr
3.
Step 3: Apply utilization factor: Required kW = (900 kWh ÷ 0.75 hr) ÷ 0.85 = 1,412 kW
4.
Step 4: Select charger rating: 125 kW units → 1,412 kW ÷ 125 kW = 11.3 → round up to 12 chargers
Answer:
The result is 1,412 kW minimum aggregate capacity, requiring 12 × 125 kW chargers. This falls within typical mine hub ranges of 1,200–3,000 kW for 10–20 truck fleets.
🏗️ Real-World Application
At the Boliden Aitik copper mine (Sweden), a 2022 pilot deployed 10 × 150 kW CCS2 chargers for 14 Volvo A30E 26-tonne haulers operating 3 shifts. Duty cycle analysis showed peak simultaneous demand occurred during 05:00–05:15 when 7 trucks returned from night shift and 3 entered pre-day-shift maintenance. The hub was sized to deliver 1,500 kW peak (10 × 150 kW), but thermal management reduced sustained output to 1,280 kW. Post-deployment monitoring revealed 92% charger utilization—validated by 4.2-min average queue time vs. target <5 min. Grid interconnection required a dedicated 2.5 MVA transformer, justified by a 12-month utility demand charge analysis showing 37% lower cost vs. shared substation.
🔧 Interactive Calculator
🔧 Open Battery-Electric Mobile Equipment (BEME) Deployment Calculator📋 Case Connection
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