The Impact of State of Charge (SoC) on Charging Times

Author: Joshua Koppenjan – Director, Customer Operations

For medium and heavy-duty electric vehicle fleets, time is money, and charging downtime is a direct hit to the bottom line. Unlike passenger cars, commercial electric trucks and buses, with their massive battery packs, require highly strategic charging to keep operations running smoothly. 

The single most critical factor influencing how quickly your commercial EV will charge is its State of Charge (SoC), or how full the battery is. Understanding the relationship between SoC and charging speed is essential for fleet and operations managers looking to optimize routes, reduce costs, and maximize vehicle uptime. 

Your Vehicle Controls the Speed 

A common misconception is that plugging into a high-powered charger guarantees a high-powered charge rate. In reality, the Battery Management System (BMS) within the EV itself dictates the charging speed (in kilowatts, or kW), communicating its limits to the charging station. 

This system is designed to protect the battery from damage and prolong its lifespan. It’s why you see charging rates, even on the most powerful DC Fast Chargers, change throughout a session, a phenomenon known as the charging curve. 

The SoC Sweet Spot for Commercial Fleets 

Electric truck batteries charge fastest when they are near empty and slow down significantly as they approach full capacity. This behavior is intentional. Imagine trying to board an empty bus: it’s quick and easy to find a seat. As the bus fills up, finding the last few empty seats takes much more time and maneuvering. Electrons behave similarly at a molecular level within the battery cells. 

For small-duty consumer EVs, the efficiency sweet spot tends to sit between 20% to 80% SoC. However, it differs slightly with medium and heavy-duty commercial EVs. Having worked with multiple kinds of OEMs using high-powered chargers at our Colton charging site, I’ve observed the peak efficiency range can typically be extended up to 95%. Some vehicle models will begin to taper as early as 80%, but the major inefficiency drop tends to occur closer to the 95%mark.  

Training semi drivers to target this 20-95% window ensures minimal downtime and maximizes throughput at high-powered chargers.  

Charging Levels for Commercial Operations 

The size and duty cycle of a commercial EV fleet necessitate a mix of charging levels. For heavy-duty operations, DC Fast Charging is critical, but slower options still play an important role. 

1. AC Level 1 Charging (L1)
   • Power: Uses a standard 120V AC outlet (typically 1.3-2.4 kW).
     
   • Commercial Use: Practically non-existent for medium and heavy-duty vehicles due to their massive battery sizes. A heavy-duty truck with a battery over 400 kWh could take over a week to fully charge on Level 1.
     
2. AC Level 2 Charging (L2)
   • Power: Uses a 208V/240V AC circuit (typically 6 kW to 19.2 kW).
     
   • Commercial Use: Ideal for depot or overnight charging. Since many fleet vehicles have long dwell times (8+ hours) at a home base, L2 provides a cost-effective way to fully recharge large batteries (200+ kWh) without high demand charges. This works well for vehicles with predictable, local routes like last-mile delivery vans or municipal buses. 
3. DC Fast Charging (DCFC)
   • Power: Delivers Direct Current (DC) power directly to the battery, bypassing the vehicle’s onboard converter. Power starts at 50 kW and can range up to 350 kW or more. The forthcoming Megawatt Charging System (MCS) standard is being developed specifically for heavy-duty trucking, targeting up to 3.75 MW. 
   • Commercial Use: Essential for high-utilization vehicles that need quick turnaround, such as long-haul trucks stopping at highway corridors or urban delivery trucks needing a midday boost. The varying SoC plays its most crucial role here, as maximizing the charge rate in the 20-80% window is the key to minimizing costly pit stops.

Beyond SoC: Other Factors Affecting Charging Speed 

While the State of Charge is the primary control, commercial charging performance is a negotiation involving several other elements: 

• Battery Temperature: The EV’s BMS actively manages the battery temperature. If the battery is too cold (slowing molecular movement) or too hot (risk of damage), the system will automatically reduce the charging speed. Pre-conditioning the battery before arrival can mitigate this. 
• Charger and Vehicle Limits: The charging speed will always be limited by the lowest maximum power rating between the charging station (kW) and the vehicle’s charge acceptance rate (also in kW). A 500 kW charger cannot deliver more than 350 kW if the truck’s maximum acceptance rate is 350 kW. 
• Voltage and Current Limits: Both the charger and the vehicle have specific voltage and current limits. As the battery charges and its voltage increases, the system must carefully manage the current to deliver optimal power without exceeding safe limits, further shaping the charging curve. 

Actionable Takeaways for Fleet Managers 

1. Prioritize the 20-95% Window: For DC Fast Charging, schedule breaks and charging stops to keep the battery within the 20-95% SoC range to get the most mileage out of your high-powered infrastructure investment. 
2. Match Charger to Dwell Time: Use cost-effective Level 2 charging for long, overnight depot stops. Reserve high-powered DCFC (or future MCS) for route-critical rapid charging. 
3. Monitor Your Charge Curve: Understand the specific charging curve of your fleet’s truck model. Each manufacturer tunes their BMS differently and knowing when your truck’s charge rate begins to taper off is vital for precise scheduling. 

By making the State of Charge the central pillar of your charging strategy, you can transform charging from a necessary delay into a predictable, optimized part of your commercial EV operations. 

Partner with Greenlane for your charging needs  

As a leader in high-performance charging infrastructure, we design and operate solutions built specifically for the demands of medium- and heavy-duty electric trucks. We offer charging reliability for all route profiles, from regional short-haul operations covering drayage and port-side logistics, to strategic fast-charging along major long-haul freight corridors.  

If your fleet is ready to implement a data-driven charging strategy that maximizes vehicle uptime and minimizes Total Cost of Ownership, we invite you to contact us.