Charging e-trucks overnight: why enough power does not guarantee a reliable charging depot

When a transport operator starts electrifying its fleet, three questions usually come first: how many trucks need to charge, how much energy do they need, and how much power is available? That is a logical starting point. If twelve trucks remain at the depot overnight, charging can look like a straightforward calculation of kilowatt-hours, charging time and charging power.

Yet a charging depot that has more than enough capacity on paper can still become unstable or prone to faults in practice. Not because there is too little power, but because multiple power-electronic systems are responding at the same time to vehicles, charging schedules and energy control.

For the fleet manager, the real question is therefore not only: can I deliver enough kilowatt-hours? The question is also: does the complete electrical system remain stable while charging power keeps changing throughout the night?

A charging depot almost never operates at one fixed power level overnight

Imagine twelve electric trucks returning to the depot between 18:00 and 22:00. One truck still has 45 percent State of Charge, another only 15 percent. Two vehicles leave again at 04:30, six at 06:00 and the others later in the morning.

The energy management system therefore does not distribute the available power equally. A truck with an early departure time may temporarily receive 150 kW, while another receives only 40 or 60 kW. As soon as one vehicle has received enough energy, capacity is shifted to another truck.

An hour later, the same installation therefore looks electrically different. Not only has the total power changed, but also the load per charger, the battery voltage of each vehicle and the number of active power modules.

This is the first important insight: overnight charging is not a constant load. It is an installation that moves from one operating point to another for hours on end.

Why this matters to a transport operator

A truck does not need to understand the term power quality. The operation will, however, feel the consequences when the electrical system does not work properly as a whole.

• A charger reports faults even though sufficient grid capacity appears to be available.
• A charging depot works well with five trucks, but becomes unstable when fifteen charge simultaneously.
• Protection devices trip at moments when nothing unusual appears in the transport schedule.
• Transformers, cables or power modules run hotter than expected based on active power alone.
• Losses are higher, so a larger share of purchased electricity never reaches the battery.
• Expansion becomes harder because the existing installation has less electrical margin than expected.

Power quality is therefore not an academic subject separate from the operation. It directly affects availability, energy losses, component lifetime, fault finding and the ability to expand the charging depot.

What does a DC charger actually do to the electrical installation?

A DC charger takes alternating current from the electrical installation and converts it through power electronics into direct current for the battery. In doing so, the charger tries to draw current from the AC network as cleanly as possible.

In reality, that current waveform is never perfectly ideal. Switching frequency, filters, control algorithms and the operating point of the converter all influence the current waveform. As a result, higher-frequency components exist alongside the desired fundamental component. These are called harmonics.

For a fleet manager, the important point is what these harmonics do. They do not provide useful traction energy to the truck, but they do flow through parts of the electrical installation. This can cause additional losses and heating.

Why a charger at 25 percent load can behave differently from the same charger at 100 percent

A common mistake is to assess how a charger performs at maximum output and then assume that this performance represents the entire night.

With smart charging, the opposite is often true. Many chargers spend a large part of the night below their maximum output. A 300 kW charger may run for hours at 70, 110 or 160 kW because the energy management system is distributing the available power across several vehicles.

At a different operating point, internal currents, switching behaviour and the ratio between useful current and distortion components also change. Efficiency and thermal loading can shift as well.

A datasheet value at nominal power therefore tells you little about all the operating points that will actually occur at a depot.

Deep dive: a higher distortion percentage does not automatically mean more distortion current

This is where the subject becomes technically more interesting. At partial load, the percentage of harmonic current distortion produced by a charger can increase. This is commonly expressed as THDi: Total Harmonic Distortion of current.

Take a simplified example. At full power, a charger draws 100 amperes of fundamental current and produces 4 amperes of relevant harmonic components. The relative distortion is then limited. If the same charger draws only 25 amperes of fundamental current at low load, while certain distortion components do not decrease by the same proportion, the THDi percentage can rise considerably.

That does not automatically mean that the absolute harmonic current is higher than at full load. The percentage can increase while the absolute distortion current stays the same or even decreases.

That is why a charging depot should never be assessed on the basis of a single percentage. You need to look at absolute harmonic currents, total load, the number of chargers and the electrical strength of the site.

This distinction matters when comparing suppliers. Two chargers may show similar percentages on paper, yet behave differently at site level because their absolute currents, phase angles and control behaviour differ.

Ten chargers together create a different system from one charger

An individual charger can operate within its specification while the complete installation still requires attention. At a depot, multiple converters are connected in parallel to the same transformer and the same grid connection.

Their harmonic currents do not always simply add one-to-one. Depending on phase angle and circuit configuration, certain components can reinforce each other or partially cancel each other out.

On top of that, the relationship changes continuously. At 22:00, ten chargers may be operating at medium power. At 02:00, four trucks may be nearly full, two vehicles may already have completed charging and the remaining trucks may receive more power.

The electrical profile of the depot therefore changes throughout the night. A measurement at one specific moment does not tell the whole story.

The strength of the grid connection determines how much of the effect becomes visible

The same chargers can behave differently at two different sites. That is because the charger alone does not determine what happens to the voltage. The impedance of the transformer, cables and upstream network also matters.

Harmonic current causes voltage drop across that impedance. The higher the relevant network impedance, the more strongly harmonic currents can translate into voltage distortion at the point of connection.

An electrically strong site can therefore accommodate a substantial amount of power electronics without noticeable problems, while the same group of chargers on a weaker connection can encounter limits much sooner.

This is also why contracted power alone is not enough. A 1 MW grid connection tells you how much power is contractually available, but it does not automatically tell you how electrically strong the system is across all relevant frequencies.

Smart charging is therefore also an electrical design choice

An energy management system is often viewed as a software layer on top of the charging depot. From an operational perspective, it decides which truck receives priority and when. From an electrical perspective, it also determines how dozens of power modules behave throughout the night.

If the system sends a new power setpoint to every charger at the same moment, a distinct power step can occur. If setpoints are changed gradually or at slightly different times, the same redistribution can take place more smoothly.

This does not mean that every rapid change creates a problem. It does mean that ramp rates, response times, priority logic and coordination between chargers form part of the electrical behaviour of the site.

The software therefore does not only determine when a truck is sufficiently charged. It also influences how the electrical installation is loaded.

The battery voltage of the truck also changes the operating point

A charger does not only deliver different power levels. It also operates across different DC voltages. That voltage depends on the vehicle type, battery architecture and State of Charge.

Two trucks that are both charging at 150 kW can therefore require a different combination of voltage and current on the DC side. Internally, the charger’s power electronics then operate at a different point.

Testing with only one vehicle type or one battery voltage is therefore limited. With a mixed fleet, the assessment should cover the operating range that will actually occur in practice.

What can go wrong if you only investigate this after commissioning?

Many power-quality issues are not visible during a simple functional acceptance test. One truck is connected, the charger delivers the requested power and the charging session completes successfully.

But that does not prove how the installation behaves when fifteen trucks are connected simultaneously, six chargers are being limited, three chargers are ramping up and the energy management system is reallocating power every few minutes.

Problems can therefore emerge only during normal overnight operation. That creates exactly the wrong situation: the installation is already in service and the transport schedule has become dependent on the charging depot.

• Faults have to be investigated under time pressure.
• The charger supplier, installer and software provider may point to one another.
• The cause may appear only with a specific combination of chargers and power levels.
• Measurements may have to be taken while the transport operation continues.
• A temporary workaround may reduce the available charging capacity.

For a transport operator, a technically unclear boundary case then becomes an immediate operational risk.

A proper commissioning test therefore checks more than maximum power

A charging depot must of course demonstrate that it can achieve its nominal power. For a depot relying on overnight charging, however, that is only one part of the test.

It is at least as relevant to check what happens at the operating points that occur during a normal night.

• Multiple chargers active simultaneously at different power levels.
• Long periods of partial-load operation.
• Ramping up and down under control of the energy management system.
• Different vehicle and battery voltages.
• The transition from many active charging points to a smaller number of chargers operating at higher power.
• Behaviour during future expansion or temporary site limitation.

Where appropriate, current and voltage quality should be measured at the relevant point of connection. This allows the complete charging depot to be assessed as one system rather than looking only at individual chargers.

What should a transport operator ask the charger supplier and installer?

A fleet manager does not need to become a specialist in harmonics. It is, however, sensible to make several questions explicit before awarding the contract.

Question	Why it matters
How do the chargers perform during prolonged partial-load operation?	Overnight charging often runs for hours below maximum charger output.
Are power-quality measurements available across multiple operating points?	A single datasheet value at nominal power does not describe an entire night.
What happens when several chargers ramp up or down at the same time?	Total site power may remain within the limit while internal loading changes rapidly.
How does the system respond to different battery voltages?	A mixed fleet can move chargers into different electrical operating points.
Where will measurements be taken during commissioning?	Site-level measurements show how all chargers operate together.
Who analyses faults involving the charger, EMS and electrical installation?	When problems occur, responsibility for system-level fault finding must be clear.
Has expansion to a larger number of trucks been included in the assessment?	An installation that works well today can behave differently after scaling up.

Growing from five to twenty trucks is not simply four times the power

Expansion is often considered linearly. Five trucks become twenty trucks, so energy demand and charging power become roughly four times larger. For a first estimate of energy demand, that may be useful. For the electrical system, however, the reasoning is too simple.

More trucks also mean more power electronics operating simultaneously, more combinations of operating points and greater dependence on central power control. The installation therefore becomes not only larger, but also more dynamic.

Scalability should therefore not be assessed only in terms of spare physical space, transformer capacity and number of charging points. Electrical interaction between systems also needs to form part of the growth strategy.

Power quality is ultimately an availability issue

For a transport operator, power quality is not an objective in itself. It is one of the technical conditions that allow the charging depot to operate predictably every night.

A properly designed system therefore looks beyond maximum charging power. It combines the route profile, required energy, available grid capacity, charger behaviour, energy management and the electrical characteristics of the site.

The final measure is straightforward: are the trucks that need to leave according to the schedule actually charged and ready, without the electrical installation constantly operating close to its technical limits?

That is why overnight charging requires more than enough hours and enough kilowatts. The charging depot must also remain electrically controlled under changing load conditions.

Do you want to include this in the design or market enquiry?

When designing or preparing a market enquiry for a truck charging depot, power quality can be considered alongside energy demand, charging profile, grid capacity, scalability and operation and maintenance. This makes it clearer in advance which behaviour of the chargers, energy management system and electrical installation must be demonstrated.

Read more about our quick scans, designs and market enquiries for businesses or view our charging and energy infrastructure projects.