EV charger throttling

Why an AC chargepoint may reduce charging current and how to distinguish load management, schedules, solar control and faults.

EV charger throttling is a reduction in the current offered to the vehicle. It can be normal control, such as protecting the home’s supply while other loads run, or it can indicate a schedule, solar setting, vehicle limit, communications problem or fault.

The chargepoint offers an allowable AC current through the control-pilot signal. The vehicle decides how much of that offer to draw within its own limits. A lower reading in the car therefore does not, on its own, identify which side imposed the limit.

Dynamic load management

A home has a finite supply and main protective-device rating. Dynamic load management measures the site’s demand and reduces EV charging before the combined load exceeds the configured limit.

For example, cooking, electric water heating or a heat pump may raise household demand. The EV chargepoint reduces its offer and restores it when capacity becomes available. The exact current depends on the measured load and the configuration, not a fixed derating percentage.

This behaviour is useful only when the measurement is correct. A reversed, misplaced or wrongly configured current transformer can cause unnecessary throttling or fail to protect the intended limit.

A static limit is different

An installer may configure a permanent maximum below the chargepoint’s hardware rating because of cable capacity, protective devices, a DNO agreement or the vehicle’s intended circuit design. Unlike dynamic control, that limit does not rise when other household loads fall.

Changing an installer limit in an app is not a harmless speed adjustment. The setting forms part of the electrical design and should not exceed the circuit, supply or connection allowance.

Schedules and smart control

A chargepoint, vehicle, energy supplier or home-energy platform may delay or reduce charging to meet a departure target or tariff schedule. The EVs (Smart Charge Points) Regulations 2021 also require relevant private chargepoints sold in Great Britain to have smart functions and a randomised delay capability.

Randomised delay can make the actual start differ slightly from the displayed schedule. The owner can also have overlapping schedules in the car and chargepoint, which may leave only a short shared window or prevent charging entirely.

Use one primary scheduler where possible. Check the vehicle, chargepoint and any supplier integration before treating a delayed start as a hardware fault.

Solar-surplus control

In a solar mode, the chargepoint varies the offered current to follow measured export. Passing cloud or a large household load can therefore slow or pause the session.

AC charging has a standards-defined minimum offered current. If surplus falls below the energy needed for that minimum, the system must pause, import the shortfall or blend grid and solar according to its mode. Rapid repeated starting and stopping can be reduced with a time delay or controlled grid contribution.

The solar sensor location and phase treatment must match the installation. A plausible app display is not enough if the EV charges from the grid while the system labels it surplus.

The vehicle can impose the limit

The vehicle’s onboard charger sets the maximum AC rate it can accept. It may reduce demand because of:

an in-car current or location setting
battery temperature or state of charge
charging-system temperature
single-phase or three-phase capability
a scheduled departure or charge limit
an onboard fault

The chargepoint cannot force the car above those limits. A high-power AC post does not make a vehicle with a lower onboard AC charger accept more.

DC rapid charging behaves differently because the public charger supplies the battery through the vehicle’s DC interface. Its taper and thermal limits should not be used to diagnose a domestic AC session.

Temperature and protective derating

Chargepoints, plugs, socket contacts and cables can reduce or stop current if temperature or another safety condition becomes unacceptable. This is distinct from ordinary load management.

Repeated thermal derating deserves inspection. Do not cool, bypass or reset the equipment repeatedly without finding the cause. Loose connections, damaged contacts, direct sun, unsuitable enclosure conditions or a cable left tightly coiled can require correction.

Network or communications loss

The smart-chargepoint regulations require covered products to retain a means of charging if communications are lost. The fallback may not preserve every tariff or optimisation feature and can be limited by the product’s safe configuration.

Cloud, Wi-Fi or vehicle API failure can stop a supplier-managed schedule even though manual charging remains possible. Check local status, internet connection and the product’s fallback mode. Do not confuse a service outage with a loss of electrical supply.

A structured diagnosis

Record the actual current and when the reduction occurs, then check in this order:

Vehicle charge limit, schedule and displayed reason.
Chargepoint mode, schedule and current limit.
Supplier or third-party smart-charging control.
Site demand and dynamic-load reading.
Solar export and sensor direction if in surplus mode.
Temperature, RCD or other fault indication.
Whether the limit is permanent, periodic or linked to another load.

Testing with other large loads off can help identify dynamic management, but protective devices and sealed settings should be left to the installer.

Commissioned load control

A competent check can confirm:

circuit and configured current rating
supply and DNO constraint
current-transformer location, direction and phase
dynamic-load threshold and fail-safe behaviour
control-pilot current offered to the vehicle
voltage drop and connection temperature under load
fault logs, firmware and communications fallback
interaction with PV, battery and other controlled loads

Throttling that prevents an agreed charge target is a control-design issue even if every component is technically working. The remedy may be a different priority or schedule rather than a larger chargepoint.