Charging loss

Charging loss during HPC charging is the difference between the energy delivered by the charger and the energy that ends up stored in the battery. The rest is lost as heat or used by vehicle systems during the charging session.

This matters when comparing charging tests, cost, charging efficiency, and real-world energy consumption. A charger may report that it delivered 80 kWh, but the battery may only have gained slightly less usable energy.

What charging loss means

Charging loss is usually expressed as a percentage:

Charging loss = (Energy delivered by charger - Energy stored in battery) / Energy delivered by charger

If a fast charger delivers 70 kWh and the battery stores 66 kWh, the charging loss is about 5.7%.

The opposite number is charging efficiency. In the same example, the charging efficiency is about 94.3%.

Where the energy goes

During HPC charging, the charger supplies DC power directly to the vehicle’s high-voltage system. This avoids the onboard AC charger used for normal AC charging, but it does not remove all losses.

The main sources of loss are:

resistance in the charging cable, connector, contactors, busbars, and battery pack
heat generated inside the battery cells during fast charging
battery heating or cooling during the session
pumps, fans, valves, control units, and other auxiliary systems
DC/DC conversion for the 12-volt or low-voltage system
measurement differences between the charger, the vehicle, and estimated battery energy

At very high power levels, resistance losses become more important because electrical losses increase with the square of current. This is one reason high-voltage battery systems can be useful for fast charging. At the same charging power, an 800-volt system uses roughly half the current of a 400-volt system, which reduces current-related losses in cables and conductors.

Charger-side and vehicle-side losses

It is important to separate two different things:

Energy billed by the charger This is normally measured on the charger side and is what the driver pays for.
Energy stored in the battery This is the useful energy added to the vehicle, although estimating it from state of charge is not always exact.

Losses inside the charging station itself may or may not be visible to the user, depending on where the energy is metered. Public DC chargers normally bill the energy delivered to the vehicle, not the energy drawn from the grid by the charger cabinet. The driver therefore mainly sees the loss between the charger output and the battery.

Why charging loss changes during a session

Charging loss is not fixed. It depends on temperature, charging power, battery chemistry, pack design, state of charge, and the vehicle’s thermal management strategy.

A cold battery can increase loss because the vehicle may spend energy heating the pack before or during charging. A hot battery can also increase auxiliary consumption because the cooling system must work harder. Charging at very high power can create more heat in the cells and conductors, especially at high current.

The state of charge also matters. At low and medium SOC, the car may accept high power if the battery temperature is suitable. Near the top of the pack, charging power tapers to protect the cells. The lower current during tapering can reduce some resistance losses, but the session becomes longer, so auxiliary systems may run for more time.

Typical range of charging loss

For a warm battery charging under good conditions, DC fast-charging loss is often only a few percent. In cold weather, during heavy battery heating, or when the vehicle keeps cooling systems running aggressively, the effective loss can be higher.

This is why two charging sessions with the same car can show different efficiency:

a preconditioned battery at 20–30°C may charge efficiently
a cold battery may use several kilowatt-hours for heating
a hot pack after hard driving may need active cooling
a long session to a high SOC keeps pumps and electronics running longer

The charger display alone does not show how much of the delivered energy became usable battery energy.

Charging loss and EVKX charging curves

Charging curves usually show power delivered by the charger to the vehicle. They do not always show how much energy is stored in the battery after losses.

When EVKX compares charging performance, the most useful data is still the charger-side curve because that reflects what the driver experiences at the charging station: power, time, and energy billed. But charging loss is relevant when estimating how much usable range was actually added.

For example, a vehicle that receives 50 kWh from the charger does not necessarily add 50 kWh of usable battery energy. The range gained depends on:

charging loss
battery temperature
actual usable battery capacity
SOC calculation accuracy
consumption after leaving the charger

This is also why range added per minute should be treated as an estimate, not an exact physical measurement.

Why SOC-based calculations are approximate

Many charging tests estimate stored energy from the change in state of charge and the vehicle’s usable battery capacity. For example, if a car with 80 kWh usable capacity charges from 10% to 80%, the estimated stored energy is:

80 kWh × 70% = 56 kWh

If the charger delivered 59 kWh during the same session, the estimated charging loss would be about 5.1%.

However, this method has limitations. The SOC display may be rounded, the usable capacity may vary with temperature and age, and some vehicles do not expose the full battery buffer in a simple linear way. For this reason, calculated charging loss from public data should be treated as a good estimate rather than a laboratory measurement.

What the driver notices

The driver usually does not notice charging loss directly. It appears as a small difference between what the charger bills and how much range the car seems to gain.

Charging loss becomes more visible when:

charging in cold weather
using very high charging power
charging to a high SOC
comparing charger energy with battery capacity
calculating real charging cost per usable kWh
testing vehicles under controlled conditions

For daily use, charging loss is usually less important than charging power, charging curve shape, charger reliability, and battery preconditioning. For detailed comparisons, it is still worth understanding because it affects cost, efficiency, and range-added calculations.

High-Power Charging
DC Fast Charging
Charging Curve
Battery Preconditioning
Charging Efficiency
State of Charge
Usable Battery Capacity