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Thermal management in hybrid vehicles

Here you will find useful basic information and practical tips relating to thermal management in hybrid vehicles.

There is an increasing number of models with hybrid technology on the market as an intermediate step towards a pure electric vehicle. These require new solutions for interior air conditioning and thermal management. This page provides you with a good overview of the topic of hybrid vehicles: From the basics and system properties, to solutions for thermal management, right up to the special features regarding maintenance and repair work.

Important safety information
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.



The term "hybrid" as such means a mix or a combination. With respect to vehicle engineering, this term means that an internal combustion engine with standard drive technology has been combined with elements of an electric vehicle in one vehicle.


Hybrid technology has three stages of complexity: From the micro hybrid to mild hybrid up to full hybrid technology. Despite all the technical differences, all of the technologies have one thing in common: The battery used is recharged by the recovery of braking energy.


are generally equipped with a standard internal combustion engine with start/stop automatic system, as well as a braking energy recovery system (so-called recuperation).



are also equipped with a small electric motor and a more powerful battery. The electrical auxiliary drive is only used as assistance when starting and for greater power delivery when overtaking, a concept known as "boosting".



can not only "boost" but also drive with just electrical energy. To this end, they are fully equipped with an electric drive train. However, this requires a much more powerful battery than a mild hybrid.

One option for full hybrids comes in the form of a plug-in hybrid. They are able to charge their batteries overnight. A positive secondary benefit for this type of vehicle is that the passenger compartment can be brought to the desired temperature at the same time. This means that the vehicle can be used directly the following morning.


Current representatives that typify full hybrid vehicles include the Toyota Prius, the BMW ActiveHybrid X6 (E72) and the VW Touareg Hybrid. In contrast, the BMW ActiveHybrid 7 and the Mercedes S400 (F04) are examples of mild hybrids.

  Micro hybrid Mild hybrid Full hybrid
Output of the electric motor / alternator 2 – 3 KW
(regenerative braking via alternator)
10 – 15 KW > 15 KW
Voltage range 12 V 42 – 150 V > 100 V
Achievable fuel savings compared to vehicles with conventional drives < 10 % < 20 % > 20 %
Functions that help reduce fuel consumption – Start-stop function
– Recuperation
– Start-stop function
– Boost function
– Recuperation
– Start-stop function
– Boost function
– Recuperation
– Electrical driving


As can be seen from the overview, each technology has different functions that help reduce fuel consumption. These four functions are briefly outlined in the following.

Start-stop function

If the vehicle comes to a stop, e.g. at traffic lights or in a traffic jam, the internal combustion engine switches off. The engine starts automatically if the clutch is pressed and first gear is engaged to drive off. This means it is ready to start driving again immediately.


Recuperation is a technology that recovers a portion of the energy from braking. Normally, during braking, this energy would be lost as thermal energy. During recuperation, however, the vehicle alternator is used as an engine brake – alongside the normal wheel brakes.


The energy created by the alternator as the vehicle slows is fed into the accumulator (battery). This process specifically increases the drag torque of the engine, slowing the vehicle.

Boost function

As the vehicle accelerates, the available torque of the internal combustion engine and electric motor are combined. This means that a hybrid vehicle can accelerate more quickly than a similar vehicle with a conventional drive system.


The boost function is used to help during moving off and allows more power to be delivered when overtaking. This power is generated by an electrical auxiliary drive that only serves these two purposes. An example: In the VW Touareg, this means a performance increase of 34 kW.

Electrical driving

If less drive power is required, e.g. when driving in the city, only the electric motor is used as a power unit. The internal combustion engine is switched off. One of the benefits of this kind of drive: No fuel consumption and no emissions.


These technologies in the vehicle also result in different requirements which you have to take into account in your daily work.

Electrical voltage in the vehicle electrical system

The requirements and tasks that the electrical drive of a hybrid vehicle must meet and should carry out cannot be accomplished with a voltage range of 12 or 24 Volts. Much higher voltage ranges are required here.


It is important to keep in mind that the high-voltage range starts with alternating voltages higher than 25 V and direct-current voltages higher than 60 V. In line with the ISO standard, this voltage range is classified as dangerous to people.


Hybrid vehicles necessitate the installation of high-voltage components. These are clearly identified with uniform warning signs. Also, all manufacturers mark high-voltage lines with a bright orange colour.


The following is a basic rule when working with hybrid vehicles:

  1. Completely switch off the electrical system
  2. Secure the vehicle from being switched on again
  3. Verify that the vehicle has been de-energised


Observe the information provided by the vehicle manufacturer!


For conventional drive concepts with internal combustion engine, the interior air conditioning is directly dependent on the operation of the engine due to the mechanically driven compressor. Compressors with belt drives are also used in vehicles, referred to as micro-hybrids by experts, that only have a start-stop function. This causes the problem that, when the vehicle is stationary and the engine is switched off, after just 2 seconds the temperature at the evaporator outlet of the air-conditioning system begins to increase. The associated slow rise in the temperature of the air blown in by the ventilation and the increase in humidity can be annoying for passengers.


To tackle this problem, in the future newly developed cold accumulators, co-called storage evaporators, can be used.

The storage evaporator comprises two blocks: An evaporator and an accumulator block. Refrigerant flows through both blocks in the start-up phase or when the engine is running. In the process, a latent medium in the evaporator is cooled until it freezes. This makes it a cold accumulator.


In the stop phase, the engine is switched off and the compressor thus not driven. The warm air passing the evaporator cools and heat is exchanged. This exchange lasts until the latent medium has completely melted. Once the vehicle continues on, the process starts again so that the storage evaporator can start cooling the air again after just one minute.


For vehicles that do not have a storage evaporator, the engine has to be started again even after a short standstill time in very warm weather. This is the only way to maintain interior cooling.

Climate control inside the vehicle also includes heating the passenger compartment if required. In full hybrid vehicles, the internal combustion engine is switched off during electrical driving. The residual heat in the water circuit is only sufficient to heat the interior for a short period. To provide aid, electrical PTC heating elements which take over the heating function are then switched on. The functioning principle is similar to that of a hairdryer: The air taken in by the interior blower is heated as it passes the heating elements, and then flows into the interior


High-voltage compressor function

Vehicles with full-hybrid technology use electrical high-voltage compressors that do not require that the internal combustion engine be running. This new drive concept means that functions are possible which can increase the comfort as regards the vehicle air conditioning.

There is the possibility to cool the heated interior to the desired temperature before starting the journey. This can be activated via remote control.


This process of cooling while stationary is possible only if there is enough battery capacity available. The compressor is controlled with the lowest possible power taking into account the necessary air conditioning requirements.


For the high-voltage compressors currently used, the power is controlled via corresponding speed adjustment in steps of 50 rpm. This means internal power control is not required.


In contrast to the swash plate principle, which is primarily used in the belt-driven compressor field, the high-voltage compressors use the scroll principle to compress the refrigerant. The benefits are that the weight is reduced by around 20% and there is a reduction in the displacement of the same amount while the output remains identical.


In order to create a torque of the right magnitude for driving the electrical compressor, a direct-current voltage of more than 200 Volts is used – for the field of vehicles, a very large voltage. The inverter fitted into the electric motor unit converts this direct-current voltage into the three-phase alternating voltage required by the brushless electric motor. The return flow of refrigerant to the intake side facilitates the necessary removal of heat from the inverter and the motor windings.


The battery is essential for the operation of a hybrid vehicle. It must provide the high amount of energy required for the drive, quickly and reliably. Usually these batteries are high-voltage nickel metal hybrid batteries, but high-voltage lithium-ion batteries are being used increasingly often. This reduces the size and weight of the hybrid vehicle batteries further.


It is absolutely essential that the batteries used are operated in a particular temperature range. Service life decreases at operating temperatures of +40 °C or higher, while efficiency drops and output is lower at temperatures below -10 °C. Furthermore, the temperature difference between the individual cells must not exceed 5°-10° Kelvin.


Brief peak loads in connection with high current flows, such as from recuperation and boosting, lead to a significant increase in the temperature of the cells. Also, high outside temperatures in the summer months can mean that the temperature quickly reaches the critical 40 °C level.


If the temperature is exceeded, faster aging and associated premature failure of the battery are the consequences. Vehicle manufacturers strive to ensure that the calculated battery lifetime is 1 car life (approx. 8-10 years). This means the aging process can only be tackled with appropriate temperature management.


So far, three different temperature management options have been used.

Option 1

Air is drawn in from the air conditioned vehicle interior and is used to cool the battery. The cool air drawn in from the vehicle interior has a temperature below 40 °C. This air circulates around the accessible surfaces of the battery pack.


The disadvantages of this option are:

  • Low cooling effectiveness.
  • The air drawn in from the interior cannot be used for an even temperature reduction.
  • Considerable effort for guiding the air.
  • Annoying noises are possible in the interior from the blower.
  • There is a direct connection between the passenger compartment and battery via air ducts. For safety reasons (e.g. outgassing of the battery) this is problematic.
  • Another factor that should not be underestimated is the risk of dirt entering the battery pack because the air from the interior of the vehicle also contains dust. The dust is deposited between the cells and forms a conductive layer together with condensed humidity. This layer facilitates the creation of leak currents in the battery.


To avoid this risk, the intake air is filtered. Alternatively, the air can be cooled by a separate small air-conditioning system, similar to the separate rear air-conditioning systems in luxury vehicles.

Option 2

A special evaporator plate enclosed in the battery cell is connected to the air-conditioning system in the vehicle. This is done using what is known as the splitting process on the high-pressure and low-pressure side via tubes and an expansion valve. This means that the interior evaporator and the evaporator plate of the battery (which functions like a standard evaporator) are connected to the same circuit.


The different tasks for the two evaporators result in different requirements for refrigerant flow accordingly. While the interior cooling system aims to satisfy the comfort demands of the passengers, the high-voltage battery must be cooled to varying degrees of intensity depending on the driving situation and the ambient temperature.


These requirements are the defining factors for the complex control of the quantity of evaporated refrigerant. The special design of the evaporator plate and its resulting integration into the battery offer a large contact surface for the heat exchange. This means it is possible to guarantee that the critical maximum temperature of 40 °C is not exceeded.


When the outside temperatures are very low, it is necessary to increase the battery temperature to the ideal battery temperature of at least 15 °C. However, the evaporator plate cannot help in this situation. A cold battery is less powerful than one that has the right temperature. It is also difficult to charge the battery when temperatures are significantly below freezing. In mild hybrids, this can be tolerated: In the extreme case, the hybrid function is only available to a limited extent. It is, however, still possible to drive with the internal combustion engine. On the other hand, a battery heater needs to be fitted in purely electric vehicles so that the vehicle can be started and driven whatever the situation in the winter.


Evaporator plates that are integrated into the battery directly cannot be individually replaced. In the event of damage, the entire battery must always be replaced.

Option 3

The correct temperature plays a key role for batteries with higher capacities. Therefore, an additional battery heater is required at very low temperatures to ensure the ideal temperature range is achieved. This is the only way to achieve satisfactory range when in the "electric driving" mode.


To realise this additional heating function, the battery is included in a secondary circuit. This circuit ensures that the ideal operating temperature of 15 °C – 30 °C is maintained at all times.


Coolant, made of water and glycol (green circuit), flows through a cooling plate integrated into the battery block. At low temperatures, the coolant can be heated fast using a heater to reach the ideal temperature. The heater is switched off if the temperature in the battery rises when the hybrid functions are being used. The coolant can then be cooled via a battery cooler located in the front of the vehicle using the airstream from the vehicle driving forward.

Special heat exchanger

If the battery cooler cannot provide sufficient cooling due to high outside temperatures, the coolant flows through a special heat exchanger. Here, the refrigerant from the vehicle air-conditioning system is evaporated. In addition, heat can be transferred from the secondary circuit to the evaporating refrigerant in a very compact space and with a high power density. The coolant is additionally re-cooled. Thanks to the use of the special heat exchanger, the battery can be operated within the most efficient temperature window.


Continuous ongoing education is required to maintain and repair the complex thermal management systems found in hybrid vehicles. Employees who carry out work on such high-voltage systems in Germany, for example, must attend an additional 2-day education workshop to become a certified "electrical technician for high-voltage systems".


This course teaches the employee to recognise the risks when working on systems of this kind and how to switch off all the current to the system for the duration of the work. People who have not attended specific training courses are prohibited from working on high-voltage systems.


General inspection and repair work (such as on exhaust systems, tires, shock absorbers, oil changes, tyre changes etc.) are also a special situation.


Such work may only be carried out by personnel who have been made aware of and trained in the dangers of these high-voltage systems by a certified "electrical technician for high-voltage systems".


Furthermore, it is absolutely essential that tools are used which meet the specifications of the hybrid vehicle manufacturer.


During an air-conditioning check and service it should be noted that the electrical air conditioning compressors are not lubricated with the common PAG oils. These do not have the necessary insulating properties. This is why POE oils are generally used which have this characteristic.


Consequently, for the air-conditioning check for hybrid vehicles, air-conditioning service units with an internal flushing function and a separate fresh oil reservoir are recommended. This makes it possible to avoid mixing fresh oils of different oil types.