EV and Hybrid Vehicle Compressors: What You Should Know

How does a car air conditioning system work?

A car air conditioning (AC) system works using a series of components to cool and dehumidify the air inside your vehicle. By compressing, cooling, and evaporating refrigerant, the system effectively regulates the environment inside the vehicle.

There are four primary components in a car AC system, and here’s how they work:

Compressor - the compressor is the heart of the AC system. It compresses the refrigerant, a fluid that absorbs heat. This turns it into a high-pressure, high-temperature gas. 

Condenser - once the refrigerant is compressed, it moves into the condenser. The role of the condenser is to cool the refrigerant gas by releasing the heat it carries to the outside air. As the gas cools, it condenses into a high-pressure liquid, ready to continue through the system.

Evaporator - cooled refrigerant then flows into the evaporator. Here, it absorbs heat from the air inside the car. As it does so, the refrigerant evaporates, turning back into a gas. This lowers the air temperature inside the car, providing you with cool, comfortable air. The evaporator also removes moisture from the air reducing cabin humidity.

Refrigerant flow - the refrigerant, now a low-pressure gas, flows back to the compressor, and the cycle repeats. A continuous flow of refrigerant through the AC system is what keeps the air inside a vehicle cool and dry.

Regular maintenance is key to keeping these components in top condition, ensuring your car stays comfortable no matter the weather outside.

What makes EV and hybrid air conditioning systems different?

While both EV, hybrid and combustion engine air conditioning systems serve the same purpose – to cool and dehumidify the cabin - they differ primarily in how they’re powered and managed. EVs and hybrids use electric compressors, powered directly by the vehicle's battery, making the component more energy-efficient and capable of operating even when the engine is turned off.

EVs and hybrids are optimised for energy efficiency, and often have dual systems for managing both cabin comfort and battery temperature. Being able to maintain AC functionality even in electric-only mode is one of the key advantages of EV and hybrid systems, which makes them more versatile and energy-efficient compared to traditional vehicles.

EV and hybrid air conditioning systems differ in some important other ways too.

What’s the difference between AC compressors for combustion and EV vehicles?

The high-voltage battery 

EV and hybrid car batteries each contain hundreds of volts. This adds an extra layer of complexity, so before you begin any AC service, you need to take extra precautions. For example, you must power down the system to reduce any risk of electric shock when working in or around the electric compressor. For this reason, we strongly recommend that you hold an approved hybrid/electric vehicle accreditation before working on high voltage vehicles.

The components

Most modern hybrid and electric vehicles now use fully electric air conditioning compressors that operate independently of the engine. These typically use a brushless motor integrated directly into the compressor housing to drive a scroll-type compression mechanism, which consists of a fixed and an orbiting scroll.

The type of oil

Conventional air conditioning systems typically use PAG or Ester oil. However, EVs and hybrids require a special polyolester (POE) type oil. This has high dieletric or non-conductive properties, that are designed to lubricate the compressor while protecting the compressor’s electrical windings.

Research has shown that as little as one percent of PAG oil in an EV or hybrid system can affect the dielectric properties, reducing the insulation resistance of a compressor from over 10 mega ohms to less than one mega ohm. An impaired winding can damage the compressor, and even shut it down completely because of small voltage leaks that may occur.

For more detail, take a look at our video on how air conditioning compressors for EVs differ from those on combustion engines.

Signs your EV or hybrid car’s air conditioning needs servicing

Weak or inconsistent cooling

If the air blowing from the car’s vents isn’t as cold as should be, or varies in temperature, it’s a clear sign something isn’t right. In EVs and hybrid cars, this could point to low refrigerant, a failing electric compressor, or even an issue with the battery cooling loop.

Unusual noises when the air conditioning is on

Can you hear grinding, clicking, or whining noises when the AC is activated? This could indicate a problem with the electric compressor, blower motor, or internal system debris. Unlike traditional systems, EV and hybrid compressors are electrically driven and can fail silently or with odd electrical noises.

Air conditioning doesn’t work at all

If the air con is unresponsive, there are a number of possible causes: a faulty compressor, a blown fuse or relay in the system, a software or sensor issue, an insulation fault or even oil contamination.

Fogged-up windows or excess humidity in the cabin

A failing air conditioning system often can’t properly dehumidify the cabin. This results in fogged-up windows, especially in humid or rainy conditions. This might be due to a clogged evaporator, low refrigerant, or even an HVAC control system issue in the vehicle.

Warning lights or error messages

Many EVs and hybrids have sensors and diagnostic systems that monitor the air conditioning system. If a ‘check AC system’ message or a warning light appears on the dashboard, the vehicle needs to be scanned at the very least. 

Strange or musty smells from the vents

If you or your customer notices a musty or sour smell when the AC is on, it's usually a sign of bacteria or mould buildup on the evaporator coil. In EVs and hybrids, this can be more noticeable because the system often runs at lower power or cycles on and off more frequently, creating the perfect conditions for moisture buildup.

In less common cases, a burning or electrical smell could indicate overheating in the electric compressor or an issue with high-voltage components.

Detect issues in the air conditioning system with Delphi's accurate diagnostics tools

Step-by-step guide: how to service an EV or hybrid air conditioning system

Refrigerant handling

To avoid system damage and stay compliant, always start by identifying the manufacturer-specified refrigerant. Check the bonnet label – if it’s missing or unreadable, check the service manual or OEM documentation instead.

Most EVs and hybrids built before use R-134a, while newer models often use R-yf, which is a more environmentally friendly option. But never mix refrigerants – this can damage the system and stop you from completing a full and correct service.

In terms of safe recovery:

• Recovery equipment - use a certified refrigerant recovery machine designed for the specific refrigerant type. EV and hybrid systems often need additional safety steps due to their electric compressors, so make sure your equipment is EV- or hybrid-safe and the recovery oil is non-conductive.
• Connect Properly - attach the machine’s low and high service hoses to the right ports. Always wear gloves and eye protection to avoid contact with refrigerant.
• Purge and Recover - follow the machine’s instructions to purge air, then recover the refrigerant. Monitor system pressures until they reach near-zero and the machine confirms the full recovery is complete.
• Handle Refrigerant Responsibly - never vent refrigerant to the atmosphere. It's illegal and environmentally harmful. Store it in approved cylinders for recycling or disposal.

Component inspection and maintenance

Cabin air filter - locate the filter. Remove the cover and slide it out. Check for dirt, debris or smells. If it looks dark or clogged, replace it with a new, manufacturer-approved filter to ensure clean airflow.

Compressor inspection - visually inspect the AC compressor for oil stains, refrigerant residue or physical damage. EV and hybrid compressors are often electric and sealed - so any external signs of leakage or wear mean servicing or replacement is needed.

Electrical connectors - inspect all visible electrical connectors. Look for green or white corrosion, burnt terminals or loose fittings. Clean any light corrosion with an electrical contact cleaner and replace damaged connectors if needed.

Leak test

Before recharging the air conditioning system, it’s critical to check for leaks - even small ones can lead to performance issues and future component damage. You should do this using oxygen-free nitrogen.

UV Dye Method – if you added UV dye during recharging, use a UV flashlight and yellow safety glasses to inspect the system. Focus on key areas: compressor fittings, condenser joints, service ports and hose connections. Any bright green or yellow glow may indicate a refrigerant leak. Clean the area afterwards to help you monitor for recurring leaks.

Electronic Leak Detector - turn on the electronic refrigerant detector, then slowly move the probe around hoses, fittings, and AC components. The detector will beep or flash when it senses a refrigerant leak.  Work slowly and patiently to ensure accuracy.

Important - even tiny leaks can reduce system pressure over time, leading to weakened cooling or compressor failure. Always address leaks immediately to protect system efficiency and avoid the need for repeat servicing.

Refrigerant recharging

Start by vacuuming the system. After repairs or leak checks, connect a vacuum pump to the high and low service ports using a manifold gauge set. Run the pump for 30–45 minutes to remove all air and moisture, which can damage components, especially in EV or hybrid electric compressors. Let the system sit for 10–15 minutes to confirm it holds a vacuum.

Next, recharge with the correct refrigerant amount. Add the right refrigerant (see above)  via the low-pressure port using a calibrated scale or automated machine for accuracy.

Throughout the recharge, follow these best practices:

• Only use non-conductive compressor oil compatible with EV or hybrid systems
• Always wear safety gear and work in a well-ventilated area.

Final system testing

1. Turn on the vehicle and set the AC to maximum cooling and airflow.  EV and hybrid compressors should function (even when the engine in the hybrid vehicle is off).
2. Measure temperature output. Place a thermometer in the centre vent and monitor the temperature. A properly functioning system should produce cold air (around 40–55°F / 4–13°C) within a few minutes.
3. Listen for irregular sounds. Check for clicking, buzzing, or grinding noises from the compressor or blower motor. These can indicate internal damage, incorrect oil levels, or electrical issues.
4. Check cooling stability. Let the system run for several minutes. Cooling should remain consistent, without fluctuations or warming. If cooling fades or airflow weakens, it may be a sign of low refrigerant, a control issue, or poor charge quality.
5. Check system pressures are holding steady and at the expected levels.

How often should an EV or hybrid AC system be serviced?

We recommend every two years. However, service intervals may vary, so always follow the manufacturer's recommendations in the vehicle’s owner’s manual.

What are the risks of overcharging or undercharging the refrigerant in an EV or hybrid system?

Overcharging can cause high-pressure damage, compressor failure and poor cooling efficiency. Undercharging results in insufficient cooling, increased wear on the compressor, and potentially, system failure. Always charge the system with the exact refrigerant amount specified in the manual.

Are there any other specific tools I need to service EV or hybrid vehicle air conditioning systems?

If you’re removing high-voltage components, you must use the right PPE. It’s also essential you use the correct oil – see our guidance above.

Although there are naturally many similarities between the air conditioning (AC) systems in hybrid and electric vehicles and those powered by a traditional combustion engine, there are also several distinctions, not least in the compressor, which is the component at the heart of both.

When it comes to their function, while both are designed to provide the vehicle’s occupants with the most comfortable cabin environment, the AC system in a hybrid or electric vehicle also has the vital job of cooling the battery that powers the vehicle.

“If the AC system in a ‘normal’ car fails, you can still drive, but if it fails in an electric vehicle it won’t run, as the battery temperature is no longer controlled,” explains Richard Groot, AC product specialist at DENSO AM. “The AC system is therefore an integral element in the vehicle’s correct operating parameters and must be maintained accordingly, this includes the compressor, which in these vehicles is electrically driven and so defined as an eCompressor.”

What is an eCompressor and how does it work?

Conventional compressors are powered mechanically by the engine’s auxiliary drive belt, but as electric vehicles don’t have an engine that needs a belt-driven drive system, an eCompressor is required.

DENSO eCompressors include:

• Compressor section – scroll type compressor to draw in, compress and discharge refrigerant

• Electric motor – a direct current (DC) brushless type, using a permanent magnet as a rotor and coil for the stator, to drive the compressor

• Inverter – drives the motor and converts DC from the high voltage (HV) battery into alternating current (AC) for the motor. The AC electronic control unit (ECU) inputs compressor rotational speed signals to the inverter via the HV ECU, to control the rotational speed of the electric compressor

The electric motor runs at more than 200 volts and is cooled by refrigerant and oil mix, so a special, insulating compressor oil is required to prevent failure of the motor. Therefore, using the wrong oil brings a big risk of damaging the motor.

Fuel saving benefits and much more

The design of DENSO eCompressors brings several benefits to hybrid and electric vehicles:

• Controlled revolution eCompressors provide high performance at low revolution speed, which saves energy and the less energy that is used to drive the compressor, means more can be used to drive the vehicle and its usable range will increase.

• A DENSO eCompressor also consumes less energy, which helps to extend the range of the HV battery pack. Although many use a LION (lithium ion) battery, a NiMh (nickel metal hybrid) alternative is used in several Toyota hybrid models, for example.

• The internal electric motor enables the AC to continue to operate when the hybrid/electric system is switched off

• DENSO eCompressors are quieter, whilst retaining the same cooling capacity

Working with eCompressors

Safety first

Safety is paramount. The eCompressor is driven by the HV battery pack with an output of between 200 and 400 volts, so they can’t simply be replaced, they must first be disconnected from the vehicle’s HV battery.

Correct oil

eCompressor oil (usually) differs from the oil used in mechanical driven compressors. DENSO ND-Oil 11 is a highly insulating POE oil and in most cases is what is required, but always check the vehicle manufacturer’s specified formulation requirement, particularly as the wrong oil can potentially cause a short circuit in the system and destroy the electric motor.

Refilling

Although most AC refilling machines can only handle one type of oil, there are machines with an ‘internal flushing’ programme, so the oil can be switched more easily. However, to prevent the accidental mixing of oil types, separate tools for adding oil to hybrid and electric vehicles, is strongly recommended.

Maintenance

The eCompressor is hermetically closed, with no shaft seal or potential risk of refrigerant leakage to the outside air, so the leak rate of refrigerant is less than that of a mechanical compressor. However, it’s critical the keep the refrigerant at the right level because problems can start when the level is overlooked.
In addition, hybrid and electrical vehicles obviously still require regular maintenance, so technicians must refer to the vehicle manufacturer’s scheduled service intervals and follow any replacement requirements.

What does the future look like?

The AC system in hybrid and electric vehicles continues to evolve and it’s likely that it will ultimately become a thermal management system, which is not only responsible for cooling the interior cabin and the HV battery, but the vehicle’s entire heating system, and subsequent thermal efficiency. As a result, because of the complexity of the system, in just 10-15 years it won’t be easy for technicians to diagnose problems, so specific knowledge and technical training will be a requirement.