Plug-In Hybrids

Definitive Guide to Plug-In Hybrid Cars

GreenCars' Guide to Plug-In Hybrid Cars

Table of Contents

What is a Plug-In Hybrid Car?

A plug-in hybrid electric vehicle (PHEV) is a type of electric vehicle (EV) that uses a rechargeable battery to power an electric motor and another fuel, such as gasoline or diesel, to power an internal combustion engine or similar propulsion system. PHEVs will typically run on electricity until the battery is depleted. Once that happens, the vehicle will automatically switch over to the internal combustion engine for power.

Two other popular vehicle types to be aware of with similar technology include: Hybrid Electric Vehicles (HEV), which are highly efficient gasoline vehicles aided in part by rechargeable batteries that are incapable of being recharged through an external power source and Battery Electric Vehicles (BEV), which run purely on electricity and have batteries that can only be recharged through an external power source.

Electric Vehicles (EV) Cheat Sheet  
Type of CarBEVHybrids 
NameBattery Electric VehicleHybrid Electric VehiclePlug-in Hybrid Electric Vehicle
Fuel TypeBattery OnlyGas + BatteryGas + Battery
Fuel SourceChargerGas + Regenerative BrakingCharger + Gas + Regenerative Braking

How Does a Plug-In Hybrid Car Work?

From playing music on the stereo to spinning the wheels, plug-in hybrids harness the needed energy from both electric and gasoline components. Although many of the electrical components function similarly to those found in all-electric cars, there are some important distinctions to be aware of. Before we jump right into how PHEVs work, let’s first take a look at the two different types of vehicle components: electric and gasoline.


We begin with the traction battery pack, which stores all the electrical energy in the car and powers all the electrical components in the vehicle. Most modern EV batteries are made from lithium because they can store a lot of energy while also remaining relatively lightweight.  Assuming every EV has the same electrical efficiency, it would be fair to say that a bigger battery stores more electrical energy in the vehicle, thus translating into more electric range. BEVs tend to have the largest batteries, followed by PHEVs then HEVs.

Electric Motor

Using the electrical energy stored inside the traction battery pack, the electric traction motor converts electrical energy into mechanical energy. The vehicle’s transmission then transfers that mechanical energy from the motor to drive the wheels. Depending on the number and placement of these motors, a PHEV’s drivetrain can either be front-wheel drive (FWD), rear-wheel drive (RWD) or all-wheel drive (AWD).

PHEVs also come standard with a useful energy recovery mechanism called regenerative braking, which is controlled either by the motor or another component known as a generator. Regenerative braking occurs when the driver takes their foot off of the acceleration pedal, prompting the generator to spin (or the motor to act in reverse) and convert unused kinetic energy into electrical energy. The energy is then stored in the battery, ready to power the motor again. In other words, this mechanism recovers energy using the vehicle’s momentum to use less electrical energy and extend the battery’s state of charge.

Charge Port & Onboard Charger

In PHEVs, the battery pack is charged through an external power source using the appropriate charging plug (found on the cord) via the charge port (found on the car). This plug is the EV equivalent of a fuel nozzle at a gas station and the charge port is like the opening where you insert the nozzle.

Working in tandem with the charge port is the onboard charger. Think of this as the computer that is responsible for converting the plug’s native electrical current into an acceptable DC power level to charge the battery. The onboard charger does the thinking and converting so the driver doesn’t have to, ensuring the battery is not harmed by the wrong type of charger.

To read more about the charging capabilities of PHEVs, feel free to skip ahead to “Recharging Plug-In Hybrids.”

Gasoline Components

As mentioned above, PHEVs also come equipped with an internal combustion engine to power the car using another fuel source (typically gasoline or diesel) once the traction battery has been depleted. This is possible through the design of the vehicle’s transmission, which can select either the combustion engine or electric motor as a power source. Since PHEVs can run on two different energy sources, there is a second fuel lid in a separate location to refuel the vehicle with gasoline. This fuel filler is the same as found on all gasoline-powered vehicles. PHEVs also come equipped with an advanced exhaust system to channel away emission byproducts generated by the chemical reactions in the engine. Depending on the PHEV model, the exhaust system is fine-tuned to release lower emissions than traditional gas vehicles and are certified to a certain tailpipe emission standard.

Recharging Plug-In Hybrid Cars

Think of a plug-in hybrid car (PHEV) as an electric car that has a gasoline-powered engine as backup. The idea here is that you can drive 28 to 60 miles (depending on the make and model of PHEV) on electric power to get to work or do errands around town without having to use gasoline. How do you charge up a PHEV? It's easy.

First of all, plug-in hybrid cars only use Level 1 and Level 2 charging. Level 1 means you can plug your car right in to any standard 120-volt electric power outlet to recharge it. The onboard chargers that come with most plug-in hybrid electric vehicles are only equipped to handle AC power, which can be extracted from standard home wall outlets. Level 2 charging will charge your car faster but you'll need to have a 240-volt outlet installed in your garage or use public charging stations. DC fast chargers are considered too powerful for most PHEVs due to the cars' lower battery capacities and onboard charger restrictions.

For Level 1 charging at home, your car comes with a 120-volt charger that can be switched from 8 to 12-amps for recharging. The 12-amp setting will charge up the car faster but they have been known to trip the circuit breaker depending on how your house is wired. Basically, with Level 1 charging, you can recharge anywhere that has a standard electrical outlet. The chargers themselves are designed to last for 30 years of daily charging.

Charging overnight is best, when electricity rates are at their lowest cost. Level 1 charging generally takes six hours to get you a full charge. If you have a 240-volt electrical socket wired up at home, your Level 2 charger can recharge your PHEV in about three hours.

If you drive your car to work, you can also recharge your plug-in hybrid at any standard electrical plug so that you can do your daily commute without using gasoline. For long road trips, you have the backup of the car's gasoline engine and gas tank to take you at least 300 miles before you have to refuel. Think of an PHEV as being the best of both worlds.

Total Cost of Ownership of a Plug-In Hybrid Car

The total cost of owning a PHEV consists of three categories:

  1. The initial cost of the vehicle
  2. The maintenance costs of the vehicle
  3. The fuel costs of the vehicle

Initial Cost

In general, EVs tend to currently have a higher manufacturer’s suggested retail price (MSRP) than traditional gas vehicles. The biggest reason for the higher price tag is the expense of the traction battery, making battery size an important determining factor when comparing MSRP among types of EVs. Due to their smaller battery size, PHEV models are a cheaper alternative to their BEV counterparts. For example, consider the 2019/2020 Kia Niro. Even the 2019 fully electric model’s starting MSRP of $38,500 costs around $9,000 more than the 2020 plug-in hybrid model and almost $14,000 more than the 2020 gas model.

Plug-in hybrids offer an affordable middle ground for those who are interested in becoming more eco-friendly and cost-effective with their transportation expenses but are still intimidated by the initial cost of BEVs. As the price of lithium-ion batteries decreases, expect the price of EVs to fall as well. It is estimated that many electric vehicles will cost pretty much the same as their gasoline-powered counterparts by 2023.

Maintenance Costs

Since PHEVs have internal combustion engines, the gas components of the car will share the same maintenance requirements of traditional gas vehicles. However, the electrical components of the car will be a lot cheaper to maintain because there are fewer moving parts in an electric motor than in a gasoline engine. The electric motor’s regenerative braking function will also help EV owners save money by significantly reducing the wear and tear of the braking system, specifically the brake pads. The maintenance cost for PHEVs will ultimately depend on the driver’s ability to consistently drive electric. As long as there is charge in the traction battery, the vehicle’s gas components will remain idle and require less maintenance over time.

Fuel Costs

Similarly, fuel costs will largely depend on how consistently the internal combustion engine remains unused. Electricity has the benefit of being more price stable and a cheaper fuel source than gasoline or diesel. Most utilities offer residential electric rates that cost only a few cents per hour, with some even offering special off-peak rates or time-of-use rates to lower fuel costs even further.

Plug-In Hybrid Cars vs. Range Extender Vehicles

Range extender vehicles (REV), also referred to as extended range electric vehicles (EREV), are effectively battery electric vehicles (BEVs) with a small internal combustion engine that is used to generate additional electricity.

REVs are typically considered as either BEVs (by the previous definition) or plug-in hybrid electric vehicles (PHEVs) due to the inclusion of a gasoline-powered engine. What separates the engines in REVs from PHEVs, however, is that their purpose is to supply the traction battery and electric motor with power rather than propel the vehicle. When the battery charge in a REV is reduced to a certain level, the engine is turned on to power a generator that recharges the battery as you drive, thus powering the electric motor.

The entire reason for REVs is to banish "range anxiety" from existence. Small powerplants are used to extend the range of the electric vehicle. It can be a gas engine or even a fuel cell. The first modern range-extended vehicle was the Chevy Volt which used a gasoline engine to recharge its electric motor. The Cadillac ELR is similar to the Volt, just a lot more expensive. Its 17.1-kWh battery will take you 40 miles before its 84-horsepower 1.4-liter 4-cylinder gasoline-motor engages to recharge the batteries. Together, the Caddy's gasoline-assisted electric powertrain averages 32 miles per gallon combined.

Another example of a range extender car is the BMW i3 REx. The basic i3 is an all-electric car, while the i3 REx includes a small gas-powered extender engine. The most advanced version of an REV today is the Toyota MIrai which is a hydrogen fuel-cell vehicle. Instead of using a gas-powered range extender engine, its fuel-cell system can generate 153 horsepower which runs the electric motor and recharges the batteries.

Is a Plug-In Hybrid Car Right for Me?

Many buyers find that PHEVs are the perfect combination of electric driving performance and efficiency without the “range anxiety” of an electric-only vehicle. A plug-in hybrid's all-electric driving range is anywhere from 28 to 60 miles depending on the make and model. PHEVs enable some drivers to perform their daily commute and errands around town solely on electric power, functioning almost as if there wasn’t a gasoline engine under the hood at all.

The same PHEV would also let them go on that cross-country road trip without being forced to find EV charging stations along their route (though they certainly have the option). As with any recommendation, determining if a PHEV is the right choice for you really depends on how you plan to use your car, but for many buyers, it’s the perfect middle ground.

How Plug-In Hybrid Drivetrains Work

Power Electronics & Auxiliary Systems

The power electronics control systems inside PHEVs function very similarly to those of BEVs. The primary function of the power electronics controller is to manage the flow of electrical energy delivered by the traction battery to the electric motor. A couple of ways the controller can accomplish this is by managing the speed at which the motor turns or by controlling the torque the motor produces.

The secondary function of the power electronics controller is to distribute electrical energy from the traction battery to the auxiliary vehicle systems, such as the lighting, heating, ventilation, and infotainment systems. Rather than the traction battery, a separate 12-volt auxiliary battery – identical to the ones found in gas vehicles – is responsible for powering these systems. This battery is also used to start the vehicle when the driver turns the key and is kept charged by the DC/DC converter, which converts high-voltage DC power from the traction battery into the low-voltage DC power required to energize the auxiliary systems.

One particular auxiliary vehicle system that deserves to be mentioned is the vehicle thermal management system. One of the biggest determining factors of a safe, long-lasting and functional PHEV battery is its ability to effectively maintain a uniform temperature distribution across its cells. Since traction batteries are designed to only operate within a certain temperature range, they will cease to work if there is no thermal system to monitor it.

This is essentially the main function of the thermal management system: to keep the traction battery within this workable range. When the battery’s electrical energy is discharged by pressing down the acceleration pedal, it generates heat inside the battery. Since this will be the primary method of discharging the battery, the lack of a proper cooling system will quickly cause the battery to overheat and lead to deterioration.

Indirect Liquid Cooling

Most modern PHEVs are manufactured with indirect liquid cooling systems for several reasons:

  1. They are highly efficient systems
  2. They can store large amounts of heat energy
  3. They are currently the most compact and lightweight solution

As the name implies, the liquid coolant in this type of system does not have direct contact with the battery. It is instead circulated through a series of metal pipes either surrounding the battery or embedded between the battery’s cells to transfer the heat away.

This method allows the cooling system to consume a small amount of energy from the battery to keep it at an operable temperature. In other words, more of the battery’s energy can be devoted to powering the motor and maximizing the powertrain’s performance all the while being uninterrupted by the weight of the system.