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
Several types of thermal management systems are available on the market, but we will only be discussing indirect liquid cooling systems because they are the superior choice. Most modern PHEVs are manufactured with indirect liquid cooling systems for several reasons:
- They are highly efficient systems
- They can store large amounts of heat energy
- 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.