Exploring the Prius Prime Low-Voltage System

Professor John D. Kelly, a seasoned automotive expert from Weber State University’s (WSU) Davis Campus, takes us on a deep dive into the low-voltage charging system of the 2024 Toyota Prius Prime PHEV. With his wealth of experience and dedication to educating future automotive professionals through the WeberAuto YouTube channel, Professor Kelly offers a detailed exploration of this innovative system and shares an unexpected discovery along the way.

Uncovering the Key Components

The 2024 Prius Prime features a sophisticated low-voltage charging system comprising four essential components: the high-voltage battery, the inverter with DC-DC converter, the low-voltage battery, and the onboard charge module (OBCM). Each part plays a critical role in maintaining the vehicle’s electrical ecosystem.

The High-Voltage Battery: Positioned under the vehicle for the first time in a Prius, this refrigerated battery employs a cooling system with refrigerant lines, a feature reminiscent of BMW designs. Two orange connectors manage high-voltage output to the inverter and input from the onboard charger, while a black connector handles low-voltage power and communication.

The Inverter with DC-DC Converter: Mounted under the hood on the driver’s side, the inverter converts high-voltage power from the battery to three-phase power for the electric motors. The integrated DC-DC converter steps down high voltage to maintain the low-voltage system. Notably, Toyota’s fifth-generation inverter design no longer allows separate replacement of the DC-DC converter, making the entire unit a single service component.

The Onboard Charge Module (OBCM): Located in the rear hatch, the OBCM charges the high-voltage battery from an external power source. It also powers 120-volt sockets and utilizes a sub DC-DC converter to maintain low-voltage battery charge during plug-in charging.

The Low-Voltage Battery: A traditional lead-acid battery housed in the rear hatch area, this unit supplies power for the car’s 12-volt systems. Although not an AGM type, it includes a vent tube due to its placement inside the passenger compartment.

Putting the System to the Test

Professor Kelly utilizes Fluke Connect-enabled tools to monitor the low-voltage charging system under various conditions. By attaching Fluke 375 FC Clamp Meters to key points, he tracks amperage outputs and battery performance.

When the vehicle is off, the low-voltage battery discharges at -0.9 amps, while the DC-DC converter remains inactive. Upon powering on the Prius, the DC-DC converter springs to life, delivering an initial surge of 84 amps, which tapers to 29 amps as the low-voltage battery recharges.

Using Toyota’s Global Techstream+ Scan tool, Kelly accesses real-time data from the hybrid control module. His custom data list zeroes in on critical parameters: high-voltage input, DC-DC converter output, and low-voltage battery performance. The system efficiently reduces 289 volts from the high-voltage battery to approximately 14.12 volts for the low-voltage system.

Stress Testing with a Carbon-Pile Load Tester

To push the system to its limits, Kelly employs an Associated 6039 Carbon-Pile Load Tester. Simulating a heavy electrical load, he measures the DC-DC converter’s ability to maintain voltage under stress. Impressively, the converter outputs 122 amps while sustaining 14.1 volts—evidence of a robust charging system.

A Surprising Discovery

During the plug-in charging test, Kelly observes an unexpected behavior: the front DC-DC converter remains active while charging the high-voltage battery. This contradicts Toyota’s service documentation, which indicates a separate sub DC-DC converter should handle this task. Kelly’s real-time measurements reveal that the front converter, typically used during driving, supplies 23 amps, with 13.5 amps directed to the low-voltage battery.

Expert Insights from Professor John D. Kelly

With over two decades of experience educating automotive students at Weber State University, Professor Kelly is a pillar in the automotive technology field. Through the WeberAuto YouTube channel, he delivers highly technical content on hybrid and electric vehicles, preparing students and professionals alike for modern automotive challenges.

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Professor Kelly’s commitment to sharing knowledge is evident in every video. His thorough explanations, combined with hands-on demonstrations, make the WeberAuto channel an invaluable resource for automotive enthusiasts and professionals alike.