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.

Connect with WeberAuto

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.

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About WeberAuto

Welcome to the WeberAuto Channel at Weber State University (WSU) - Davis Campus in Layton, Utah, U.S.A. The purpose of the channel is to help prepare the next generation of service professionals in the automotive industry.

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