Teaching Hybrid and Electric Vehicle Low Voltage Battery Systems
We are pleased to share an insightful article written by Curt Ward, Professor at Joliet Junior College. Curt brings a wealth of experience and knowledge in automotive education, and we are confident that you will find his perspectives both valuable and useful.
As I write this article, the summer semester is nearly complete, and I have just returned home from a fantastic Automotive Educators Conference in Sacramento California. My congratulations to NACAT, CAT, and American River College on a great event. One of the most overlooked systems on a hybrid or electric vehicle is the low voltage battery system. Without a properly functioning low voltage system the high voltage system will not work at all. This article will highlight the operation and the diagnostics of the low voltage battery and the DC-DC converter. Included is a case study to demonstrate real-world repairs. The inclusion of this topic and related lab activities, will enhance a basic electrical course or a stand-alone hybrid and electric vehicle class.
All hybrid and electric vehicles have a low voltage battery. The battery chemistry will be lead-acid, absorbed glass mat (AGM), or lithium. Most systems have a nominal voltage of 12-volts; however, Tesla uses some 16-volt and 42-volt batteries (See Figure 1 – Tesla 42-Volt Battery). Although battery chemistry may vary, the fundamental cell construction is the same. Each cell will have positive and negative plates and electrolyte that will allow electron flow if a circuit is connected between them. The low voltage batteries will be rated by their ability to supply electrical power. The most common rating is cold-cranking amperes, however, cranking amperes, reserve capacity and ampere hour ratings are also used. Many of the AGM batteries used in Toyota hybrid vehicles are rated using the Japanese Industrial Standard (JIS).
The lead-acid and AGM batteries state of charge (SOC) can be checked with a voltmeter. This is sometimes called the open circuit voltage test. The voltage of the battery does not necessarily indicate whether the battery can perform satisfactorily, but it does indicate if the SOC is high enough to continue testing. The voltage of the Tesla lithium low voltage battery is best tested using the capacity test that is built into the Tesla Diagnostic Toolbox.
The load testing of the low voltage battery varies with the chemistry. The lead-acid battery can be load tested with a carbon pile tester in much the same way as a non-hybrid or non-electric vehicle. An AGM battery should be tested with a conductance tester as a carbon pile tester may damage the battery (See Figure 2 – Conductance Tester). The lithium battery is primarily tested with a scan tool. Most lithium batteries can set a fault code if a problem is detected. The scan tool data will show voltage, state-of charge, and internal resistance that can be used to identify weak or degraded cells before a failure occurs.
The DC-DC converter serves the same function as an alternator; it charges the battery and provides the needed current for the operation of the low voltage system. In most cases the output of the DC-DC converter can be tested in much the same way as an alternator is tested. A current clamp can be placed around the output wire of the DC-DC converter, and the low voltage system can be loaded by turning on various electrical loads such as exterior lights, the rear defroster grid, windshield wipers, and the blower motor. The measured output can be compared to the manufacturer’s specifications (See Figure 3 – DC-DC Converter Output Test).
Case Study: A 2010 Honda Insight was towed to the shop for a dead 12-volt battery. The battery had been previously replaced, and the new battery continued to go dead. The Check IMA and Check Engine lights were both illuminated in the instrument cluster (See Figure 4 – Honda Instrument Cluster). When attempting to check for codes, the scan tool would not communicate with the vehicle. The fuse that supplies Pin 16 of the data link connector did not have power, which was also the power feed to the motor control module (MCM). The MCM controls the charging of both the high and low voltage batteries. After carefully jumping power to pin 16 of the data link connector with a breakout box, the scan tool was able to communicate and all systems returned to normal operation. After the fuse box was replaced due to an internal open and the keys were relearned, all systems were verified to be operating as designed.
The proper operation of the hybrid or electric vehicle low voltage system is critical to operation of all systems on the vehicle. Many of the diagnostic procedures are similar to those we already teach in electrical fundamentals. The addition of a hybrid electric or electric vehicle to the class will add interest and safely expose the students to newer technology.
I will finish this article with the same offer I make after each of my presentations. If you are interested in getting started in the process of adding hybrid and electric vehicles to your curriculum or want more information, please feel free to reach out. I am more than willing to sit down in-person or online and share my experiences. Are you looking for a classroom textbook? Reach out to Pearson and ask for a review copy of the Electric and Hybrid Electric Vehicle text that Jim Halderman and I co-authored. It is a comprehensive text covering all the latest information on the subject.