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First master 12 Volts before tackling high voltage drive systems

Monday, January 1, 2018 - 09:00
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High voltage drive systems aren’t really new anymore; after all they’ve been on the market in the United States since late 1999. Shortly after those first hybrid vehicles entered the market, specialized hybrid vehicle training started being offered to the aftermarket as well. Over the years I’ve been fortunate enough to attend classes from numerous providers nationwide, and I was even luckier to be able to develop and run training classes on the subject for technicians, shop owners, and educators all over the country. One lesson that all of those experiences taught me was something that I instinctively already knew as an educator, but sometimes forgot as a technician. That lesson was that unless you have a solid understanding of the “basics” all of the advanced training in the world won’t help you very much. This article will take a look at some electrical system “basics” and explain how that information can transfer to high voltage system understanding and diagnostics. Before you flip the page thinking, “I already know the basics of electricity,” stop and give the subject a chance! While this article will cover some electrical basics they will all be tied into the high voltage system operation and diagnosis and a review of a topic never hurts! 

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Series connected NiMh battery modules in a Toyota Prius battery pack

This article will focus on the following electrical “basics” and explain how they relate to the high voltage drive systems: 

  • Ohm’s Law – trust me, this is still important!
  • Watt’s Law
  • Series & Parallel Circuit design
  • Voltage drop
  • Magnetic Induction
  • Counter Electromotive Force (CEMF)

Ohm’s Law and HV Design
As basic as it may seem, Ohm’s Law is the foundation of virtually all electrical diagnosis. No, I’m not implying you’ll be running calculations to figure out the missing part of the equation! What I am saying is that without a firm understanding of the relationship between Ohms, Amperage and Voltage you’ll struggle to understand even the most basic electrical circuit. For instance, have you ever heard a technician say they thought a blown fuse was caused by a corroded (bad) connection? An understanding of Ohm’s Law would have made the technician realize that with a few minor exceptions that simply isn’t a possible reason for the blown fuse. 

So, what is Ohm’s Law? It’s typically written out as “E = I x R”, where “E” represents Voltage (Electromotive Force), “I” represents Amperage (Intensity), and “R” represents Ohm’s (Resistance). Another way to write it is Voltage = Amperage x Resistance. So, what do you really need to know about Ohm’s Law for electrical diagnosis (both 12V and high voltage)? Focus on the relationships. If you have a fixed voltage and resistance goes up (bad connection, corrosion, etc.), then amperage MUST go down (it’s not an option, it’s the LAW). If amperage goes down then you won’t see blown fuses as a result. If on the other hand you have a fixed voltage and resistance goes down, then amperage MUST go up (again, it’s the LAW). In 12V systems, high resistance can cause symptoms like dim lights, while low resistance is likely to result in blown fuses. 

In high voltage systems, high resistance may cause symptoms such as limited power output from the hybrid/electric drive system (caused by lower amperage for the motors). In high voltage systems a “typical” low resistance failure is very unlikely to result in a blown fuse. This can be a little confusing because Ohm’s Law would indicate blown fuses should occur when resistance drops enough. The low resistance does cause an increase in amperage as you’d expect, however the fuses used in high voltage systems are typically “slow blow” fuses. The amperage does increase as it did in the 12V example, however the slow blow fuse doesn’t immediately fail. Instead, the computer system senses the increased amperage and attempts to protect itself from damage. In addition to these self-preservation tactics, the system will set one or more diagnostic trouble codes. So, if you see a high voltage system trouble code stored that references excessive current flow you should immediately think “low resistance.” With that knowledge, you can start testing the related components which are most likely to have a low resistance failure mode such as motor windings.

Toyota Prius battery data PIDS with a low performing module group

Watts – Electrical power
The next logical step after understanding Ohm’s Law is to take a look at Watt’s Law. While Ohm’s Law explains the relationship of Voltage, Amperage, and Resistance; Watt’s Law explains how Voltage and Amperage work together to create Power. Watt’s Law is typically stated as “P = E x I.”  “P” = power in Watts, “E” still represents voltage (Electromotive Force), and “I” still represents the amperage (Intensity). In a typical 12V system this law helps explain why a headlight appears dimmer if there is a high resistance connection. Let’s walk through these two to understand the dim light scenario. First, a high resistance circuit develops due to something like corrosion in a wire or connector. Ohm’s Law dictates this increase in circuit resistance will cause a drop in amperage flowing through the circuit. Watt’s Law would then indicate that a drop in amperage will reduce the watts output from the bulb. As we all know, a lower wattage bulb will put out less light, hence the dimmer appearance of the bulb.

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