Fig 1.

Danger: High Voltage!

This month John continues to explore high voltage components, their operation and some diagnostic options

Fig 2.

Published:  25 June, 2020

If you're a ‘go get ‘em’ garage owner, then you’ll have been keeping a close on the new technology hitting our roads. You would have been forecasting what this means for your business, and starting to build a strategy comprising the changes you’ll need to make in the coming months to ensure your team are prepared and your income guaranteed.
    
While the number of EVs on our roads is increasing the market has been missing that special something. Well, perhaps not any more. This month, the long anticipated EV Mini has been released. It’s got that certain je ne sais quoi you’d expect from a Mini, and the starting price of £24,400 has ensured buoyant sales. The interesting thing being that this is just the first of A LOT of popular EVs that are being released this year and there’s no doubt that this will raise the awareness of your non EV customers and move the market closer to a tipping point. You just need to make like a boy scout.

Be prepared
Depending on your time in the industry, you’ll no doubt have experienced such technological revelations as the death of points and the introduction of transistorised ignition, computers to control fueling and catalysts to save the planet, not to mention more sensors and actuators than you could shake a stick at, and fault codes to tell us what to change (NOT).
    
We’ve taken injectors out of the inlet manifold and shoved them directly in the cylinder (GDi,) and then created cars that have both manifold injection and direct injection combined. We’ve downsized engines, added a myriad multi turbo options, and more emission saving devices than you’d have ever imagined conceivable! It’s almost enough to make your head spin! There is good news though.
    
Every time a new technological change emerges it gives you the opportunity to move with the times and get ahead of your competition. Today, you’ve got that opportunity in spades. It’s just a case of dipping your toe into the world of EV, becoming familiar with the technology and embracing the opportunities.

Same tech different model
As always, a little research often relieves any technical anxiety that may exist around new systems and EVs are no different. You’ll find the same type of components on a Toyota Prius as you will a BMW i3. They all have high voltage; batteries, relays, inverters, DC-DC converters and motors. It’s not quite you’ve seen one and you’ve seen them all, but when you compare one manufacturer with the next it’s plain to see the commonality between the operational characteristics of the systems and components, which is comforting for those delving into this technology.

Back to diagnostics
We took a quick look at HV batteries last month and I figured it’d be a good idea to take a look at how the high voltage made its way from the battery to the Power Electronics unit (inverter and DC-DC converter). Enter the high voltage relay pack.
    
High voltage relays exist to separate the HV battery from the rest of the HV components. They’ll be open circuit with the ignition key removed, and closed with the vehicle's ready light illuminated.
    
Fig.1 shows an example of a high voltage relay pack, Fig. 2 a wiring diagram and Fig. 3 the relay control circuits and high voltage current when scoped.
    
While the relays are spec’d for high voltage and high current they have three independent 12v control circuits. The eagle eyed among you will also have noticed that while we have two HV cables (one +, and one -) that there are three relays utilised within the system. Let’s take a look why.

Three into two goes
If you look carefully at Fig.3,  you’ll see that while we have three relays, relays one and two are connected in parallel. Look closer still and you’ll also find a resistor ( r ) in the current path on relay two. So why three relays? It’s all about control.
    
Take a look at the scope and you’ll see that relay three is engaged followed by relay two. Relay two having a resistor in series with it, ensures that current flow to the power electronics is reduced. Why? Good question.
    
Were this not the case then it would be possible for a very high current to flow as the contacts closed, and the possibility of an arc forming between the contacts. This is obviously undesirable and could lead to a reduced service life for the relays.
    
Relay three is closed followed by relay two, current between the HV battery and power electronics raises to just over 10 amps initially, and reduces as the voltage at the power electronics becomes closer to that of the battery. As the PD between the battery and the power electronics is the same-ish. Relay one can be closed without arcing. Relay two is no longer required and can be disengaged. Hey presto your Ready light will now be burning brightly and the vehicle ready to drive.
    
Just in case some of you are asking “Why does the relay control circuit voltage rise from 12v to 14v at point A?”  Well, as the ready light is on the DC - DC converter has come online to charge the 12v battery and power the consumers on the vehicle. It’s all very cool and I’d go into more detail but we’re out of time for this month, so you’ll have to keep your eyes on our future articles for that one.

Where next?
Once you dip your toe in the water you’ll see that the fundamentals of EV technology are straightforward, and where appropriate training, tooling and information are employed can be painlessly integrated into your workshop.
    
You’ll benefit from an increased confidence throughout your team, and additional revenue from work previously sent elsewhere. What’s not to like about that? Not much!
    
Need some help with your EV training and qualifications? As always I’m here to answer your questions. If you’d like to find out how Auto iQ can help your garage with our training and consultation programs then feel free to call on 01604 328 500.
 




Fig. 3

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