Variable CP duty cycle

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ben.martins
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Joined: Tue May 16, 2017 1:02 pm

Variable CP duty cycle

Post by ben.martins »

Hello all,

Having recently spent some time with HEVRA, I just wanted to add a little something here following a quick look at a solar charger and how it effects the CP signal.

As per the normal method, the EVSE indicates to the vehicle the amount of possible current that the EVSE can deliver by using a PWM signal over a 1kHz signal. The duty cycle percentage can then be converted to current using the math channel -

posduty(A)x 0.6

where A is the channel with the CP signal.

Please note that this formula only works on EVSE’s that allow up to 51A. For an EVSE over 51A an alternative formula should be used. For more information, please see the guided test within the EV section of PS7 to view the charger communication tests.

What makes this EVSE different is that the system will adjust the CP duty cycle to inform the vehicle of the amount of current available. The current which can be delivered by the charger is based on the amount of sunlight available at the time, which this time of the year in the UK is somewhat questionable! Luckily, we had a brief break in the clouds and we were able to see the CP duty cycle change to allow the car to take more current.
Variable CP duty.png
In the above capture –

Channel A (Blue) – CP signal
Channel B (Red) – Current from the EVSE
Yellow Channel – Math channel to indicate current available based on CP duty cycle

As we can see towards the end of the buffer, we see the change in the math channel indicating that the vehicle that there is more current available, which is reflected in the actual current draw.

With more and more sophisticated chargers coming out a variable duty cycle is going to be more common and as technicians we should be prepared for what is normal. I do feel though that we won’t quite know what ‘normal’ is for a number of years!

I hope this helps and thanks again to Peter Melville at HEVRA for setting up this capture.

Kind regards

Ben

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