Strange current fluctuations when utilizing two EV chargers

[leonar40]

For the last 5 years I've had a Clipper Creek EV charger that I used to charge my Model 3 without issue. Recently, I had a second 240V NEMA 14-50 plug installed in my garage. The installer ran a subpanel into the garage with a 100A breaker from the main, and then has each NEMA 14-50 on its own 50A breaker within the subpanel. I am now using the Tesla mobile charger with 240V plug to charge the Model 3, and the Clipper Creek charger to charge my wife's Honda Clarity. When using them one at a time, they work perfectly. However, I noticed today that when I tried to start charging my Model 3 while her Clarity was already charging, the current was fluctuating up and down, as high as 20A and as low as 0A, back and forth. The car or Tesla charger were not reporting any errors. So I started to investigate. I unplugged the Clarity, and then the Tesla locked right in at normal max amps that I have it set at (24A). I then plugged in the Clarity, and the Tesla started to fluctuate current again.

Any ideas what is going on? Both circuits are on separate breakers in the subpanel and appear to work properly when used by themselves. Also, voltage appears to be solid at 240V on both according to the app for each car.

Thanks for any ideas that you have!

 

[jjrandorin]

Did you have a load calculation done to verify that you can, in fact use that much power at once in a continuous load situation?

 

[leonar40]

I seem to have solved this by playing around with it. I've always run my Model 3 at 24/32A just to keep heat down on the charger. However, the Clarity's system is much more primitive and doesn't allow you to set a max current, in fact you can't even see the current at all, so I assume it's pulling the full 32A that the Clipper Creek is rated for. All I did was increase the Tesla charger's rate to 32A and they both appear to co-exist just fine now. It appears that trying to have the Tesla charger throttle the current while the other one was on at full rate was causing some kind of instability in the circuit.

 

[Rocky_H]

That doesn't make any sense to me electrically, so I don't think you have really found or solved the issue.

 

[coryjfowler]

I am wondering if that subpanel creates a voltage drop the Model 3 does not like to see. I would large add a line filter to the EVSE that has the issue most often which should provide a path to ground for stray harmonics and other noise.

 

[davewill]

What do you see the voltage do on the Tesla's screen? It's normal for it to drop a few percent as the amperage ramps up, but a sudden large drop could indicate there's a bad connection somewhere. It couldn't hurt to check all of the connections, both at the outlets and in the subpanel to make sure they are tight, and that there are no signs of overheating (melting, scorching). Does it seem to make any difference which car you plug in first?

 

[coryjfowler]

What do you see the voltage do on the Tesla's screen? It's normal for it to drop a few percent as the amperage ramps up, but a sudden large drop could indicate there's a bad connection somewhere. It couldn't hurt to check all of the connections, both at the outlets and in the subpanel to make sure they are tight, and that there are no signs of overheating (melting, scorching). Does it seem to make any difference which car you plug in first?

If you have access to a thermal camera, bad/resistive connections should show up HOT when current is flowing.