Interesting home charging issue

[Idenphones]

We normally charge our MS at home using a Juice Box 40 that we purchased for our previous vehicle.

when everything is working right it charges at 7kw @ 32 amps

Over the past few charges I noticed that it starts off at 32 amps but after an hour or so it drops off to 24 amps or about 5kw

the interesting thing is that the MCU says 32 amps when you start to charge then It drops to 24 amps all by itself.

If you leave it alone it will return to 32 amps by itself sometime after charging is completed.

If you stop the charge and hit the + to advance it back to 32 amps everything is back to normal again.

Voltage and current capabilities is not an issue at the Juice Box so before I request service I will try another EVSE
Just curious if anyone else has seen a similar issue.

 

[Kandiru]

If both JB and UMC lead to same result then it is the car. My bet is your battery cooling is about to go kaput.
Any little yellow triangles around the instrument cluster energy display screen when you accelerate?
How is she behaving on superchargers?

 

[Idenphones]

A couple of months ago I noticed that little yellow triangle while coming to a stop using regen braking. Only did it a couple of times and that was a few months ago.

Only supercharged a few times and it seemed normal (I guess) 100+ kw with the battery at 40-50% to start

The attached picture is what I see on the JB app. The single line top to bottom at around six pm is me stopping the charge then restarting after nudging the MCU limit up

 

[aesculus]

when everything is working right it charges at 7kw @ 32 amps

You have not stated what circuit breaker and wire size your JB is connected to and rated at. If it is warmer lately you could be hitting the high end of power draw and the car could be reducing the power it consumes.

 

[Idenphones]

Sorry about that.

About 15 feet of #8 Cable on a 40 amp breaker that is on a 200 amp service.

All connections clean and tight. Under load, while charging, I am reading 235 volts and 236 when not charging.

When the issue appears the voltage at the output of the JB stays at 235 volts.

 

[aesculus]

About 15 feet of #8 Cable on a 40 amp breaker that is on a 200 amp service.

Technically you should be able to hold 32 amps since you are right at 80% of the 40 amp breaker.

Not sure of the wire type or the temperature as talked about above. You are technically on the edge in some configurations.

And what is the cable from the charger to the Tesla, the UMC or some other cable? Perhaps the Tesla thinks that cable is causing a bigger voltage drop than it expects. That is really hard to measure because you cannot monitor that aspect directly.

 

[brkaus]

If it's repeatable, sometimes there is a message on the dash when the car reduces the charging amps.

 

[Amacharola]

I set up a juicebox at work a few months back. There is a setting in the JB app that will only feed the Tesla a certain amount of juice. You can choose to disable this and allow the tesla to control the amp draw. Try that and let us know.

 

[Idenphones]

I will give that a try but it has been set at 12kw in the app for the past couple of years. The same EVSE was used on our previous van a Pacifica Hybrid for a couple of years with no issues.The Pacifica charged at 32 amps as well.

The JB has been working just fine until just recently but maybe its not the JB's fault.

I am also going to fire up the Tesla mobile EVSE ,when the right adapter gets here to see if it has the same issue when charging above 24 amps. The adapter that I have for it is a 30 amp Dryer Plug and that sets the limit to 24 amps.