Automatic Change from 40 Amp to 30 Amp charge rate

This happens when the input voltage drops a certain percent from its initial voltage when supplying more current. This is called voltage sag. For example typical household voltage will be 240 V at 1 amp of charging. If the voltage sags to 230 or so the car notices and drops the amp draw limiting to 30 amps from 40 amps... This may occur due to long thin wiring. In this case the wire would heat up. The car is trying to prevent this wire from heating up. Tesla wants to prevent a fire in your house.!!!!

Unfortunately the car cannot actually determine the cause of the voltage drop which might be from your AC turning on rather than a long thin wire to the charger. This is a safety feature which was too sensitive in prior software updates.

There is another cause, the car has a location memory which requires the max charging amps to be changed on the touch screen AFTER starting to charge.

This started for me since day one (12 months ago). Input voltage at the house is between 238 and 242, solid. Service center has done multiple chargers, charge ports, and even a different mobile charger. It does it like clockwork probably 3/10 times, regardless of household load, season of the year, position of the moon, etc. HOWEVER, it only did it once in three weeks when I borrowed the service center's mobile charger to test with. Based on that alone, we weren't willing to blame it all on the charger. But I will say that the service center is only doing 208v for commercial voltage so maybe there is a batch of mobile chargers that doesn't play nice with 236v+. Go figure, I just keep an eye on it and turn it back to 40 when I notice. I started noticing it less when I went to scheduled charging. If it's done by the time I wake-up, I have not complaints as long as it only happens at home. On the road, it's considerably more irritating when charge time actually matters.

I have the same issue. I haven't been able to determine the cause either (although "position of the moon" is a good guess Supra).

It is possible for the supply voltage to the vehicle to sag due to many factors - electrical loads kicking on in the house (A/C, hot water heater, etc.) or even external to your residence (loads from neighbors or businesses in the area) or even regional loads. I struggled with this when trying to do precision research work using standard power feeds. There are also sometimes "drop-outs" which are due to heavy loads switching in and out. You can only see these with specialized monitoring equipment -they can be too quick even to see on a voltmeter. If you happen to have a storage oscilloscope on the line when this happens, you might see it. If the monitoring circuit in the vehicle is controlling charging current based on these transient loads, you can indeed see charging reductions based on phenomena too quick to notice when monitoring manually.

It can also be something as simple as not having a heavy enough conductor in the charging circuit. A 14-50 line should have at least 6-gauge cable all the way back to the main panel.

Mine does this all the time and I have the proper wiring and nothing running when I charge at 1am.

Interesting reading ... has Tesla commented yet?

It's more than just the voltage drop when current is applied; disruptions and fluctuations in voltage can cause it as well. See my FAQ in the charging section (linked in my signature, below) for more information. It could be an overloaded transformer, undersized service conductors, misbehaving appliances on your service or at a neighbor's house, etc.

FAQ: Home Tesla charging infrastructure QA

Look for the question entitled "AFTER VERSION 5.8.4, MY CAR "BACKS OFF" FROM THE CONFIGURED CHARGING CURRENT BY 25%. IT WORKED FINE BEFORE, SHOULD I BE CONCERNED?"

Having the right size wiring isn't the only factor.

Mine did this after the latest software update. I think about a week ago. I hardcoded the amps to 38 and haven't had an issue since. I think the software threshold for voltage flux changed in the latest update...

Mine did this for the first 3 months of ownership - after the second master charger replacement no issues for a year

Mine does it whenever there is an additional load or voltage sag. Pretty much a non-issue because voltage sags are very common in my neighbourhood and have been since I moved in many years ago..

Cut your max current down 1 amp at a time until it stops.

Mine did it because of the unusually long run (350'). I keep mine set at 34A and it doesn't cut back to 30A.

Nothing odd about the run, less than 75ft of 6-something, whatever was recommended. As I said, does it regardless of household load. I was told once that the car was sensing 480v on the line and that's why it dropped to 30, but if I really had 480v surges in the house, id know about because every device would explode.

I recall some software updates being better than others, but I keep coming back to the mobile charger and an over zealous utility giving me power at 244v when charging starts. I like the 38amp idea though, thanks.

supratachophobia said:

Nothing odd about the run, less than 75ft of 6-something, whatever was recommended. As I said, does it regardless of household load. I was told once that the car was sensing 480v on the line and that's why it dropped to 30, but if I really had 480v surges in the house, id know about because every device would explode.

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Not necessarily. It depends on how long the surge lasted.

Yes, if you applied 480V RMS to a 120V/240V appliance, they would blow up.

However, the "240V" you have in your home is actually an RMS measurement over time. If you were to take an oscilloscope measurement of one cycle of AC 240V, you'd see that the sine wave peaks at about 340V or so (sqrt(2) * 240) with no load. In addition, reactive loads will change instantaneous voltage as well.

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supratachophobia said:

I recall some software updates being better than others, but I keep coming back to the mobile charger and an over zealous utility giving me power at 244v when charging starts. I like the 38amp idea though, thanks.

Click to expand...

For what it's worth, my transformer is set to provide 248V to my home, and the current does not back off.

Please read the FAQ, it describes all the different contributing factors. In many neighborhoods, transformers are shared and it can be a neighbor's appliance that causes problems.

Barry said:

Cut your max current down 1 amp at a time until it stops.

Mine did it because of the unusually long run (350'). I keep mine set at 34A and it doesn't cut back to 30A.

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I'll try that.

350'? The laundromat down the street?

Nigel Tufnel said:

350'? The laundromat down the street?

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Funny!
No, I'm in a condo building with deeded parking spots. My spot is at the SE corner of the building and the electrical room, of course, is at the NW corner of the building.

I have had similar problems at home and the local service center read codes for HIGH voltage, not low. There is apparently a +/- voltage tolerance and exceeding either causes it to drop back from 40 to 30 A.

FlasherZ said:

Not necessarily. It depends on how long the surge lasted.

Yes, if you applied 480V RMS to a 120V/240V appliance, they would blow up.

However, the "240V" you have in your home is actually an RMS measurement over time. If you were to take an oscilloscope measurement of one cycle of AC 240V, you'd see that the sine wave peaks at about 340V or so (sqrt(2) * 240) with no load. In addition, reactive loads will change instantaneous voltage as well.

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For what it's worth, my transformer is set to provide 248V to my home, and the current does not back off.

Please read the FAQ, it describes all the different contributing factors. In many neighborhoods, transformers are shared and it can be a neighbor's appliance that causes problems.

Click to expand...

I run several rack-mount servers and UPS units (48V with AVR). Fortunately or unfortunately, the UPS's aren't flagging anything unusual with regards to spikes/drops in voltage. And I would suspect they are every bit as sensitive as the Model S. But as someone suggested, the dropping by 1-2amp is a great solution. At least then you arent stuck at 30amp without knowing.

supratachophobia said:

I run several rack-mount servers and UPS units (48V with AVR). Fortunately or unfortunately, the UPS's aren't flagging anything unusual with regards to spikes/drops in voltage. And I would suspect they are every bit as sensitive as the Model S. But as someone suggested, the dropping by 1-2amp is a great solution. At least then you arent stuck at 30amp without knowing.

Click to expand...

Yes, there are ways to work around it. I think Tesla's algorithm might still be too sensitive, but they want to err on the side of safety - and I *guarantee* that anyone who experiences this has at least one problem in their electric infrastructure; bad appliance, undersized transformer, undersized service conductors, questionable charger, etc. I have used Model S in many diverse electrical situations, and I've been able to narrow it down (with some patience) in each of the cases.