40 amp charging dropping to 30 amp

40 amp charging dropping to 30 amp

I have set my charger at 40 amps plugged in to my home 220, and I have noticed that it keeps slipping back to 30 amps. Has anyone else had this experience. Today I manually pushed it back up to 40 amps but do I need to keep watching it every time I charge?

jjs | August 4, 2014

Is your 220 circuit rated at 50amps or 40amps? In the U.S. continuous load on circuits should only be 80% of stated rating. Hence 50 becomes 40, 40, 32. I know that Tesla has built in intelligence into their charging hardware/software.

If your circuit is rated at 40amps then the answer is Tesla is preventing your house from burning down. (Actually would probably trip your breaker first.) Quite a nice thing to do! :) If it is rated at 50amps then I would check with a service center.

I have had somewhat of a similar issue. Occasionally my home charge setting was getting overridden, but the other way. I had it set at 20 and it was bumping up to 30. The circuit is interpreted by the Tesla charger as a 40amp circuit, but I prefer the charge to take place at 20amps.

DanielR | August 4, 2014

For over 15 months, our 220 circuit automatically provided 40 amps and charged at 28-30 miles for every hour plugged in. Last month we had the master charger unit on the car replaced. Since then, the car charges at 30 amps, unless I manually bump it up to 40 amps. 60kWh battery pack.

michael1800 | August 4, 2014

What is the SOC when your amps are noted as lower than manually set?

SCCRENDO | August 4, 2014

Have this from time to time. I mostly charge at 40 amps but when there is a power surge in the neighborhood as a result of the latest firmware updates it will drop the amperage. The fix is to unplug, set the car amperage back to 40 and then start charging again. Call your Service Center to pull logs and they will be able to tell you. Occasionally its the UMC cable that is at fault and they can check it out.

dowopdave | August 4, 2014

Thanks. I checked with my electrician and he said it was rated at 50.

RichardKJ | August 4, 2014

I usually get 40 amp charging from my NEMA 14-50, but recently in the warm weather when the house A/C is running the car drops back to 30 amps. I assume that is because the car senses the voltage drop from the additional heavy load and drops back. It will stay at 40 amps unless I push it back up.

By the way, nominal voltage these days is 240 not 220.

dowopdave | August 4, 2014

I'll keep an eye on it. It is pretty darn hot here in Central Florida!

DonS | August 4, 2014

After a garage fire (that was not part of Tesla's hardware), Tesla added voltage monitoring to charging process. A large voltage drop from changing the charge current from 0A to the 40A load indicates a high resistance path, possibly due to bad connections.

It will take some detective work to figure out if it is your wiring or if the utility feed is inadequate. Or you can just charge at the lower current. On a thread about the HPWC, someone just posted that he was able to measure the incoming voltage and it didn't meet the utility company specs, so they upgraded the at the pole to fix it.

carlk | August 4, 2014

It has happened to me a couple of times. I don't think it's voltage/current issue since I have 50A breaker and 6AWG wire and I always charge late at night. The only thing I can think of is perhaps the garage was getting too hot.

David70 | August 4, 2014

That's happened to me several time when the garage has stayed above 90F overnight.

Kimscar | August 4, 2014

What DonS said. I have an 80 AMP HPWC. My 80 AMPs would drop to 60. Long story short I measured the voltage at the Service panel at the bus bar right after the meter. You can look at your Tesla voltage also. Start at 5 amps and not the voltage. increase your charging amperage. If you see the voltage drop significantly say more than 12 volts or close to that you probably have a problem. Now if you have air conditioning that kicks on and off that might drop the voltage enough to put you out of the Tesla tolerance they set on the voltage thus reducing the current.
Your problem may be in the house wiring or the utility. In my case after taking my measurements at the service panel and everywhere else I called the utility. They hung a data recorder that records voltage against time and saw the problem on there end. I am now waiting for a new transformer to be mounted on the pole in my back yard.
Note if you are not familiar with electrical circuits and versed in safety don't go trying to make measurements yourself as the wrong move could kill you.
Pretty cool as it appears that Tesla starts out with a voltage measurement with no current. They then begin to increment the current while checking voltage. If at any time they exceed the limits (perhaps over a period of time) they put in for voltage drop they then drop the current.
Start of by looking at the volage displayed on the Tesla screen as charging starts up. After charging at max for example if it is working turn on the AC etc.
As for me I am waiting for the city to put in my transformer.

HenryT2 | August 5, 2014

Yep, happens to me occasionally. But I always noticed after it was probably doing it for a little while. I'll have to check whether temps have been high on those days.

Rocky_H | August 5, 2014

I would think this is probably a heat managing issue. First, it's hot in your garage. When you start charging, that generates some heat in the batteries, so the car usually turns on the battery cooling system within a minute or two, which is now blowing more heat into the air in your garage. So with that also increasing the temperature inside your garage, the cooling radiators are getting less effective, so the car is probably managing the other side of it, by slowing the charging so it creates less heat that needs to be vented.

Kimscar | August 5, 2014

I could be wrong but I don't think heat is the issue here. I charge just fine at 60 AMPs with temperature in the garage at 105. For people saying it's how in the garage what is the car saying for temperature?

Rocky_H | August 5, 2014

I think you've convinced me to change my vote. 240V at 40A isn't all that high versus what the battery can take, so it probably would have a lot of leeway to handle that. I'll go with your suggestion, Kimscar, to step up the charging amps a little at a time, and see if the car starts showing some drop in voltage.

Kimscar | August 5, 2014

@Rocky_H it would be interesting to see your results. One thing to do is keep the A/C off. When charging at your max current turn on the A/C and see what happens to voltage and current

AndyO | August 5, 2014

The voltage and amperage on the right hand side of the charging screen includes the power going to support systems. I've seen it hit 12 amps (at 240V) when A/C is running at max and charging has been completed. Shut off A/C and it goes to 0-1 amps. The miles-per-hour charge rate on the left would be indicative of actual charge rate except that it's a long term average. It's still good to not be using A/C if you're trying to get the quickest charge. It's just hard to measure the immediate impact.

Rocky_H | August 5, 2014

@Kimscar, I'm thinking maybe that was intended for the OP, not me. I never have a problem with charging, and that's probably because the run from my electrical panel to my 14-50 outlet is about 3 feet. I charge at night now, so I don't watch it, but when I first got it, I would have it charge when I plugged it in. The voltage stayed at 240 or 241V through the whole range from 0 to 40 amps.

Kimscar | August 5, 2014

@Rocky_H. Yes it should have been directed to the OP.
@AndyO. You are correct about the A/C draw in the car up to 12 amps. I should have been more clear. I was referring to the home A/C. With the car charging by turning on home A/C you put an instant load on the line. That could possibly cause the voltage to drop enough to have the car reduce the current, if the current stays the same check voltage at car to see if A/C increases drop in voltage.

AndyO | August 5, 2014

@Kimscar. Gotcha. Sorry.

Dcp9142 | August 5, 2014

I recently bought a second UMC. My old one always charged at 40 amps from my 14-50, the new one consistently dropped back to 30 amps.

Both had grey faced 14-50 connectors.
Both well plugged in, yes I did try unplugging and re-plugging.
My outlet was electrician installed and is two feet from my panel.
No voltage drop, 241-245 volts when I look.

Tesla replaced the second UMC.

The third one happily charges at 40 amps.

mcptwo | January 17, 2015

Our madel S is currently charging at from 227 to 232 volt at 39 to 40 amps. Since we are about 500 feet from the main panel at the road and an other 80 feet out to the garage, we expect about a 6 volt drop in voltage from 240 to about 234 volts. The additional drop in voltage may be because PG&E provides less than adequate voltage to begin with and more of our neighbors are home in the evening. Most of the time, I will find that the charging rate has dropped to 30 amps after an hour or so.
Does anyone know how Tesla's software was written? Is there a low voltage limit? Or some other factor that triggers the lowering of the charging rate to 30 amps.
I am absolutely confident that the #6 copper wire running to our garage is adequate. But I would like to solve this issue and get back to the consistent 40 amp charging we had prior to the software update.

Brian H | January 17, 2015

Yeah, twitches and glitches in supply will make the car dial down the amps.

icarreras | January 18, 2015

It happens to me every time the first floor AC unit (bigger that the second floor one) turns on. It has happened with a couple of loaners as well. Electrician says everything in the house is fine, that it is most likely some type of protection of the car. I charge at 2:00 AM now, when the AC is usually off, and problem solved.

sonofomar | January 18, 2015

@icarreras: "...and problem solved." This statement and the thread implies 40A is better than 30A or lower. Is that known? I've searched the forum for a definitive answer, but I've seen varying opinions on what the optimum amperage for charging is, whether you're looking for maximum charging efficiency or least stress on the car. On the latter, the answer seems to be "don't worry about it", Tesla has you covered. On the former, it's apparently complicated. Power losses will increase with the square of amperage, but I've read that the inverter may have a sweet spot for efficiency at 20-30A. I'm not an electrical engineer, but I do care about charging efficiency. I want to be sure that our solar panels have the best chance of covering all our annual usage. AZ summer AC needs will come, and the next few months of building net metering credits are critical. Can anyone supply a link to the definitive answer?

AoneOne | January 18, 2015

From:, for the example 40 miles @ $0.12/kWh:

40 or 80A takes 13.2 kWh
24A takes 13.4 kWh

So, it doesn't make a real difference in efficiency. Personally, if I have the time, I like charging at lower currents to minimize I^2*R losses in the wiring and connectors.

bish | January 18, 2015

Mine would drop from 40 to 30 amps whenever the house air conditioning started(Similiar to @icarreras). But when charging at 38 amps it does not drop down to 30.

@AoneOne, twinkle twinkle little star, power is equal to i squared R.

AoneOne | January 18, 2015

@bish: cute rhyme.

Haggy | January 18, 2015

Ever since my car was new, on more days than not, by the time I got into the car in the morning things were at 30A. Some days it was still at 40A. The longer the charge, the more likely it was that it dropped.

I read the explanations and they made sense so I didn't give it much thought. Nobody's power is perfectly clean. After a while I noticed that if it was charging at 30 and I stopped it and restarted it at 40, I got an message about a problem with charging. Then it dropped to 30 and worked.

I took it in for service for an unrelated issue and figured I'd ask about it and have them check my cable while they were at it. They told me that there's a problem with the charger and that even if I hadn't told them they would have fixed it on that visit. They also said it would have been a matter of time before they called me and told me to bring it in.

Long story short, they changed the charger on the car and it's charged at 40 and stayed at 40 ever since. No exceptions.

bevguy | January 18, 2015

Unless you are in a big rush why charge at the full 40 amps? Considering all the other construction shortcuts in most houses today, I don't[put full faith in the electricians.
Since you usually have at least 12 hours to charge do it at 30 amps. You have nothing to lose. What difference does it make whether you charge in 6 hours or 8 hours if you are asleep? | January 18, 2015

Could charging current be in part due to the state of charge of the battery pack? The higher the percent of full charge, the less current is drawn as the rate of charge decreases once the battery pack gets up to something like 60% (guess) of full charge.

AoneOne | January 18, 2015

Looking at some old supercharging records, I got 32kW at 92%, dropping to 4.4 kW at 98%, so I wouldn't expect the 40A (9.6 kW) to be tapered unless you're charging to 95% or more.

In my experience, a range (100%) charge did not change the 40A charging current setting, even if it might have tapered the charge when it was nearly full. The reset to 30A seems to be associated only with a charging cable or electrical supply problem.

Haggy | January 19, 2015

I don't have 12 hours to charge. My rates drop at 11 pm and go back up at 7 am. If I have at least 50 miles of range left, that's enough time but doesn't necessarily leave lots of margin.

Ruby110 | January 19, 2015

I had this problem twice. Both times Tesla replaced the charger and the problem went away.

Cyclist71 | January 20, 2015

I had a similar problem to Ruby110, receiving my S85 in June 2014 and noting a problem in August with charging dropping to 30Amps, then one morning it had not charged overnight and service confirmed a dead charger after my attempt to charge at a friend's charger was also unsuccessful. The repair was done in my garage and worked great until late November when it dropped to 30amps again. I am awaiting a new charger.

So, don't blame it on the house A/C and contact your local service center. They can monitor it remotely. I am a bit concerned that I've needed two chargers in less than a year, but still love the car.

Hopefully Ruby110's and my story will both end happily with charger two...

Cyclist71 | March 27, 2015

It was not the charger again, but the high-voltage junction box, and now that most of the potholes are filled and the salt washed off the roads, the S85 is back on the roads, and what a sweet ride! Still just makes me smile. Service could not be better, picked up in front of my house on a flat bed and delivered back two days later with tires rotated and courtesy inspected.

KL | March 27, 2015

This is part of Tesla's voltage monitory algorithm for safety reasons. At home, I have my DIP switches set for an 80A breaker on my HPWC. I can sustain 64A at 235V. At a recent hotel's destination charger, it had a similar setup, but while it started the charging cycle at 64A sustained, it dropped it to 48A when the voltage dropped below 220V.

So this is pretty normal. Reasons why your voltage may be dropping?

- You have bad wiring/not suitable wiring in your house and you have a greater path of resistance as temperature rises in the conductor.
- You turn on other high-load appliances and load increase to the point where voltage drops.
- The distribution circuit you're on in your neighborhood is taxing your utility's transformer to the point where it cannot supply the amount of power needed to match load, therefore voltage drops, e.g., there are 5 other Model S owners with HPWCs on your street and you all charge at the same time.

- K

Magemarq | March 27, 2015

I have 240 volt at home on a 60 and it stayed at 40 amps no problem. At work the voltage is 208V and it will drop to 30 amps every second or third charge unless I reset to 40. I just traded my 60 for an 85D due any day now and am curious if this will change.

collier0680 | September 19, 2015

I have had this same issue and no one can figure it out. I can hold steady at 237-238v and almost to the minute after 45 minutes it drops to 32A. It does not matter when I charge it whether 1am of 6pm.

Also the other day my stated range dropped by 30% while the car was parked in my parking garage at work after 10 hours. They tell me this is is manually readjusting and could happen anytime. I don't know about you but if I knew the battery could just drop 30% by being parked i would not have bought the car, considering I didn't have enough charge to get home that day after that adjustment

Tropopause | September 19, 2015

collier0680- I read your thread. You lost 15 miles rated range. That is not 30% of the battery.

Either way, according to the letter you provided from Tesla, your SOC recalibrated and that should normally be viewed as a good thing because it means your old reading was inaccurate.

Many here complain about rated range decreasing with time due to needing recalibration. Now it sounds like Tesla has addressed this issue.

If you don't want your car, please give it to me. I will help relieve you of this burden.