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14-50 vs HPWC dropping 25%

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Although we are not privy to the Tesla charger's algorithm for current throttling, it is likely it works something like this:

First, a little electrical circuits 101. (I realize I am applying DC circuit equations to an AC circuit here but, due to the primarily resistive load of the batteries, this should be a reasonable conceptual approximation.)

V=IR (Voltage is current * resistance)

P=VI (Power is voltage * current)

Now, power dissipated in the branch circuit (circuit breaker, wire to the 14-50, the 14-50 outlet itself, and the UMC) is what needs to be monitored. Power = heat. Too much heat = fire. So, power must be controlled.

Combining the above gives us P=I^2*R. Since power goes up with the square of current, reducing current even a little can significantly reduce heat. Since the charger can control the resistance in the circuit, this is the only action it can take.

Of course, the charger can't measure power directly. It only knows input voltage (V) and the current (I) drawn from the circuit. It can calculate the power that the charger uses but not the power dissipated in the circuit. So, how does the charger know if any power is getting dissipated in the circuit itself?

Let's say the charger notices a voltage drop. It could be a result of heat in the circuit causing its resistance to go up. Or it could be caused by any number of other issues cited above (loads in the neighborhood, appliance kicking on, etc.). Well if the charger notices a voltage drop, it could respond by momentarily reducing its current draw. Since the resistance (R) in the branch circuit is constant over a short time period, a reduction in current should result in an increase in voltage. If this happens, it is likely there is a wiring issue in the branch circuit. However, if voltage does not increase, the issue is likely somewhere else in the grid.

So, if the charger detects the first situation (perhaps by performing multiple tests over an interval), it could then decide to permanently reduce current.

Again, this is just my speculation as to how this works but if I were designing such a system this is how I would do it.
 
I tried 38 amps and 35... no dice. still cuts to 30. I may just call Tesla. Their abundance of caution is... something... something. but I can't force it back to 40 every 30 min.

Do you actually need it at 40A in order to get a charge in the time you have available, or can you do it with 30A? Just curious, you're perfectly within your rights to want it to charge at 40A even if you don't need it -- I'd want the same.
 
Since there is a time-delay before it cuts back to 30A, i'm going to predict that you have a high resistance junction from a slight defect in a crimped lug or screw terminal somewhere in the circuit. When cold it has a lower resistance and can flow the full 40 with minor voltage drop, but after a half hour at full current the junction temperature heats up and this causes the resistance to rise and the voltage drop increases above the limit and the car kicks down the current.

Unfortunately you can't tell by looking at a crimped lug or screw terminal that it is slightly defective--but a thermal scan with an IR camera would show up under load.
 
Again, this is just my speculation as to how this works but if I were designing such a system this is how I would do it.

There is a voltage drop component to it, that's the one that's predictable and happens 70-80% of the time in certain situations, and it's usually because of a long wiring run with smaller conductors - a 400' run from transformer to home, a 200' feeder from service panel to subpanel / 200' run to a 14-50. That one's easy to diagnose because just a few questions and a quick inspection from an electrician can determine this.

What's not easy to determine is the other case - I call this out in the other threads all the time. I charge at a shop on some hunting ground where the transformer is dedicated and the only load besides my charging is a 30A water heater. About a year ago, I noticed that merely turning the fluorescent lights on in the shop would cause the car to drop to 30A occasionally. As it turns out, a bad ballast in one of the fixtures was causing the reduction in charging current. I didn't bother to do any more experimentation, but since then I haven't had any drops in charging current at all.
 
FWIW, I would occasionally get reductions to 30A. I realized a few days ago that I had set my pool pump to start around midnight, and that clock had drifted, so it really started around 12:30am, well within the time when my car is charging (it starts charging at 12:05). I reset my pool pump's time so that it starts before the car charges, and so far no current reduction.

Anyways, for those that have overnight reductions, check to see if any big loads in your house start around midnight too.
 
No pool pump, but I am going to try charging, starting at 1 AM and see if that changes anything...

We did just go to an electric water heater (heat pump kind, but that doesn't work until they repair it), so it's drawing the 9500 watts vs the 500 it should in heat pump mode... my basement is 74 right now, so that sucks.
 
I use a mobile cord with 14-50 40 amp service at home to charge. No issues for 2 years now the charging keeps interrupting. Tesla checked my cable and replaced under warranty, but the problem continues. It's really weird... Any ideas? It's gotten so unreliable I can't be sure the car will charge overnight. When it interrupts, I unplug, re plug and it'll charge for maybe 1/2 hour then repeat.
 
I use a mobile cord with 14-50 40 amp service at home to charge. No issues for 2 years now the charging keeps interrupting. Tesla checked my cable and replaced under warranty, but the problem continues. It's really weird... Any ideas? It's gotten so unreliable I can't be sure the car will charge overnight. When it interrupts, I unplug, re plug and it'll charge for maybe 1/2 hour then repeat.

My first thought is an aging or overloaded power company transformer feeding your home. Transformers become less efficient over time and in some cases may have new loads placed on them that then degrades thier output voltage.
 
OR, transformer shared with neighbors and their loads have changed, OR you got a new appliance that is creating a new load profile. See charging FAQs for details, but we can't keep blaming it on UMC or Tesla firmware. Yes, they are both relatively intolerant - but intentionally so.
 
Charged last night from 50% to 90% at 40 amps... the whole time. Started charging at 11:00 PM.

Next time I'll run visible Tesla, and start at 10PM, but this may be the key to charging at 40 amps for the duration... starting late at night.

My guess is PP&L under-serves the area, what with my new Tesla, Heat Pump, and Electric water heater, and 100 amp sub-panel for my shop, etc etc etc lol.

But their equipment says "it's all within spec"...
 
Charged last night from 50% to 90% at 40 amps... the whole time. Started charging at 11:00 PM.
Next time I'll run visible Tesla, and start at 10PM, but this may be the key to charging at 40 amps for the duration... starting late at night.
I always start my charge sessions late at night. (11:30 PM). This allows the batteries more time to cool down before charging. This may increase your battery life in the long run.

If the battery spends less time at a high state of charge it may also help increase battery life.
 
But their equipment says "it's all within spec"...

That's ROFL funny! I hope PP&L doesn't come out at one point in time, take a bunch of metrics and pronounce you OK, when what you are experiencing is time and situation dependent load variation? I hope they plugged in a monitor and got a week's data. But I'm guessing it is the former, not the latter. The last thing they want to be bothered with is expensive equipment replacement to satisfy a single customer.
 
So wait... I have to contact my electric company to have the quality of my electricity checked?

Have you checked the voltage at the car during charging? Can you also catch a voltage reading when charging is interrupted? Have you reviewed your wiring, panel, and other loads, or hired an electrician to do so?

Depending on the results, you may need to call your power company next.

Good Luck,

GSP
 
I do only buy the cheapest electricity ;)

maybe I need to switch to mid-grade.

Charged again, starting at 10 PM, 40 amps whole time.

looks like I found the issue - I have to charge after 10 PM if I want to charge at 40 amps the whole duration. Most likely caused by summer A/C units?
 
I do only buy the cheapest electricity ;)

maybe I need to switch to mid-grade.

Charged again, starting at 10 PM, 40 amps whole time.

looks like I found the issue - I have to charge after 10 PM if I want to charge at 40 amps the whole duration. Most likely caused by summer A/C units?

An easy way to monitor your line voltage is with PowerChute from APC. I showed a data printout from the program when we had a voltage sag every day at the same time, and they traced it to a faulty switch and corrected the problem.

ftp://ftp.apc.com/apc/public/software/unix/linux/pcplus/452/manual.pdf
 
But their equipment says "it's all within spec"...

Yes, unfortunately it's "within spec" to the extent that Edison incandescent light bulbs and resistive heating elements care. Most basic power company analyzers - as you've noted - take once-per-minute measurements and do nothing to look for abnormal voltage fluctuations. If their meter isn't sensing the power perfectly when loads kick in, then it says everything's fine.

It's a shame; it will take power companies a while to figure it out. The move to more electronics for lighting will help.

Here's a story from another member:
Incoming utility cable capacity - A cautionary tale - Page 2

He had to boil the transformer before PG&E would replace it. :)

- - - Updated - - -

So wait... I have to contact my electric company to have the quality of my electricity checked?

If you're experiencing this problem and you've ruled out a lot of other factors (charger, EVSE/UMC/HPWC, big loads in your home, etc.), then yes, you may need to.