Can't get full 80Amps out of Tesla Gen 2... How to monitor the J1772 negotiation??

[ZSC100]

Wouldn't the resistor mess up the voltage pull down?


Would the capacitor help? Are you suggesting it to smooth the waveform?
I am weak with building circuits, so would love an explanation from an EE if you could.

Can't say what will help without seeing an Oscope visual of the signal, but yes, if the signal is low or high in voltage a large resistor sized correctly may pull it up or down past a threshold that it is likely on the edge of. Same with noise. You'd be placing the resistor inside the housing of the EVSE, and I assume you have access to the circuit board or is it potted/sealed? Sorry I'm not at all familiar with Tesla stuff, but I'd love to help you troubleshoot and if you fix you can post a new thread with the fix for the community.

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[garsh]

Lower current inherently creates less heat, that is just physics...

Not sure about "charging efficiency" being better at higher currents though...you'd have to define what you mean.

Hello physics! Meet real-world engineering.

Charging has overhead. The vehicle's on-board charger uses a fixed amount of power regardless of how many amps it's handling. That's what causes charging to be less efficient at lower power levels.

As for the charging rate resulting in degradation, I was going by a blog post written by Tesla back in 2008. But I misremembered - they said to stay under C/2.

Avoiding very high charge rates. Charging faster than about C/2 (two hour charge) can reduce the cell's life.​

 

[galstaf]

Can't say what will help without seeing an Oscope visual of the signal, but yes, if the signal is low or high in voltage a large resistor sized correctly may pull it up or down past a threshold that it is likely on the edge of. Same with noise. You'd be placing the resistor inside the housing of the EVSE, and I assume you have access to the circuit board or is it potted/sealed? Sorry I'm not at all familiar with Tesla stuff, but I'd love to help you troubleshoot and if you fix you can post a new thread with the fix for the community.

Circuit board is not potted and easy to get to. I already know which wires are the CP and PP from their voltages... they are pretty easy to access. I have confirmed there is no voltage drop from inside the EVSE to the end of the cable thru the adapter.

I don't have an oscilloscope unfortunately. But I could probably order a cheap one as it is only a 1kHz signal if that is necessary.

What I don't know is appropriate sizing for either the capacitor / resistor if you could make a suggestion. CP is about 12V and PP is about 4.5V if that helps.

 

[ZSC100]

Hello physics! Meet real-world engineering.

Charging has overhead. The vehicle's on-board charger uses a fixed amount of power regardless of how many amps it's handling. That's what causes charging to be less efficient at lower power levels.

As for the charging rate resulting in degradation, I was going by a blog post written by Tesla back in 2008. But I misremembered - they said to stay under C/2.

Avoiding very high charge rates. Charging faster than about C/2 (two hour charge) can reduce the cell's life.​

True, kind of,
The computer part of the OBC does use a fixed amount of power regardless of what's going on, but it's negligible. The important part of the wasted power in OBCs is the % waste relative to charge current from the power electronics in the OBC. And there is a specific step increase in waste when trying ot charge at Level1 b/c the charger has to boost the voltage x4+ to get enough into it's DC capacitors to charge versus with Level2 on boost of x2+ is needed. So, pretty much any Level2 charging you do the waste is linear and relative to the charge current.

And yes, 80A charging on the Lightning is literally a breeze for the battery, there's no reason to not charge at 80A if you can.

 

[tls]

There is no setting on my version for 72A. That is the later model with the rotary dial to set amperage. Mine is the 4 linear DIP switches and the next lowest after 80A is 64A (which does work fine)..

There are two different versions of the Gen1 wall connector - part number ending in A or B, or part number ending in C or D. They both have DIP switches per the manuals. The setting for 24A on one is the setting for 72A on the other. Odd, no?

Anyway it sounds like that is not your problem. One of the posters in that thread I pointed out earlier put a scope on the pilot line and described the PWM waveform at 100% duty cycle as "dirty". That's what I know. That plus it comes up here periodically and nobody can ever get a Gen1 unit working at 80A.

 

[ZSC100]

Circuit board is not potted and easy to get to. I already know which wires are the CP and PP from their voltages... they are pretty easy to access. I have confirmed there is no voltage drop from inside the EVSE to the end of the cable thru the adapter.

I don't have an oscilloscope unfortunately. But I could probably order a cheap one as it is only a 1kHz signal if that is necessary.

What I don't know is appropriate sizing for either the capacitor / resistor if you could make a suggestion. CP is about 12V and PP is about 4.5V if that helps.

You need a OScope, do you know any, do you know any EE students in the area who can help, and might take the measurements for you in exchange for a Starbucks gift card

Yes you can order a cheapie, but it might be hard to get good images off of it.

like this:
https://www.amazon.com/FNIRSI-DSO152-Handheld-Oscilloscope-Bandwidth/dp/B0FDPYNQBC/ref=sr_1_3

What I want to see is the signal (2 full wave forms with the signal scaled 80% of the screen showing the zero ref) for every charge speed you can select.

Correspond the images with file name like (EVSE CP Signal - 48A - 2/13/2026), also check the PP signal for each instance, reference should be the ground pin.

 

[galstaf]

You need a OScope, do you know any, do you know any EE students in the area who can help, and might take the measurements for you in exchange for a Starbucks gift card

Yes you can order a cheapie, but it might be hard to get good images off of it.

like this:
https://www.amazon.com/FNIRSI-DSO152-Handheld-Oscilloscope-Bandwidth/dp/B0FDPYNQBC/ref=sr_1_3

What I want to see is the signal (2 full wave forms with the signal scaled 80% of the screen showing the zero ref) for every charge speed you can select.

Correspond the images with file name like (EVSE CP Signal - 48A - 2/13/2026), also check the PP signal for each instance, reference should be the ground pin.

I can give it a shot!

I guess I am curious as to how you tell what to do from the waveform.
Could you give me a couple of IF THEN scenarios?

 

[chl]

Charging has overhead. The vehicle's on-board charger uses a fixed amount of power regardless of how many amps it's handling. That's what causes charging to be less efficient at lower power levels.

I've never seen a complete analysis of the energy throughput end to end of the Lightning AC charging system.

But the wires are the same, so their resistance is the same, no matter what rate you are charging at. So, as an electrical engineer, I can say with certainty that a lower current rate will have lower energy losses, due to I^2R (current x current x resistance) losses.

From the math you should be able to see that the power lost due to resistance is proportional to the square of the current.

Thus, it stands to reason that the energy losses due to resistance (heat) in all the components from EVSE to AC-DC converter to battery would be higher with a higher current.

And, of course some are actively cooled by the Lightning, which also requires energy to operate, proportional to the temperature.
---

Although I often find one can't trust AI answers, this time AI did a fairly good job explaining why a lower current is more efficient than a higher current, although the difference in how much energy is wasted may be small:

---AI below--

Charging an electric vehicle (EV) at 40A is generally more energy-efficient than at 80A, though the total difference in energy wasted is often minor for daily use. While higher amperage (80A) charges the battery faster, it produces more heat (losses) in the charging cable, onboard charger, and battery, leading to lower efficiency compared to moderate speeds.

Efficiency Comparison: 80A vs. 40A

• 40A (Lower Power): Offers higher efficiency, generally in the 90–94% range, because the onboard charger works closer to its optimal range and generates less heat.
• 80A (Higher Power): While faster, this can decrease efficiency due to higher heat losses. The difference in wasted energy between 40A and 80A is often considered negligible for daily use, but 80A will result in higher heat generation.
• Optimal Speed: Studies suggest that Level 2 charging at lower amps (around 32A-40A) is often the "sweet spot" for maximum efficiency.

Sources of Energy Waste

Regardless of the amperage, AC charging typically loses 5% to 20% of energy as heat, with 10% being a common benchmark.

1. Onboard Charger Efficiency: The converter that changes AC to DC is most efficient at specific loads (often 50-70% of maximum).
2. Heat Resistance (I^2R): Heat loss increases with the square of the current. A 2x increase in amperage (40A to 80A) results in roughly a 4x increase in heat loss in the cables.
   [My emphasis]
3. Battery Management: Higher current can require more energy to operate cooling fans/pumps, increasing wasted energy.

Key Takeaways

• Best for Efficiency: A 40A or 48A charger is generally sufficient and more efficient than an 80A charger.
• Cost Impact: The extra energy lost at 80A is usually negligible for daily driving, though charging at lower amperage can save a few percentage points of energy.
• Recommendation: If you are not in a hurry, charging at a slightly lower amperage than maximum is slightly better for both energy efficiency and battery longevity.

---
I'd wonder if anyone has quantified it in the Lightning?

 

[galstaf]

I've never seen a complete analysis of the energy throughput end to end of the Lightning AC charging system.

But the wires are the same, so their resistance is the same, no matter what rate you are charging at. So, as an electrical engineer, I can say with certainty that a lower current rate will have lower energy losses, due to I^2R (current x current x resistance) losses.

From the math you should be able to see that the power lost due to resistance is proportional to the square of the current.

Thus, it stands to reason that the energy losses due to resistance (heat) in all the components from EVSE to AC-DC converter to battery would be higher with a higher current.

And, of course some are actively cooled by the Lightning, which also requires energy to operate, proportional to the temperature.
---

Although I often find one can't trust AI answers, this time AI did a fairly good job explaining why a lower current is more efficient than a higher current, although the difference in how much energy is wasted may be small:

---AI below--

Charging an electric vehicle (EV) at 40A is generally more energy-efficient than at 80A, though the total difference in energy wasted is often minor for daily use. While higher amperage (80A) charges the battery faster, it produces more heat (losses) in the charging cable, onboard charger, and battery, leading to lower efficiency compared to moderate speeds.

Efficiency Comparison: 80A vs. 40A

• 40A (Lower Power): Offers higher efficiency, generally in the 90–94% range, because the onboard charger works closer to its optimal range and generates less heat.
• 80A (Higher Power): While faster, this can decrease efficiency due to higher heat losses. The difference in wasted energy between 40A and 80A is often considered negligible for daily use, but 80A will result in higher heat generation.
• Optimal Speed: Studies suggest that Level 2 charging at lower amps (around 32A-40A) is often the "sweet spot" for maximum efficiency.

Sources of Energy Waste

Regardless of the amperage, AC charging typically loses 5% to 20% of energy as heat, with 10% being a common benchmark.

1. Onboard Charger Efficiency: The converter that changes AC to DC is most efficient at specific loads (often 50-70% of maximum).
2. Heat Resistance (I^2R): Heat loss increases with the square of the current. A 2x increase in amperage (40A to 80A) results in roughly a 4x increase in heat loss in the cables.
   [My emphasis]
3. Battery Management: Higher current can require more energy to operate cooling fans/pumps, increasing wasted energy.

Key Takeaways

• Best for Efficiency: A 40A or 48A charger is generally sufficient and more efficient than an 80A charger.
• Cost Impact: The extra energy lost at 80A is usually negligible for daily driving, though charging at lower amperage can save a few percentage points of energy.
• Recommendation: If you are not in a hurry, charging at a slightly lower amperage than maximum is slightly better for both energy efficiency and battery longevity.

---
I'd wonder if anyone has quantified it in the Lightning?

The full wall-to-pack loss budget has at least four terms:
(1) fixed auxiliary/controls power while the vehicle is awake and charging,
(2) onboard charger (AC→DC) conversion losses that vary with load,
(3) resistive/contact losses between EVSE and charger, and
(4) battery thermal conditioning (heating/cooling) energy.

Especially when it is super cold in winter, number 4 dominates.
At low charge power, the session takes longer, so fixed overhead and coolant pumps/fans run longer per kWh delivered.

In extreme cold or heat, battery conditioning can consume a large, near-constant amount of power; if you charge slowly, that conditioning can take a bigger fraction of the available input and/or extend the session, reducing efficiency. In those conditions, charging at higher Level 2 power can be more efficient simply because it shortens the time the vehicle spends running heaters/chillers and other auxiliaries.
Net: there is usually a “sweet spot,” but it’s not universal and it moves with ambient temperature, starting pack temperature, and SOC (state of charge).

 

[chl]

The full wall-to-pack loss budget has at least four terms:
(1) fixed auxiliary/controls power while the vehicle is awake and charging,
(2) onboard charger (AC→DC) conversion losses that vary with load,
(3) resistive/contact losses between EVSE and charger, and
(4) battery thermal conditioning (heating/cooling) energy.

Especially when it is super cold in winter, number 4 dominates.
At low charge power, the session takes longer, so fixed overhead and coolant pumps/fans run longer per kWh delivered.

In extreme cold or heat, battery conditioning can consume a large, near-constant amount of power; if you charge slowly, that conditioning can take a bigger fraction of the available input and/or extend the session, reducing efficiency. In those conditions, charging at higher Level 2 power can be more efficient simply because it shortens the time the vehicle spends running heaters/chillers and other auxiliaries.
Net: there is usually a “sweet spot,” but it’s not universal and it moves with ambient temperature, starting pack temperature, and SOC (state of charge).

All of those (1) to (4) are necessary uses of energy in the process.
Any wasted energy will be higher at higher currents - can't get around that - the power wasted is proportional to the square of the current.

So if we are talking about energy efficiency, higher current is less efficient.

If you are talking about the "efficiency" in terms of time, then I agree, lower currents will require more time, maybe if you are on a schedule, that is the most important factor and the fact that it costs a bit more due to wasted electrical energy may not matter, since in that case, time is money, maybe more money than the cost of the wasted kWh.

I can see that as a reasonable reason to use a higher current.

Another place where time charging is money:
My utility gives me a reduced cost per kWh from 1am to 5am, so if I needed more time to reach 80% then I would probably want to raise the current so that it finishes by 5am, but not so much that the cost difference from wasted energy at the lower price eats up the savings.

It is just a fact of the way electricity moves through wires - higher current means higher wasted power, proportional to the square of the current...so twice the current means four times the wasted power to resistances.

In the case of our L2 charging currents, as I have said, it will be relatively small in the scheme of things, the cost of the wasted energy will be small. Our resistances pale in comparison to utility transmission lines, for example, as do our loads. Utilities step up the voltage for transmission lines so the current can be lower for the same power distribution and they can minimize the power losses due to wire resistance. The resistance of wire goes up with distance in a linear fashion. But the power lost to resistance goes up with the square of the current. So it can be a big deal!

Now one thing I omitted from consideration is wire size.
If you are wired for 80A max then you have thicker wire which has lower resistance to current due to the larger cross section.

So that 80A wire will transfer energy more efficiently - waste less - at a lower current, say 40A, than if the circuit was wired for 40A max with smaller wire.

Even with larger wire for carrying 80A, the power loss is greater running 80A through it than a 40A current through 40A wire.

There are cost tradeoffs of course to using heavier gauge wire than needed, it's more expensive.
Over time, the cumulative savings in less wasted energy due to resistance may justify the up front extra cost of larger wire.

The power wasted grows faster with current than the resistance drops due to wire size so it may not be worth it unless the facility will be using high currents for long periods of time, such as in a 24/7 data center or factory.

Wire is sized for current carrying capacity not for energy waste, but it is something to consider for planning purposes.

Warming the battery is going to take the same amount of energy no matter what the current is,
whether you are providing 80A or 40A.

But at a lower current, less energy would be wasted in the form of resistance power x time.

There is always less energy is available to charge the battery during the battery warming period either way.

But more energy is wasted from resistance at a higher current.

So while you may charge your battery faster with higher current, you will waste more energy in the process.

 

[mr.Magoo]

All of those (1) to (4) are necessary uses of energy in the process.

But more energy is wasted from resistance at a higher current.

So while you may charge your battery faster with higher current, you will waste more energy in the process.

Then again, as an electrical engineer you would also know that it's not quite that simple.

Sure, very simplified you can say that higher current generates more heat, i.e. energy loss.
Keeping all the components in the truck alive during charging is using more energy (DC/DC converter is pulling 0.5A at 380V) compared to the additional heat/energy losses you get when going from 40 to 80A so going by those two factors alone, 80A is better since energy loss is at least breaking even with your energy use but you're cutting the time in half.

But, then you also have to consider exterior temperatures and if you need to either heat or cool the battery as well, and you should also weigh in the installation cost.

In the end this feels like a theoretical exercise since we're talking about cents / 1kWh or less difference in the end.

 

[garsh]

So if we are talking about energy efficiency, higher current is less efficient.

Unless you have an extremely long cable run, the fixed losses of the vehicle's on-board charger are going to dwarf the ohmic losses in the cabling.

AC Charging at the fastest-available current level will minimize losses.

 

[mr.Magoo]

AC Charging at the fastest-available current level will minimize losses.

Well, if you don't charge at all you completely eliminate charging losses !

 

[chl]

Unless you have an extremely long cable run, the fixed losses of the vehicle's on-board charger are going to dwarf the ohmic losses in the cabling.

AC Charging at the fastest-available current level will minimize losses.

MAXIMIZE losses!

power = I X I X R

 

[chl]

Then again, as an electrical engineer you would also know that it's not quite that simple.

Sure, very simplified you can say that higher current generates more heat, i.e. energy loss.
Keeping all the components in the truck alive during charging is using more energy (DC/DC converter is pulling 0.5A at 380V) compared to the additional heat/energy losses you get when going from 40 to 80A so going by those two factors alone, 80A is better since energy loss is at least breaking even with your energy use but you're cutting the time in half.

But, then you also have to consider exterior temperatures and if you need to either heat or cool the battery as well, and you should also weigh in the installation cost.

In the end this feels like a theoretical exercise since we're talking about cents / 1kWh or less difference in the end.

Sorry, you lost me a bit...

But yes it is simple basic electronics 101,

And yes it may be a small difference in cost, but if you had a fleet of vehicle it would add up.

My only point is simply that higher current is not more energy efficient.

Just for the sake of avoiding that misconception.

Time wise, yes more efficient.

No point belaboring it any further eh?

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