For Tesla End-To-End Charging, There’s Supercharging & Destination Charging. For Everything Else, There’s The Mobile Connector — Part I

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Put your propeller caps on folks because it’s time to take a deep dive into the magic behind the charging cable that came in the trunk of your Tesla Model S/X/3. The notion behind this post is to explain how Tesla has evolved its new generation charge cords and outlet adapters to make charging as safe as possible. Although the Gen 2 Mobile Connector has been around since the Model 3 started shipping in 2017, details of the tech behind the new charging cable and its updated adapters is just coming to light.

Note: If you don’t own a propeller beanie you can always skip straight to the end of Part II of this series for a non-technical executive summary.

With Supercharging and Destination Charging, Tesla has end-to-end control over the charging process from the charger*, to the charge cord, to the car, so safety is fully within the company’s control. This is also true of Tesla’s home charging solution, the Tesla Wall Connector (formerly known as the High Power Wall Connector or HPWC). In fact, a Destination Charger is nothing more than a Tesla Wall Connector installed at a *destination* like a motel, restaurant or other public location.

Tesla-Kettleman City-Supercharger
Tesla Supercharger Station. Photo Credit: Tesla.
Tesla Destination Station. Photo Credit: Tesla.

But Teslas can and do charge at a variety of other locations and from a variety of power sources. That’s where that cable in the trunk known as the Mobile Connector (formerly known as the UMC or Universal Mobile Connector), comes in. The Mobile Connector (MC) allows an owner to charge their Tesla from non-Tesla power sources. But in a case where the connection to the power source is not controlled by Tesla, what measures are in place to ensure a fast but safe charge? Of concern is the safety of the circuit delivering the current, which includes the structure’s wiring supplying the power, the outlet, the charging cord itself and the attached adapter.

A primary difference between Supercharging and Destination Charging versus charging with the MC is the variability of the outlet that the MC gets plugged into. With Superchargers, a specialized Tesla team facilitates connecting each Supercharger to grid power and that’s it. It’s a permanent connection. And once an electrician connects a Destination Charger or Wall Connector to a power source that’s usually a permanent fixture as well.

On the other hand, the business end of a Mobile Connector is often being plugged and unplugged, plugged and unplugged, to potentially a wide variety of outlet types and their associated voltages and current carrying capacities. We could characterize the MC as being promiscuous, and sometimes we’re not sure which side of the tracks the receptacles it mates to come from. The outlet is the weak link in a charging scenario that Tesla has far less control over. A Tesla car can potentially draw far more current than an outlet may be rated for. So when the company redesigned the charging cord for the release of the Model 3 they kicked it up a notch by incorporating a method to monitor the health of unknown receptacles.

Much of the redesign centers around the charging cord adapters that Tesla supplies with its Mobile Connector, so let’s make sure we understand these devices. Every Mobile Connector kit comes with one or two adapters that allow plugging the charging cable into two different types of outlets; a 120 volt 15 amp household outlet also known as NEMA 5-15, and a 240 volt 50 amp outlet also known as NEMA 14-50. (For the purposes of this conversation, we will ignore the J1772 adapter.) To emit a proper propeller-head persona you should know that NEMA stands for National Electrical Manufacturers Association. That’s simply a group of US based electrical equipment manufacturers that have banded together in order to establish uniform standards for safety and compatibility, including AC plugs and receptacles.

NEMA nomenclature such as 5-15 and 14-50 is a bit arbitrary, but the second number always refers to the current carrying capacity of the plug/receptacle. Tesla includes these adapters simply because they fit two of the most common outlets in the US. 120V/15A household outlets are ubiquitous, and 240V/50A outlets are seen everywhere from the plug used by an electric kitchen range to RV campgrounds on the road. Both Tesla and third-party companies manufacture adapters for many other NEMA outlet types, but at a price. We’ll get to those in a bit.

Note: While you have your beanies on let’s further explain that devices drawing power continuously for extended lengths of time, like electric vehicles do, are only allowed to utilize 80% of an outlet’s rated amperage. For example, a 5-15 outlet — which is rated at 15A — should only be utilized by an EV to draw a maximum of 12A. Conversely, a 50A NEMA 14-50 outlet should only be allowed to draw 40 amps of current continuously. This is a safety rule that comes to us from the NEC, or National Electrical Code. The Tesla Mobile Connector knows about this rule and charges the car within NEC limits. Read further for exceptions, and how to avoid them.

With that primer out of the way, we want to explain how the car knows which adapter has been connected to the Mobile Connector. It is the adapter itself that informs circuitry embedded in the MC as to the outlet’s capacity, and the MC in turn signals the car as to how much current is safe to draw. However, the charging cable and adapters that came with all pre-Model 3 Teslas, compared to the updated Mobile Connector that comes with all Model 3s — and since January 2018, with all Tesla cars — are completely different animals. Tesla has evolved its charging cables and adapters … for the better.

GEN 1 Universal Mobile Connector

The legacy Universal Mobile Connector (UMC) that came with the Model S and X up until 2018, now retroactively dubbed the Gen 1 UMC, employed adapters that had tiny resistors embedded inside. A resistor is a simple electronic component that causes the voltage in a circuit to be lowered (i.e. voltage drop). Depending on the rating, or value of the resistor, the voltage drop will vary. Each type of Tesla adapter (5-15, 14-50, etc.) has a resistor with a different value.

The Gen 1 UMC electronics have a lookup table that match the voltage drop it reads from the adapter to the exact adapter type. Once the adapter type is determined (and thus the outlet type), the UMC then signals the car to limit the amount of current it attempts to draw based on the current rating of the outlet. It’s a rudimentary but effective system. (By the way, the resistor is a bypass circuit and does not reduce the voltage or power to the car.)

GEN 2 Universal Mobile Connector

When Tesla introduced the Model 3, along with the new car came a new-generation charging cable. Tesla actually dropped the “U” from “UMC,” referring to the updated charge cord simply as the Mobile Connector, or MC (although the term UMC certainly remains in the lexicon). This new charging cable, dubbed the Gen 2 MC, incorporates several design changes.

First, although the Gen 1 cables are capable of carrying up to 40 amps of charging current, the Gen 2 cable maxes out at 32 amps. There was no official word from Tesla as to why the capacity was reduced, however the Gen 1 charge cords were problematic at times, requiring Tesla to replace many of them due to overheating. Tesla therefore may have been playing it safe in limiting the Gen 2 cables to 32A.

Secondly, the lone resistor has been eliminated in the Gen 2 design, having been replaced by a full blown circuit board. Tesla wanted a bit more data from the Mobile Connector and it took more than a humble resistor to provide it

The tiny circuit board lives in the very center of the adapter and contains a proprietary chip (perhaps an ASIC or SoC?), memory, a temperature sensor, and supporting components. Using a circuit board makes the adapter more extensible, and Tesla makes use of the board’s capabilities by employing it to measure the temperature of the adapter’s plug, allowing it to reduce the charging current if the adapter gets too hot. This feature was the key to make charging with the Mobile Connector as safe as possible.

Something I’ve learned from both personal experience as well as when doing the research for the Model 3 Home Charging Guide is that the weak point in electrical circuits are the connecting points. If there is a poor connection at the outlet (plug not fully inserted, weak springs on receptacle contacts, burned or pitted prongs, etc.) the same amount of current (or more) will try to pass through less surface area, thereby creating excessive heat.

So it was clever of Tesla to add this feature. The sensor is strategically located to determine if the plug or outlet is getting too hot due to heat being conducted from the outlet to the charging adapter. It’s pretty cool that even though Tesla doesn’t control the end-to-end circuit when using the Mobile Connector, it can now account for the possibility of a wall outlet not being up to snuff. The MC can even cut back on the amount of current the car draws until the temperature of the plug/receptacle normalize. There’s some nice first principles thinking at work there!

Another perk of using an intelligent circuit in the adapter is that each adapter is serialized. A unique ID is stored in the board’s memory chip, presumably so that if any problems arise Tesla can trace the lineage of the adapter. The memory also stores the adapter type, which dictates the receptacle’s maximum current capability, mimicking the function of the resistor employed in the Gen 1 UMCs. The memory is common EEPROM (pronounced double-ē-prom) and stands for Electrically Erasable Read Only Memory. Quite simply, EEPROM stores information that doesn’t change very often.

Tip: Avoid placing strong magnetic fields near Gen 2 adapters, as that could possibly de-program the EEPROM. If that were to happen the red “T” logo on the MC would blink and the car would not charge. The adapter would then have to be replaced.

Fun With Dick and Jane

So let’s see how this all plays out by illustrating a sample charging session with a Gen 2 Mobile Connector using a Gen 2 NEMA 14-50 adapter. Lets’ be a fly on the wall in Dick and Jane’s garage as they arrive home in their new Model 3:

Jane: As she steps out of the driver’s seat, she turns to Dick and says, “Honey, be sure to plug in the car. I have to check on the kids.”

Dick: “Sure honey. I love plugging in our hip, state-of-the-art, pollution-free automobile.” He grabs the Mobile Connector from the trunk, affixes the 14-50 adapter to the end, and plugs the assembly into the NEMA 14-50 outlet in their garage.

As soon as the MC is plugged into the outlet, the electronics in the MC energize. An AC to DC rectifier circuit in the MC supplies the DC required to run the electronics in both the MC and the adapter. The MC then pings the adapter for authentication. The adapter’s circuit board responds with a digitally-encoded message consisting of its unique identifier, and that the outlet is capable of handling up to 40 amps of current (per the 80% rule).

Dick then plugs the Mobile Connector into the car’s charging port, the MC informs the car to draw up to 32 amps (the maximum current the cord is designed to handle), the lights on the cord turn green, and charging commences.

Dick: “Okay, honey. The car is happily charging and will be fully charged long before you need it for work tomorrow.”

Jane: “I love you honey.”

Dick: “I love you.”

And of course Dick and Jane lived happily ever after (even though it’s pretty clear Jane is hogging the car in this marriage).

Now of course this would never happen to a storybook couple like Dick and Jane, but if the outlet or adapter were to overheat for any reason, the adapter would alert the MC of the temperature increase and the MC would tell the car to slow the current draw. The chip in the MC adapter talks to the MC once per second to ensure a rapid response. As a further safety measure, if communications are ever interrupted (unlikely), the car will automatically reduce the current draw to 8 amps.

In Part II of this post, we will explore what options are available if a Gen 2 outlet adapter is required that Tesla does not supply. As you’ve learned, the Gen 2 adapters are more complex than their predecessors. This presents a significant challenge to third-party suppliers wishing to replicate what Tesla has accomplished for outlet types that aren’t available from Tesla.

The inevitable question is: Will a complete line of outlet adapters be available for the Gen 2 MC? Stay tuned, same Bat-time, same Bat-channel (i.e. tomorrow, here on CleanTechnica). We’ll also cover a simple trick to determine if a third-party Gen 2 adapter is either suitable or problematic for your car.

*The term “charger” strictly defined refers to the circuit that rectifies Alternating Current (AC) from the grid to the Direct Current (DC) that batteries operate on. It is DC current that charges the battery. Usually the onboard charger in the car handles the AC-to-DC function, with the notable exception being superchargers, which deliver more current than a practical onboard charger could possibly handle. In that case the rectifier circuitry is built in to the supercharger itself. All other Tesla “chargers”, like the Tesla Wall Connector, Destination Chargers (and even the Mobile Connector) are technically considered “Electric Vehicle Supply Equipment” or EVSE and shouldn’t technically be called chargers. Ouch. Tell me, have you ever once heard Indiana Jones say EVSE? Even the Acronym is a mouthful.

So, NEW RULE: Instead of being bullied by the Proper Term Police to say, for example, “Destination Electric Vehicle Service Equipment,” or even “Destination EVSE,” just say “Destination Charger.” People will know what you’re talking about.

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Steve Bakker

is a semi-retired teacher, writer, and technologist who is currently passing time by attempting to cure his ignorance as to how electric cars work.

Steve Bakker has 26 posts and counting. See all posts by Steve Bakker