EV Charging Standards: CCS, ISO 15118 & Plug and Charge

Introduction

Most EV charging explainers stop at the connector. The parts that decide whether a vehicle actually charges live one layer down: the Control Pilot signal, the proximity resistor in the cable, and the communication sequence between vehicle and charger. This article works through that layer, from AC charging up to ISO 15118-20, with notes on where implementations break in the field.

What a charging standard covers

A charging standard is not one document and not just the plug. It defines how a vehicle and a charging station agree to move energy safely, spanning the physical and signaling layer up through the communication protocol. The connector geometry is defined separately in IEC 62196 and only referenced, which is why the same protocol can run on mechanically different connectors.

Two worlds sit underneath, but they share the same Control Pilot: a 1 kHz PWM signal plus a set of voltage states. In AC charging the PWM duty cycle encodes the available current, and no digital communication is needed. In DC (ISO 15118) the PWM is fixed at 5% to flag DC mode, while the charge values are exchanged digitally over Power Line Communication (PLC).

Meet the connector

Before the signals, the hardware. In Europe the standard inlet is CCS2, Combined Charging System 2: a Type 2 AC section (three phases L1/L2/L3, neutral N, protective earth PE, and the two signal pins CP and PP) with two DC pins added below for fast charging. The two signal pins do the talking: the Control Pilot and Proximity Pilot are where everything that follows happens.

On CCS2, the connector is locked electrically once it is seated, and energy only flows after the lock is confirmed. Through the whole session the system continuously monitors the lock, the isolation, the voltage, the current and the contact temperature, and stops if any of them drifts out of range. CCS1 uses a mechanical latch and the release button instead of an electric actuator.

A DC connector's current limit is thermal: it is set by how hot the contacts get. The standard puts a number on it: IEC 62196 limits the terminal temperature rise to 50 K above ambient (around 90°C at the contacts in practice). To hold that, CCS inlets carry temperature sensors right at the DC power contacts, typically a Pt1000, and the station reads them and eases the current back as the contacts warm, derating or stopping before the limit. The contacts are where the temperature is measured and where the limit sits. To push more current, the highest-power stations liquid-cool the cable, circulating coolant through it to carry heat off the conductors, so a cable light enough to handle can still pass hundreds of amps.

Connector variants worldwide

CCS2 (Europe) supports three-phase AC; CCS1 (North America) is single-phase with higher current and a mechanical release button. The protocol is essentially the same across both; the differences are mechanical and a different PP value. CHAdeMO is the Japanese DC standard, now largely faded from European passenger EVs.

Scope note: renco hands-on experience centres on CCS, DIN 70121 and ISO 15118. NACS (SAE J3400), the Tesla-derived connector spreading across North America, and GB/T (China) are noted here for completeness rather than as areas of deep field experience.

From the field: the first Tesla Model S reached Europe with a CCS1 inlet and shipped with an adapter; the Model 3 was the first with native CCS2. Early Teslas carried both CAN (for Superchargers) and CCS/PLC. Older Superchargers still use a short ~1 m cable designed for the Tesla left-side inlet, which can force a right-side-inlet car to block two bays.

AC charging: the Control Pilot and PWM

Control Pilot states

Before any current is discussed, the Control Pilot establishes the connection state. The CP oscillator swings ±12 V; a resistor network in the vehicle inlet pulls the voltage down in steps to signal state. The vehicle sets the state voltage; the station sets the duty cycle.

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The Proximity Pilot codes the cable limit

Before the station can advertise a charging current, it has to know what the cable can carry. A proximity resistor (PP) in the Type 2 connector encodes the cable's maximum current, since neither vehicle nor station knows a detachable cable's cross-section. The lowest limit in the chain always wins: a 13 A cable on a 16 A station limits to 13 A.

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PWM sets the available current

With that limit known, the Control Pilot carries its second message. It is a fixed 1 kHz square wave, and the station varies its duty cycle to advertise the maximum current the vehicle may draw, never more than the cable allows. IEC 61851-1 defines the mapping as follows : 10 microseconds of the 1000 microsecond period corresponds to 0.6 A. The vehicle then decides how much of that to use.

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DC charging and ISO 15118

What separates the generations is not the basic charge but the features layered on top. Smart charging lets the station and vehicle agree a charging schedule, when and how fast, not just a current limit. Plug & Charge authenticates and bills the car automatically from a certificate. Bidirectional power transfer (V2G) lets the battery feed a home or the grid back. Each standard below adds more of these.

For DC the picture spans five documents. They build on one another rather than replace each other, so a charger may speak several and falls back to the simplest one both sides share, from the basic DIN 70121 up to the full ISO 15118-20.

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Alongside these, the family splits the physical layer by medium: ISO 15118-3 covers wired power-line communication, ISO 15118-8 covers wireless, and ISO 15118-10 defines the single-pair Ethernet (10BASE-T1S) link that the Megawatt Charging System uses. Separate parts (-4, -21, -23) hold the conformance tests.

Signal Level Attenuation Characterization (SLAC)

When you plug in, several vehicles and chargers can share the same stretch of powerline, so the car first has to work out which charger it is actually connected to. The station signals it is ready by putting the 5% duty cycle on the Control Pilot, and the vehicle starts SLAC, the matching process defined in ISO 15118-3 over HomePlug Green PHY, the low-power powerline-communication layer (a cut-down HomePlug AV) that CCS uses to turn the Control Pilot wire into a small network.

The vehicle sends a short burst of sounding packets (about ten). Every charger within earshot on the powerline measures how strongly those signals arrive and averages the attenuation. The vehicle then picks the charger with the lowest attenuation, which is the one its own cable is physically plugged into rather than a neighbour on the same line, and sends a match request. That charger replies with a network key, so the two form a private powerline network and run the encrypted ISO 15118 exchange over it. If nothing matches, charging never starts.

DC charging sequence

With SLAC done and a private link in place, the real negotiation begins. This is the high-level message sequence of ISO 15118, run over the powerline before any power flows: the two sides agree a protocol, exchange the battery's limits, authorize payment, test the cable's isolation, match voltages, and only then transfer energy. The vehicle is the master and drives every step; the station answers and may limit or stop, but never advances the sequence itself. In the exchange below, blue is what the EV asks for and grey is what the station replies. Most DC stations also log the whole session as an ASCII file over an internal Ethernet port, which is often the fastest way to see where a charge stopped.

Smart charging and charging schedules

ISO 15118 does more than start and stop a charge: it lets the station and vehicle agree when and how fast to charge. After the charge parameters are exchanged, the station sends one or more charging schedules, the power it will allow over the coming period (up to 24 hours), and the vehicle selects a profile and follows it. That is what makes off-peak charging, site load management and grid-friendly charging possible with no action from the driver. ISO 15118-2 introduced this scheduled mode; ISO 15118-20 adds a dynamic mode, where the station can keep updating the schedule during the session, and extends bidirectional power transfer to better support distributed energy resources (DER).

Plug and Charge

Plug & Charge removes manual authorization: the vehicle and station authenticate and bill automatically. Payment credentials are not stored on the car: a contract provider holds them on an external server and the car carries only a certificate (a reference, not a card number), which the station validates against that server. It therefore needs connectivity on both sides.

Public Key Infrastructure (PKI)

Behind that certificate sits a Public Key Infrastructure (PKI): certificates must be issued, renewed and, when compromised, revoked, across contract providers, vehicles and stations. That lifecycle, not the in-car logic, is the harder part of Plug and Charge to operate, and a large part of why it is not yet everywhere.

Adoption and TLS gap

Adoption is still thin. A 2024 security study of 325 public DC chargers across four European countries found that only about 12% implement TLS at all, the encryption Plug and Charge depends on, even though support for ISO 15118-2 had climbed to roughly 70 percent of chargers built in 2023. Many still run decade-old modem firmware. So a vehicle can be fully Plug-and-Charge capable and still meet a station that cannot do it securely.

Universal Plug and Charge

‍There is also no single certificate ecosystem yet. Different contract and operator PKIs do not automatically trust each other, so the industry is working toward a universal Plug and Charge scheme that lets any vehicle authenticate at any station without per-network registration. Until that lands, cross-network Plug and Charge stays patchy.

Autocharge

A lighter-weight alternative, Autocharge, skips the certificates: the operator registers the vehicle's identifier, usually its MAC address read over PLC at plug-in, and starts the session automatically. It is quick to deploy, but less secure: the MAC address is not cryptographically signed, so it can be spoofed, and it is not interoperable across networks. Plug & Charge (ISO 15118) instead gives each vehicle a signed digital certificate that the station verifies, which is harder to fake but needs more infrastructure to run.

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Beyond passenger cars: MCS, wireless and BPT

For the charging protocol, trucks, buses, passenger cars and off-road/NRMM use the same CCS/ISO standard; only the power levels differ. Charge stations are not built specifically for off-road vehicles, so when it is CCS2, it is genuinely CCS2. Implementing it is not mandatory but is the cheapest, most available path; DC station hardware has fallen from tens of thousands of euros to under about 10k, and the grid connection is now often the costliest part.

Megawatt Charging System

MCS, the Megawatt Charging System, is the high-power connector for heavy vehicles such as trucks. It builds on CCS and is folded into ISO 15118-20, but it changes two things: it replaces power-line communication, the fragile link in CCS DC charging, with single-pair Ethernet (10BASE-T1S, defined in ISO 15118-10), and it uses a much larger connector. The ratings step up accordingly. Where CCS tops out around 350 to 400 kW, MCS is specified up to 1250 V and 3000 A, a ceiling of about 3.75 MW, with the chargers deploying now delivering roughly 1 to 1.6 MW. That is about ten times the CCS charging speed: a heavy truck that would need hours on CCS can go from 20 to 80 percent in 30 to 40 minutes, within a driver's mandatory 45 minute rest. The connector specification, SAE J3271, was published in March 2025.

Wireless Inductive Charging

ISO 15118-20 also covers wireless inductive charging. It suits identical vehicles on fixed routes (buses and depot trucks) using a ground plate, a vehicle pad, antennas for alignment, and a handful of CAN messages equivalent to the CCS start/stop/target-current handling. The standardised inductive power classes are defined in IEC 61980, from about 3.7 kW up to 11 kW at around 90 percent grid-to-battery efficiency, well below conductive fast charging; a 22 kW class is still in development, and higher figures sometimes quoted in the press come from experimental systems.

Bidirectional Power Transfer

BPT lets a vehicle discharge to a home or the grid, defined in ISO 15118-20, and it is where standardization currently bites.

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From the field: a controller can implement BPT and announce it, but the version identifier (major/minor, e.g. 3.0) is the same as the earlier build without BPT. The station cannot tell which it is talking to, so it falls back and BPT does not engage. The documentation describes how it should work, but in the field it is effectively unusable: validated on the bench, not proven with real stations.

Where implementations break

There is no single failure that dominates. Interoperability has improved as CCS consolidated into one dominant standard, so the days of integrating several incompatible systems are largely behind us. What remains is a handful of failure modes that recur, on the bench and in the field, and locating them is where most of the real work goes.

PLC is the fragile link.

‍The high-level communication rides on power-line communication, which is sensitive to its electrical environment. The signal level has to be aligned on both the vehicle and the station: too high and it distorts, too low and not every bit is readable. Because PLC sends each message frame by frame, a single failed checksum forces a retransmit, and enough retries lose the message and drop the session. Getting that alignment right is not quick. We have seen controller programmes spend a long time on it, with unexplained stops along the way, before charging became reliable. The standard is catching up here: ISO 15118-3 Edition 2 is adding explicit limits for maximum cable attenuation and minimum PLC receiver sensitivity, the very parameters whose absence made alignment such a moving target.

Same connector, incompatible software.

‍A matching plug does not guarantee a matching protocol. We have seen the same connector carry two protocol versions that will not talk to each other, and station updates that fix one vehicle while locking out an older one. ISO 15118-20 adds its own trap: a bidirectional-capable build can carry the same major and minor version identifier as the earlier build without it, so the station cannot tell them apart and quietly falls back. Features implemented ahead of the final standard are another source of "works on the bench, rejected in the field".

Stations fail silently.

A station can show as available, in the operator app and on the connector, and still not charge. Communication starts, the sequence runs, and after a minute or two it errors out with nothing to explain why, or the Control Pilot is simply absent and the unit is out of order. We have stood at sites where one station charged normally and the one beside it, reporting the same healthy status, never completed a session. From the outside there is no way to tell; the fault is internal to the station.

Stops with no error code.

The hardest cases stop charging with no DTC and no message on either side, often overnight, with the vehicle and the station each pointing at the other. The only reliable way to localise it is comparison: run the same station with known-good vehicles and see whether the fault follows the suspect unit. When it does, that evidence is what ends the finger-pointing, which can otherwise drag on for a year or more between suppliers who each insist the problem is not theirs.

Read the log.

Nearly every DC station keeps an ASCII session log reachable over an internal Ethernet port. Pulling that log is often the fastest way to see how far the sequence got and where it stopped, and it is usually the first thing we do when a session fails.

These are exactly the failures our charging toolchain is built to reproduce and isolate before a vehicle ever reaches a public station.

Implications for the VCU / BMS

These failure modes are not abstract: they decide what the vehicle side actually has to get right. To be compliant, the vehicle side needs PLC capability in the controller (e.g. the New Eagle OpenECU M580 and NX3 platforms provide it) and the full negotiation sequence implemented end-to-end, with the vehicle acting as master. Charge power is negotiated as in the V2G sequence above: parameters are exchanged over ISO 15118 and aligned with the station before current flows.

Charging development and validation at renco

Building that stack, and proving it against the failure modes above before it reaches a vehicle, is what renco's charging toolchain is for:

• RENCharge VCU software stack: renco's production charging stack for the VCU, handling CP/PP signalling and the ISO 15118 sequence.
• CCS listener: a passive analyzer that taps the CP/PLC line and records the full V2G exchange, for diagnosing charging faults.
• CCS Sim box: a bench simulator that emulates the EV or the EVSE side, to test either end without a real vehicle or station.
• HIL smart-charging validation: hardware-in-the-loop testing of charging, including smart charging, against simulated stations before vehicle power-up.