• AC charging
• DC charging
• The battery
• The handshake
• Why sessions fail
• Payments, software, hardware

AC charging: the charger is in the car

A Level 2 "charger" is not a charger. It is a switch with opinions. It delivers grid AC to the vehicle and the vehicle's onboard charger rectifies that AC into DC for the battery. The onboard charger is the bottleneck, and it varies by model: 7.7 kW on many cars, 11 kW on some, 19.2 kW on a few. Two cars on the same 11 kW unit will charge at different speeds and neither one is broken.

The wall unit and the car talk over the control pilot, a 1 kHz square wave on a dedicated pin. The duty cycle of that wave tells the car how much current the circuit can supply. The car answers by pulling the pilot voltage down through a resistor to say it is connected and ready. Only then does the relay close. A separate proximity pin tells the car a plug is in the socket, so it won't drive off with the cable attached.

Practical consequence: Level 2 hardware is cheap and simple, and almost all of the cost of a Level 2 project is in the concrete and the copper.

DC charging: the charger does the work

A DC fast charger skips the car's onboard charger entirely. The cabinet takes AC from the grid, rectifies it into DC with a stack of power modules (commonly 15 to 75 kW each), and feeds that DC straight to the battery pack through the contactors. More modules, more power, more money, more heat, more things to fail.

Because the modules are shared, a four-dispenser site may not deliver full power to four cars at once. Power sharing is a feature, not a defect, but it is also why "four 150 kW stalls" sometimes means "600 kW split four ways when busy." Ask how power is allocated before you sign.

Communication is where it gets interesting. CCS and NACS/J3400 use HomePlug Green PHY power line communication over the pilot pins, running DIN 70121 or ISO 15118. CHAdeMO uses CAN. ISO 15118 is what enables Plug and Charge, where the car presents a certificate and authorizes itself with no app or card at all.

Pack voltage matters too. Most EVs run a roughly 400 V pack, newer ones run roughly 800 V. Chargers have a voltage window. A car outside that window either charges slowly through a boost stage or doesn't charge at all.

The battery, in the terms that affect your revenue

A pack is a few thousand cells wired into modules, managed by a battery management system that owns every decision about current, temperature, and safety. Chemistry drives behavior. NMC packs hold more energy per pound and take fast charging well. LFP packs are cheaper and longer lived, tolerate a daily 100% charge, and tend to taper harder at the top and suffer more in the cold.

Charging follows constant current, then constant voltage. The pack takes a high, steady current while there is room, then the current tapers as it fills so cell voltage stays inside safe limits. That taper is why the marketing number is 10% to 80%, and why the last 20% can take as long as the first 60%.

• State of charge is the single biggest driver of session speed. A car arriving at 60% will never see peak power, no matter how big your charger is.
• Temperature is the second. A cold pack accepts very little current until it warms. Cars that precondition on the way to a charger do far better, which is why navigation-triggered preconditioning matters.
• C-rate is power divided by capacity. A 75 kWh pack at 150 kW is charging at 2C, which is demanding. The same 150 kW into a 150 kWh pack is a gentle 1C.
• Degradation comes from heat, time spent at high state of charge, and repeated fast charging at high state of charge. It is real, and it is slower than the internet believes.

The handshake, step by step

A DC session is a negotiation, and every step is a place to fail.

Why sessions fail

Not actually failures

• Taper. The car arrived at 70% and is doing what it should.
• Cold pack. Power climbs as it warms. Nothing to fix.
• Power sharing. Two cars, one power cabinet.
• The car's onboard limit on Level 2. You cannot sell 11 kW to a 7.7 kW car.

Real failures, roughly in order of how often they bite

• Payment authorization. The terminal cannot reach the processor, the pre-auth is declined, or the card is a type your terminal was never certified for.
• Connectivity. The cellular modem sits in a metal box at the edge of a parking lot. Signal there is worse than you assumed on site visit day.
• Communication timeouts between car and charger, often specific to one vehicle model and one firmware version. These are the hardest to diagnose and the most common cause of "it works for everyone but my car."
• Isolation and ground faults, usually water in a connector or a cable that got run over.
• Cables and connectors. The single most abused component on the site. They are consumables. Budget for them.
• Contactor and power module failures, which take the stall down until a technician with parts arrives.
• Firmware updates that fix one vehicle and break another, or that break the payment terminal driver.
• Blocked stalls, whether by a gas car or by a driver who finished an hour ago.

Payments, software, and hardware: where the journey actually breaks

Three vendors usually meet inside one steel cabinet: whoever built the hardware, whoever runs the charge management system (CMS), and whoever processes payments. The driver does not know or care. The driver knows the screen said "authorizing" for ninety seconds and then nothing happened.

Where it breaks

• Any hop that needs the network fails when the network is down, and the network is down more often than the site survey implied. Decide in advance what the charger does offline: refuse sessions, or start them and hope you can bill later. Both answers cost money. Pick one on purpose.
• Pre-auth holds look like charges to drivers. Expect calls about the $75 you did not actually take.
• Terminal certification is tied to a specific processor. Changing processors can mean changing terminals, which can mean a truck roll to every stall.
• Missing meter values turn into billing disputes you will lose.
• Version mismatches between hardware firmware and CMS expectations cause subtle failures that each vendor blames on the other. OCPP 1.6J is common, 2.0.1 is where things are going, and "OCPP compliant" means almost nothing without a documented profile.
• Refunds and chargebacks have to be possible for whoever answers the phone, which is usually the site host who has no access to any of these systems.