Charging Protocols

Charging an EV battery is not like filling a gas tank. You cannot just pour energy in at a constant rate. The battery's chemistry, temperature, and state of charge all dictate how fast it can accept energy at any given moment. Understanding charging protocols helps you diagnose charging complaints and educate customers.

Level 1 uses the portable charge cord that comes with the vehicle, plugged into a standard 120V household outlet. It delivers about 1.2 to 1.4 kW — enough for 3 to 5 miles of range per hour of charging. A full battery charge from empty takes 40 to 60 hours on Level 1 for most EVs. This is practical for plug-in hybrids with small batteries and for overnight top-offs, but it is not a realistic daily charging solution for a full EV that is driven heavily. No special installation required — but the outlet should be on a dedicated 20-amp circuit.

Level 2 uses a dedicated 240-volt circuit — the same voltage as a clothes dryer or electric range. A wall-mounted EVSE (Electric Vehicle Supply Equipment) communicates with the vehicle and supplies AC power at 16 to 80 amps depending on the unit and circuit capacity. Most home Level 2 chargers deliver 7 to 11 kW, providing 25 to 35 miles of range per hour. A full charge from empty takes 6 to 12 hours. This is the standard home charging setup and the most common public charging station. Installation requires an electrician to run a dedicated 240V circuit to the charging location.

DC fast charging stations convert AC grid power to DC and deliver it directly to the battery, bypassing the vehicle's onboard charger entirely. Charging rates range from 50 kW on older stations to 350 kW on the fastest current hardware. A 150 kW session can add 100 to 200 miles of range in 20 to 30 minutes. Connector types: CCS uses the J1772 plug with two DC pins added below. CHAdeMO is a separate round connector used primarily on older Nissan Leafs. NACS (Tesla connector) is becoming the North American standard. The vehicle and station negotiate the maximum power level based on the vehicle's battery capacity, temperature, and state of charge.

Here is the most important concept for understanding EV charging speed. Think of filling a glass of water. When the glass is empty, you can pour fast. As it gets full, you have to slow down or it overflows. Batteries work the same way. At a low state of charge — say 10 to 20 percent — the battery accepts maximum charging power. As the state of charge increases, the BMS gradually reduces charging speed to protect the cells from overcharging. By the time the battery reaches 80 percent, charging has slowed significantly. From 80 to 100 percent, charging is very slow. This is why most fast-charge recommendations say charge to 80 percent — the last 20 percent takes almost as long as the first 80 percent. This is not a fault. It is the chemistry.

Battery temperature — cold batteries charge slowly, hot batteries get throttled. State of charge — speed decreases as the battery fills. Station capacity — a 50 kW station cannot charge faster than 50 kW regardless of what the vehicle can accept. Onboard charger capacity — for Level 2, the onboard charger limits the rate. Battery health — a degraded pack may accept less current. When a customer complains about slow charging, check all of these factors before looking for a fault. Most slow-charging complaints are normal system behavior, not a malfunction.