Why Charging current is getting limited and is it due to Battery BMS or Vehicle VCU in ev vehicles?

Limitations of Charging Current from Fast Charger

In electric vehicles (EVs), both the Battery Management System (BMS) and the Vehicle Control Unit (VCU) play critical roles in regulating charging current, but from different perspectives and with distinct responsibilities.

Battery Management System (BMS) Strategy:

The BMS is responsible for protecting and optimising the battery pack. It limits charging current based on:

Cell voltage and temperature: Prevents overcharging and thermal runaway.

State of Charge (SOC): Ensures cells are charged uniformly and safely.

State of Health (SOH): Adjusts charging parameters based on battery ageing.

Fault detection: If any cell anomaly is detected, BMS can restrict or halt charging.

Cell voltage imbalance: If nearing maximum voltage thresholds.

Vehicle Control Unit (VCU) Strategy

The VCU is the central brain of the EV, coordinating multiple subsystems including the BMS, motor controller, and onboard charger. It limits charging current based on:

Power availability from the grid or charger: Adjusts current to match source capacity.

Thermal management: Coordinates with cooling systems to allow safe and fast charging.

Driving mode and user settings: May prioritise slower charging as per battery state.

Communication with charging station: Utilises protocols such as ISO 15118 to negotiate the optimal current.

Other Common Power Limitations while Charging EV Vehicles:

Charger Limitations: The charger itself is limited to a maximum supply limited kW.

Load management: Shared Power in the charger itself in case of a Fast Charger.

Input Power Limitations from Grid: AC 1-phase vs. AC 3-phase supplied from the grid to the charger. Even unstable grid conditions like voltage drop can cause the charging current to reduce.