Overnight Slow Charging

Overnight slow charging refers to a daily charging routine where electric vehicle (EV) owners leave their cars plugged in overnight using an AC slow charger for an extended period. Typically lasting between 6 to 10 hours, it is the most common use case for home chargers. With the maturity of Battery Management System (BMS) technology, overnight slow charging is widely regarded as a safe and battery-friendly habit under standard operating conditions.

Technical Safeguards

Modern electric vehicles are equipped with a sophisticated Battery Management System (BMS). Once the battery reaches 100%, the BMS automatically cuts off the charging current or shifts to a minimal "trickle charge" mode to compensate solely for self-discharge losses, fundamentally eliminating the risk of overcharging. Dedicated home chargers also feature built-in protection against overvoltage, overcurrent, and short circuits, immediately shutting off power if an anomaly is detected. Throughout the session, the BMS maintains continuous real-time communication with the charger's controller to ensure charging safety.

Safety Assessment

While overnight charging is generally safe, distinctions must be made based on the charging method:

AC Slow Charging (Recommended): AC slow charging operates at lower power outputs (typically 3.3 kW to 7 kW), delivering a steady current with minimal battery temperature rise. Overnight charging has a negligible impact on the cycle life of lithium iron phosphate (LFP) batteries, while the annual capacity degradation rate of ternary lithium batteries remains within the normal threshold of under 2%. As long as certified, standard-compliant charging equipment and original cables are used, regular operation will not cause wiring to overheat or catch fire.

DC Fast Charging (Not Recommended): DC fast chargers deliver high power outputs (≥60 kW), causing battery temperatures to rise quickly to between 40°C and 50°C during charging. Prolonged overnight fast charging places the thermal management system under continuous heavy load, which can reduce battery cycle life by approximately 5% to 10% over time. DC fast charging is best suited for quick top-ups on the go rather than extended overnight charging.

Optimisation Recommendations

To preserve battery health and optimise running costs, EV owners can observe the following best practices:

Set a charging limit: Set the daily charging limit to 90% via the vehicle's companion app. At this level, the battery's State of Charge (SOC) remains in its optimal operating window, striking an ideal balance between daily driving range and long-term battery lifespan.

Utilise scheduled charging: Use the scheduled charging feature on the home charger to set charging times during off-peak electricity tariff hours (e.g. 23:00 to 07:00). This lowers running costs while taking advantage of more stable grid voltages during off-peak periods.

Avoid extreme temperatures: In ambient temperatures below -10°C, preheating the battery before charging is recommended. Conversely, after prolonged exposure to hot sun, allow the battery temperature to drop below 40°C before plugging in to avoid triggering BMS high-temperature protection, which could interrupt charging.

Perform periodic deep-cycle calibration: Carry out a full charge-discharge cycle once a month (slow charging to 100% after dropping below 20%) to help calibrate the BMS's SOC measurement accuracy.

Manage long-term parking: If leaving the vehicle parked for more than two weeks, maintain the battery SOC between 50% and 70%. In this state, self-discharge is minimised, preventing the battery from entering a damaging deep-discharge state.

Safety Precautions

The following safety details should also be noted during overnight slow charging: Inspect the charging port for moisture or debris before plugging in, and prevent rainwater from entering the socket during connection and disconnection; avoid using informal extension leads or trailing cables, as they pose serious electric shock and fire hazards; try to avoid staying inside the vehicle for long periods or running high-power appliances (such as the air-conditioning system) while charging—while not strictly prohibited, doing so increases the parasitic load on the battery.