Emergency lights are designed to provide illumination during power outages. Whether they need to be plugged in all the time depends on their type and the intended use.

　　For traditional lead - acid or Ni - Cd/Ni - MH battery - based emergency lights, it is generally recommended to keep them plugged in continuously. These batteries have a relatively high self - discharge rate. For example, a Ni - Cd battery may self - discharge at a rate of about 10 - 20% per month when not in use. By keeping the emergency light plugged in, the battery is constantly trickle - charged, which helps to maintain its state of charge and ensures that it is ready to function immediately during a power outage. However, continuous charging also requires proper charging circuitry to prevent overcharging. Most modern emergency lights are equipped with intelligent chargers that can regulate the charging current and voltage to avoid overcharging the battery.

　　In the case of lithium - ion - powered emergency lights, the situation is a bit different. Lithium - ion batteries have a lower self - discharge rate compared to Ni - Cd and Ni - MH batteries, typically around 5 - 10% per month. While it is still common to keep them plugged in for continuous readiness, overcharging can be more detrimental to Li - ion batteries. If an emergency light has a well - designed battery management system (BMS), it can be left plugged in. The BMS will cut off the charging when the battery is full and prevent overcharging. But if the BMS is faulty or not present, continuous charging can lead to overheating, reduced battery lifespan, and potential safety hazards.

　　In some applications where the emergency light is used in an area with unstable power supply or where there are concerns about electrical surges, it may be advisable to unplug the emergency light during periods of normal power supply and only plug it in when a power outage is expected. This can protect the emergency light from damage due to power fluctuations. However, this approach requires careful monitoring to ensure that the emergency light is charged and ready when needed.