The appropriate charging time for a rechargeable emergency light depends on multiple factors such as the battery type, capacity, and the charging system.

　　As mentioned earlier, for Ni - Cd and Ni - MH batteries, the charging time is relatively long. A Ni - Cd battery with a capacity of 4Ah and a charging current of 0.4A would take approximately 10 hours to charge based on the theoretical formula. But considering real - world factors like battery self - discharge and charging inefficiencies, the actual charging time is usually around 12 - 14 hours. Ni - MH batteries, which have a higher energy density than Ni - Cd batteries, may have a slightly different charging time. For example, a 6V, 5Ah Ni - MH battery charged with a 0.5A charger might take around 10 - 12 hours, but again, the actual time can vary.

　　Lithium - ion batteries, due to their different charging characteristics, have a shorter charging time. A typical Li - ion battery in an emergency light, say with a capacity of 3.7V and 2Ah, when charged with a proper CC - CV charger, may take around 4 - 6 hours to fully charge. The constant - current phase, where the battery is charged at a relatively high current until it reaches its full - charge voltage, usually takes a significant portion of the total charging time. Then, the constant - voltage phase kicks in to top - off the battery without overcharging.

　　It is essential to note that using a charger that is not compatible with the emergency light's battery can lead to incorrect charging times. Using a charger with too high a current can cause overheating and damage the battery, while a charger with too low a current will result in long charging times and may not fully charge the battery. Always refer to the manufacturer's instructions regarding the appropriate charger and charging time for the specific emergency light model.