Most electronic cigarette batteries will come with an LED light feature, whether located on the actual battery button or a separate location on the battery. The purpose of the LED light is to alrt you to several issues:

On/Off LED Function:
Most batteries include an off and on feature. This can be activated using the e cig button. If you press the battery button and no light shows and the battery does not produce any power it may be turned off. To turn the battery on simply depress the battery button 3 times quickly (or 5 times for some versions), this will turn the battery on and also will turn the battery off is performed when the battery is on.

Recharge Required LED Function:
If the battery needs charging the LED light will blink in quick succession when you press on the battery. This means there is no charge left and you need to re-charge it.

Faulty Battery / Clearomizer Tank LED Function:
Sometimes if the battery has a fault the LED light will blink in quick succession every time you use it even though it is fully charged. It will also act in the same way if a faulty clearomizer tank is used.