When your BBC micro:bit is connected to your computer with the micro USB, it doesn't need another power source.
The micro:bit can also be powered using 2 x zinc or alkaline AAA batteries. To insert the battery connector, pinch together the two wires at the base of the connector and bush into the JST socket on the board. To remove, pinch the wires and wiggle the connector to release.
- Do not try to charge normal (non-rechargeable) batteries
- Please do not mix different types of batteries or mix new and used batteries
- Please use batteries of the same or equivalent type as those reccomended
- Please insert the batteries the correct way round (with the correct polarity)
- Please remove spent batteries form the battery holder
- Do not short-circuit the battery supply terminals, for example by placing a metal object across the terminals
- Only use Zinc or Alkaline batteries with your BBC micro:bit
- Please do not use rechargeable batteries unless you have confirmed the voltage output, as some rechargeable batteries can output a voltage that exceeds the operating range of the micro:bit.
- If you find it difficult to remove the battery pack, please do not force it. It may that you are using a third party battery case like https://www.sparkfun.com/products/14299 with a connector that isn’t designed to be removed. If this is the case, please leave this connected if possible and use alternative battery packs with removable connectors in future.
The micro:bit will automatically switch to USB power when it detects a connection and to battery power if the USB connection is removed. It does this by using a low-Vf diode to switch between sources. The diode prevents back-powering of any source from any other source, so you may have the two power sources connected at the same time.
If you are using your micro:bit with a tablet or mobile, you connect to it with Bluetooth and will also need batteries to power it.
Please do not plug your micro:bit directly into the mains or into a portable charger. They supply a voltage stronger than the micro:bit can safely handle.
You can read about the power supply on the tech site