Hi! We don't recommend using external batteries for the Hub because of the high current it can sometimes require while using certain actions. We recommend a source that supplies at least 1 amp of current.
This said, you could try using an external battery such as one used for a mobile phone, but if you're using a lot of actions at once, the hub could turn off.
Also your external battery might run out quite fast if it's a small one.
Hope this helps!