cheap spare batteries for festival

Hi . I have galaxy s3 lte. Going to a four day music festival in uk. Need power to phone. Had a look on ebay uk for cheap spare batteries - seen loads for under a fiver (


Welcome to Android Forums, RobRobRobRob!:)

I have never used those "cheap" batteries before so I really can not recommend one, but do be warned that when purchasing them you quite often get what you pay for.
While they seem like a great deal, they can often overheat and damage your phone.
I have read so many accounts of the cheap batteries causing real damage to your phone, so please be careful.