Time for a new iPhone 4 battery, but which one?

my iphone 4 approx. 18 months old, , battery has started deteriorate. it'll barely make 12 hours full empty. i've seen few guides , looks easy replace, i'm struggling decide battery buy.

range in price £4 £10 on ebay. claim genuine, many don't, , photos same!

question how know if 1 genuine, , matter?

 

you know it's genuine if apple replaces battery.

 

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