I paid £2.99 at a BP for this (link is to show what I bought).
My batteru was dead literally could not even switch on the phone, came with a cheapo battery as well.
Plugged it in, stopped the wifi and data services to preserve the battery, used it to call and on stand by for a total of 3 hours and the battery was still going.
So for £3 a fooking massive praise for this product