The battery is not fit for purpose (Sale of Goods Act). Tell the seller that you don't care how many he sells, you want your money back.
BTW, under Distance Selling Regulations, if you have have had it for less that seven working days (counting from the day after it was delivered) you can demand a refund without giving a reason. Traders on Ebay (he is a trader because he sells so many) MUST obey the law.
t, August 2008