I didn't buy a relay off ebay , I used a proper professional local car Auto sales place in wakefield .
Was about £40 , but a professional one used in caravans, converted car homes etc .
Intelligent relay .
Also the battery I got is just a heavy duty vehicle battery that is designed for this multiple charging .
Superb so far , Its nice not having to plug into the mains to re-charge .
May have problems if we suffer the -10 for 2 weeks