thanks for all the replies, well the manual says if you choose the recon option to remove the battery from car before using this due to the 15.8v possibly being a problem to the electronics, however looking at their charge process table step 1 desulphation shows as passing up to 15.8v through either way....

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E53 X5 3.0d SUV prod.date 11/2004 eng.variant M57N Europe Right hand drive N