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I want to attach a leisure battery to my Scheiber BIP
Scheiber's diagram shows tabs 14 and 15 as battery tabs. There already two wires there! Any suggestions?

Hi Trevor, welcome to the forum.
What is the part number of your BIP?
Mine is 30.65100.84 code 0470146 and it is coupled with a Schaudt Booster WA 1214-8 to charge the leisure battery when connect to the towing vehicle.

I do not have terminals 14 and 15 on my BIP, the battery is charged when on electric hook up via terminal 12 for positive and the earth block for the negative.

I have another wiring diagram for a 30.65100.00 which has terminal 14 for the positive input to the fridge and terminal 15 is the positive of the "motor" battery. It is also marked up as terminal 12 for the battery positive, protected by fuse 6 (15A). I'm guessing that's your one.