Massive electrical drain

[Carl Blackwell]

Hello. My 2011 3.0 XF just stopped today at a T junction. Electrics totally dead. Tried to jump start the car but the drain through the jump leads was warming the leads. The battery is a yuasa replaced 4 months ago. Fitted the old battery that I had fully charged. When connecting positive terminal I get a good spark. Not a total short but a good drain on battery.  With fully charged battery connected still no electrics apart from indicators flashed. Any ideas how to proceed to determine fault. Could the alternator be faulty and pull such a large current.