If you use the new batteries to open the boot you stand the chance of damaging them, the totally flat batteries on the fatty will pull down the power from the new batteries and may not open the boot...

As Kot says, use another car with the engine running, you may find you have to leave it connected for a while to bring up the volts...

 Pete
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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand Sold
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Sold
2002 L322 Vogue 4.4 V8 Epson Green (Fatty Offroader) Sold
-Click for Project Fatty off roader-