It sounds from the description that it cranks but won't catch. If so, that is unlikely to be a battery problem. If it was a battery problem it wouldn't crank

Also not a good idea to fiddle with jumper cables while they are connected and the jumper vehicle is running. You can send a voltage spike to the vehicle being jumped and electronic modules can be damaged that way

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue