Positive to positive. You would be amazed how many times that gets missed. Correct sequence is the positive cable on the flat battery first, then both cables on your good battery (ensuring the negative cable at the other end isn't contacting anything) then attach the last negative clamp to the other vehicle, not at the battery. Any unpainted metal surface under the hood will work. That ensures that any sparks aren't around the battery itself. Only use the negative terminal on the flat battery as a last resort. Don't connect any cables if the flat battery is leaking or cracked. Don't lean over a battery while you connect the cables.

If you don't have a good connection, ensure everything is turned off and then fix the connection. Don't be moving the cables while the lights are on, or someone is trying to start it.

To remove the cables, reverse the order above. Negative connection at flat battery, both of the connections on the good battery, then positive connection on flat battery.

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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