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Join Date: Feb 2009
Location: Aylesbury, UK
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12 ft is quite long. The thicker and shorter the better otherwise the volt drop will be significant. Minimise the effect by turning off all unnecessary electrics of the dead car. Depress the clutch to remove the load on the starter motor.
BMW have used 4AWG from the battery to the starter and that must be 15ft or more. Should be OK then
As with all jump starts you should connect the cars together with the donor engine running above idle to get the voltage up to 14 to 14.5V. If the dead car has a really flat battery then you should allow the battery to charge up for a few minutes.
Only jump for a few seconds at a time. Make sure the jump leads don't get too warm and you'll be fine.
I had to start a dead car using nothing but bellwire once. I just left the wire connected for 30 minutes at which point there was enough juice in the battery to start it.
Last edited by m3matty; 5th January 2010 at 03:32 PM.