(16-10-2016, 11:13 AM)ghost123uk Wrote: Continuing my learning curve in preparation for the very forthcoming M59 2.0Ltr HDi "Desire" ownership.
I think I have read somewhere that if one has a flat battery (for whatever reason) one has to follow certain procedures if "jump starting" from another battery / vehicle.
Is this correct, and if so what is the procedure?
I have heard a lot of stories for donor ECU burnout when connected to flat battery and donor engine running.
It seems that they are related to this. Because battery is flat and you connect cables from donor to it there is
large current flowing to flat battery. Above 50A. Flat battery is taking a lot of current. Donor alternator starts to
produce even more current as it is starting to charge that flat battery. Then you try to start other engine. It that second
there is even more current acquired. Above 150A. Donor alternator starts to produce even more current. Then
engine form flat battery starts. His alternator scenes that battery is flat and starts to produce current, a lot
of current. In that second donor alternator scenes that there is no more large current required and it stops
to produce current. That sudden stop of producing large amount of current creates a voltage spike that is sent
on donor wires to closest computers. ECU is always close to engine battery and alternator. Voltage spike
burns ECU because it is closest to alternator. Car with flat battery will not have problems with voltage spike
only donor car.
So this procedure is the best: Donor engine OFF !!! Connect jumper wires. Then wait 5-10 minutes!
In that time flat battery will settle down taking charge form donor battery. Then you start flat battery
engine. When engine starts as soon as possible remove jumper wires.
Even better procedure is this: Donor engine ON. Connect jumper wires. Then wait 5-10 minutes.
Donor engine OFF ! . Then start flat battery engine. When engine starts as soon as possible remove jumper wires.
It would be best to have full external battery not in car and connect that battery with jump wires. As soon as
engine starts remove jump wires.