Jumping a car and attaching cables after starting donor car's engine

Question

I was jumping my friend's car today in a place with no cell phone reception.

He argued that the jumper cables should be attached after starting the donor car's engine. He said that it would decrease the risk of inadvertently depleting the donor car's battery. I thought this argument made sense.

We did so, and jumping the car was ultimately a success.

I came home tonight, and read that the jumper cables should have been attached first, before starting the donor car.

I am wondering: is there any risk of a donor car's battery being drained before starting the engine?

Answer 1

Yes...

But you also run the risk of damaging the donor car’s alternator.

So, the method I use is to connect the cables and then start the donor car and run at 2000rev/min for 5 to 10 minutes. This puts some charge into the other battery. Then turn off the donor engine and start the other car.

People will say “but I have always run the engine on the donor car when starting...”

Fine, I have replaced alternators damaged due to the energy surge many times. The owner always says “it stopped charging just after jumping another car”. They were never happy about what a 2 minute kindness cost them...