AndrewG:
Its actually fine to connect the leads battery to battery and will create a better connection.The only reason it wasnt really recommended to put the leads direct to the terminals was to avoid sparking and causing an explosion from old school lead acid batteries giving off gases, sealed batteries have now more or less aleviated this problem.With the engine off connect the red positive lead to the + terminals and the black - lead to the flat batteries last. Start the engine and allow to idle for a minute before turning over the flat truck. No need to rev the engine as full power from the alternator will be there as its detecting a drain with the connected leads.
Ive done this twice with my 3month old FH to start trucks in our yard and its been fine. New trucks may well have many complex electronic systems but these are all surge protected. Battery technology may well hve moved on with gel and silver calcium batteries but charging circuits have barely changed over the years…
I’m a little old school on his sort of stuff and was taught to use an earth point on the vehicle rather than the battery simply because you could get a larger surface area to contact, plus, in the olden days the terminals were often covered in grease.
Disagree with the point on picking up he revs though. If you’ve got a battery that’s really low, it can drag the voltage on he good battery down to he point where it stays the good vehicle. IME though, that tends to suggest a faulty battery.