Originally Posted by USNavyTaco
So just to be clear,and to learn... an alternators output is NOT proportional to the RPM of the engine? i thought that is why you give the GOOD car in a jump a little gas? to increase alternator output?
It's a little complicated, but at low RPM an alternator's output can 'droop' even with maximum field current. The field current is how the regulator or computer controls the alternator output.
So you bump the RPM up a bit above idle, and that lets the alternator put out what the regulator/computer is requesting .
The regulator/computer is mostly just looking to keep the system voltage at the optimum level. It varies a little with temperature (higher voltage in cold weather). Usually you don't see normal operating voltages outside of 13.5 to 14.5 volts.
So, the other car would have tried to boost its alternator output until it saw 13.5-14.0 volts, which it would have probably seen just from your jumper cables being connected anyway.
tl;dr = Your truck is fine.