Checking the 4 Nozzles volume gives 2.44 l/s at 60 meters head.
The RPM is close to 3275 sufficient for the car alternator being able to produce the necessary watts.
You may have different problems:
One : The Jets are not properly set to maximize the torque.
Two : The generator may have too much field current.
Three : The regulator is no producing its voltage, REMOVE THE LOAD AND MEASURE THE output voltage with a small lamp ( like 10 watts ), It should be 14.2 to 14.6 volts -- during this test the Hydro and alternator may Rev up to 6000 RPM, OK for a few minutes.
Four : The Pelton Wheel to Nozzle placement may need adjustment.
Five : The Pelton may have the wrong spoon profile.
SIX : The Hydro is loaded so much that is NOT allowed to generate the necessary RPM, which means the REGULATOR is not the proper one.
For low power Hydro systems, the car alternator can not use the regulator it comes with, because it presents a too high current output which the battery tries to absorb BUT the hydro can not produce much higher power forcing the turbine to slow down and since the battery is loading the alternator, the load just allows the turbine to produce enough power to indicate that is being loaded down.
For small hydros with alternators it is best to have a regulator that can have current limiting to insure that the full power can ge generated and the hydro is now Bogged down by the battery load.
Can you test for this last item ( SIX ). ? - AND REPORT to me ?.
680 watts @ 14.4 volts = 47 Amps but the field current may be taking around 8 to 10 amps, so the output is around 37-39 Amps but the field setting is saying produce 80 to 100+ amps -- all depends on the alternator capacity.
Regards
Nando