I ran a system similar to what you are trying from 1990 to 2007. I just upgraded last year. Here is a brief outline of that system: Total length of 4 inch pipe 4400 feet, total head 160 feet, nozzle size 1/2 inch, turgo turbine pitch diameter 4 inch, 66 amp Delco alternator.

The Delco produced about 37 amps at 13.7 volts at the battery. This system used a controller to vary the field coil amperage so no diversion load was used. The field coil used less than 4 amps.

The brushes and slip rings lasted about 6 months. Actually only one brush and ring lasted 6 months the other brush would last 12 months and the corresponding ring was never replaced. The bearings were only a problem once. I may have gone through three different alternators because of winding problems. They were all reconditioned units, about $100 each.

The main problem with an auto alternator is they are meant to be turned in the 5000 rpm range. A 4 inch turbine will not turn it that fast under load.

While changing brushes and rings isn't a big thing I usually found the problem would occur at inconvenient times.

Eventually I got tired of replacing the brushes and ring so I changed over to a 3 phase induction motor system. You may find an induction motor system will cost you about the same as using an automotive alternator since you would only need a one horse power 3 phase motor instead of the alternator.