Race car I assume.

We ran two batteries. Car was light to begin with, had mounts for two, just made sense. Not having to charge the batteries at the track (usually) was a nice side benefit to running an alternator. I did charge them up to 100% between weekends.

The main reason I ran an alternator was to keep the voltage consistent run to run, hoping that anything with a wire attached to it would also be consistent - ignition, timers, solenoids, etc.. Wasn't concerned about whether it was 13.7 or 14.7, just that it was the same every pass.