Joe and 1930 Coupe have covered it well. You need to buy a good voltmeter and hydrometer to monitor your battery and set the adjustable brush to match your driving. If your engine fires right off (as it should) and you drive for at least 30 minutes between starts, then setting the adjustable brush for 2 amps should work fine.
If you have to crank for 30 seconds before it starts, then only drive for 15 minutes and shut it off, then you probably would be better with an 8 amp charge.
By far the easiest and best solution to long battery life is to install a voltage regulator. I've got at least 10 years on my junk yard battery and have only checked the water 3 times, and only added a few ounces about 4 years ago.
Letting a battery set with a low charge will greatly shorten it's life. People don't want to spend money for a voltage regulator, but will spend money replacing the battery ever few years. I don't get it.
