I say, why not try it?
The things to keep in mind are:
- How much is really lost to the alternator? Some say 10%. But that includes the friction of the alternator spinning. To completely get rid of the alternator's drag, you have to get rid of the alternator completely.
- Long trips will use more electricity. But do we know how much? Are these going to be during the day or during the night? How far are we talking? Will you be running accessories like A/C or heater? Watts x hours = watthours. If you can reduce your electrical use you can drive farther on one charged battery. Of course if you reduce your electrical use you also don't benefit as much from this mod.
- Battery size needs to be big enough to fulfill its purpose. But bigger batteries are more expensive.
- Lead acid batteries last the longest if you keep them topped off. Run them dead and you can lose 50% of their capacity just like that.