Searching around using The Google
I find that there are many older automotive alternators designed to hook up to separate regulators, so it would be simple and cheap
to get a fairly high output alternator around 100 amps for around $100. Now if there was still a simple manually adjustable regulator you could have a pretty cheap charging
system that could work with any type of batteries and could be manually adjusted for good charging characteristics. Plus, it would be relatively bulletproof and easy to fix or replace when it does break down. In researching this a bit I came across Maine
Sail's Musings Regarding External Regulation, a good write up on his experiment
using a dumb regulated alternator and cheapo WalMart batteries, with good success so far (I don't think I can link to this as it is on another forum). Thought it was worth mentioning.