I posted info on this a few times in the past; I don't know if anyone
[besides myself] has built and used it, but there sure was a lot of views on
the webpage.
My domain has changed, so here's the new URL for anyone who wants to have a
look;
http://www.amsterdamhouseboats.nl/voltage_regulator.htm

Rap follows;
This is about an alternator regulator you can build yourself for a few
dollars, suitable for charging deep cycle wet or gel batteries for boat,
camper, or whatever.

Schematic and photo are on the webpage.

There are several reasons why you might want to make your own regulator as
opposed to buying one.

There are some good regulators on the market, but they are expensive. None
of them do all the things I wanted, or I would have bought one instead of
doing all the work of developing my own until it actually worked.

With a manually adjusted regulator, you can tailor the load to the engine,
for instance if you have a small engine driving several large loads, you
might want to reduce the charging load while other users are on the engine.
For instance a generator engine that may or may not be carrying a large A.C.
load.

With this set up, you can regulate two or more alternators at one time, on
the same engine or on different engines. Three-step charging has to be done
manually by adjusting the knob a couple of times during the charging
process, which the expensive regulators will do for you automatically.

And of course you can charge ni-cad banks with this easily, and run
compensation charges on lead acid batteries.

Mainly, my design incorporates temperature protection of the alternator[s],
which is unavailable on any off-the-shelf three stage regulator that I've
seen. They tell you to install an alternator so that it can't be
overloaded. However, if your alternator will develop full power at low
engine RPM [desirable if it's on your propulsion engine], then it will be
capable of melting itself at higher RPM. To stop it from doing this,
original in built regulators limit the current when they heat up. The very
best external regulators have current sensing, which you adjust to the
maximum rated output of your alternator. However, in higher ambience
temperatures, or if belt slippage is causing extra heat to be conducted down
the shaft into the alternator, you'll be able to stir it with a stick.

Some authorities say to install an alternator that is so big, it will never
be overloaded. But if you have a large battery bank, and a large load
possibility [inverter, say], you'll need an alternator that is much bigger
than you may want to install if you follow that logic.

If you're having any kind of mechanical trouble, you may want to be able to
charge a large battery bank slowly, with less load to the mechanical system.
You may want to run your engine without any load at all. A manually
adjusted regulator gives you all kinds of control. It's not for everyone,
but if you're a control freak like me then you'll love it.

My intention is to have a robust electrical system on a moderate budget. By
building my own regulator, I also have the knowledge to repair it easily. I
use secondhand alternators; I keep a spare or two, since they are so
affordable. So far, I haven't needed one.

By using two 80 amp alternators, instead of a single large one, there is
inbuilt redundancy, and greater cooling area . Marine applications running
gasoline [Petrol, Benzene, or whenever it may be called where you live]
require special spark proof electrical equipment in the engine room! I speak
here for diesels.

I've been using a couple of these for several years now without trouble.

The rest of the rap as well as schematic is here;

http://www.amsterdamhouseboats.nl/voltage_regulator.htm

Please link to it freely; it helps get the commercial section of my website
up the search engine rankings!
Regards to all, Mark Holden