HA! that's the same guy I bought mine from. (the first one on the page to be exact) But, I bought it through his Ebay sales at a significantly lower cost. I wasn't too concerned with the losses, since I calculated those losses into the alternator design ahead of time.

You don't have to buy a large three phase rectifier. You can simply make your own from six ordinary diodes. (stud diodes would be great) The question is, how big are you building?

Yup, bumping up to 48 volts was all about managing amps. (and future expansion)

Irregardless of the amperage ratings of a diode, the forward voltage drop across all sizes is fairly consistent, around 1.4v +/- 0.2 is typical. A stud diode is just a larger, panel-mounted power diode. Take a look at this page to get a better idea: http://www.semikron.com/internet/ds.jsp?file=471.html

Uh oh, you're getting a little too deep into electronics for me to keep up now. What circuits I have built in the past were typically copied from a schematic. Yes, I can put them together, but I can't always tell ya how it works. IC's, transistors, and many other goodies are a bit of magic to me. So for instance, I don't measure inductance, rather, I use much more crude methods, like physical tests/trials/experiments.

The only instance I can think of where I said, "trade current for voltage" before, was when talking about DC to AC inverters. Where as, 24V @ 200A would convert to 120V @ 40A (if it were 100% efficient) But, I can think of three instances where I have said, "trade voltage for current" though.

The first one, most relevant to the subject, is the wind turbine. As it's speed increases, it's voltage also increases proportionally when not connected to a battery bank. When it is connected to a bank, (through the rectifier) the voltage will rise as speed increases until the voltage supplied by the turbine matches the bank voltage. After that, the bank will not allow the voltage to rise. (at least not until the bank is "full") Basically what happens is, the voltage the turbine would normally be generating becomes amperage to the battery. Truly, it's trading voltage for amperage.

The second I can recall is in reference to my solar charge controller, a maximum power point tracking (MPPT) controller. This device takes high voltage and converts it to a lower usable voltage, at higher amps. Again, a trade of volts for amps. It's a bit more advanced than that, and actually makes far better use of available power from a solar array. While not relevant to this thread, I'll explain it's operation. First, the specs on the sticker of my solar panel:

MAXIMUM POWER (Pmax) 175.0 W
OPEN-CIRCUIT VOLTAGE (Voc) 44.4 V
SHORT-CIRCUIT CURRENT (Isc) 5.40 A
RATED VOLTAGE (Vpmax) 35.4 V
RATED CURRENT (Ipmax) 4.95 A these last two are the important ones as you'll see.

If you multiply the rated voltage and current, you'll find it equals the maximum power of 175 watts. But, what happens when we connect the panel directly to our 24v battery bank? If you guessed that the voltage will come down to the bank voltage, you are correct. You can see that, even when short circuited, the panel still can only produce 5.4 amps. Amperage is pretty consistent across it's operating range, and changes very little. (unless you approach it's Voc, where amps drop off sharply) So, let's do the math. Say your bank voltage is 26v while charging, and the amps would probably be around 5.1A, multiplied, that's 132.6 watts. HEY!?!?!?! Where's my 175 watts this panel promised? They didn't lie, it's just that you can't get 175 watts unless you operate the panel at 35.4V as the specs show.

So what the MPPT controller does, is it sits in between the panel and the bank, and it takes the higher voltage and regulates it to a lower voltage/higher current BUT closely watches the input from the solar panel to assure it's operating at it's peak power production. (it's constantly comparing the incoming voltage and current, and alters it's "draw" to achieve maximum wattage) Think of it a little like a wastegate on a dam. Open it too far, and the water drops to much. Open it too little, and you don't get enough flow. Clever device.

The third and last mention of trading voltage for amps, was in the case of a solar well pump. It operates on a very similar arrangement as the last, as it uses an MPPT controller, combined with a single-to-three phase converter to regulate speed and torque. But in all essence, it's trading high voltage for manageable voltage with peak current to run the pump.

As for rectifiers, yes there are many combinations available, but keep in mind the ratings are maximum ratings. They can be used at lower voltages and amperages. (In fact, it's preferred to keep heat down, and extending it's life) Yes, you need two diodes per phase. The wiring arrangement would look like this: