I saw this question this morning, posted on a Detroit Diesel forum of which I am a member:
"I'm building a hybrid car (isn't everyone these days?!) and have a
question. I got this old (1966) GMC Fishbowl transit bus with a 6v71
in it. It has the original 250amp alternator in it. The big engine
gear driven one. The bus is stored 200 miles away so I cant get
details, but it seems every transit I have ever seen has the same
Anyhow, what is the max dc voltage I can get out of it without having
to rewind it?
I want to use it to recharge the battery pack in the car from the cars
Quick math tells me that I can get 3250 watts from it (13vx250a)
Is there a way to switch that around to 3250w=52vx62.5a?
What are my limits with this?"
I actually started composing an answer, but stopped myself because all I would accomplish would be to start a flame war. Call me jaded, but I think this guy is looking for affirmation and would not accept an injection of reality.
The reason I think this is because of his opening assertion that, to paraphrase, everyone wants a hybrid vehicle. That's like someone who states that "Everyone knows that global warming is reality"; what they are really saying is "This is my worldview, and I'm not gonna listen to anyone who says anything different". So, rather than start a flame war on somebody else's site, I'm gonna hash it out here, just a little bit.
Automotive (including commercial vehicle) alternators are inefficient critters; generally less than 50% efficient. I have used them for short-term power, and continue to do so, but I found a few years ago that they are far below acceptability for any kind of long-term power needs because they will use literally double the fuel per kilowatt/hour when compared with the common AC synchronous alternator found on generator sets. That's the first point, which I have proven to myself in actual testing, measuring watts over time versus fuel usage.
Batteries, likewise, are inefficient. As a general rule, whether you are charging lead-acid, nickel-cadmium, nickel-metal-hydride or whatever current battery technology, you will have to put approximately one-and-a-half times as much into a battery as you can take out. That is a 25% power loss. Add that to the 50-60 percent loss one would incur from using an automotive alternator, and we would only be using 15-25% of the power output at the shaft of our prime mover.
Let's pause here to discuss offgrid power systems. The typical alternator aka "generator head" used in a genset is about 85% efficient. This is not too bad, as long as we use the AC power directly and manage loads so that battery power is not used for the large loads. We have to accept the battery inefficiency if we want 24 hour power, because it is far less efficient to run the generator for light loads, than to run the generator for just a few hours and save our large loads for when the genny is running, adding bulk battery charging, water heating etc. to get close to maximum capacity of the generator. We can also offset some of the inefficiency by using the heat produced by the generator for space and water heating.
OK, back to hybrid vehicles. Hybrid vehicles are really not the answer to the fuel-efficiency problem. In Toyota's own words, they are a lifestyle vehicle, much like a sports car is a lifestyle vehicle (SUVs are not, btw, but that's a subject for another day). The whole point of driving a hybrid is not to save money, but rather to make a political statement and appear to be "saving the world" (and I'm sure a large percentage of hybrid vehicle owners have themselves convinced, too). Besides their inherent inefficiency, the battery banks in these vehicles have a finite lifespan, are very, very expensive to replace, and are poisonous to the environment once depleted. In fact, I suspect that battery replacement will be so expensive that dead batteries will mean the entire car gets recycled. This is another win for the automaker because it means he gets to sell another car, but a loss for the environment as well as for the owner who has to buy a new car sooner than he otherwise would have had to (although most will never notice, because they don't keep a vehicle until it wears out anyway).
The thing about hybrids is that, in the real world, they don't give the fuel efficiency that they are theoretically capable of. Sure, you will probably achieve 45-55 mpg, but a same-size, diesel powered conventional vehicle will do as well, while lasting substantially longer and giving fewer problems, witness the Volkswagen TDI vehicles. Hybrids will only deliver the 62 mpg or so that some are rated at, under ideal conditions. And that is using the best technology available, including optimized batteries and purpose-built motor-generators. If something like that can't beat a conventional diesel in real world conditions over the life of the vehicle, then certainly something using an automotive alternator and off-the-shelf batteries isn't going to be competitive.
One last thing: someone is gonna bring up diesel-electric trains. If you research that subject, you will find they are built that way because of concerns of traction control and economy of production. A mechanical or hydraulic transmission that would handle the kind of loads a train engine is subjected to would be huge, maintenance-intensive and expensive, compared to a simple electric motor drive; and the continuously-variable nature of an electric motor makes it easier to start and stop heavily loaded trains without loss of traction.