and only run the engine with a battery connected to the alternator (or install an overvoltage protector - a varistor that hangs across the alternator output to keep it from committing suicide if there's no battery attached) and you should be fine.
Just for a bit of info......
I have access to basicaly an endless supply of Mitsubishi internal (2) fan alternators from Subaru's and have played with them a lot. These alts are on a lot of different makes and models with the main difference being the mounting bracketry/ flanges.
I have tested this " Run without a battery " theory for blowing them up for many hours and have NOT been successful with these units.
Some of them are the 2 wire units and some the 3 for the car computer turning them on and off but they are all basically the same and NONE of them have failed.
I have run them energised with no load and I have run them energised Overloaded and they don't fail ( or crash the windings) no matter what state they are in. I guess the best way to test these things is try to make it happen rather than avoid it and If haven't been able to create the fault so far, I would guarantee that it is not a problem on these alts.
I think a lot of these fears and warnings linger for many years and become out of date. The alts I have tested are up to 20 yrs old and done several hundred thou KM on the cars before I got to them. It is rare to sell these as replacements, hence why I can get so many. While I have mainly tested the Mitsi alts, I have run the older Bosch external fan type quite a bit in the same way and haven't had a failure with those in 12 or 24V versions either.
I have also run these alts flat out for hours trying to see how much heat I can get in them and they can get hot enough not to be able to touch yet still put out over rating amps and do so seemingly indefinitely. The twin fan design is great because one fan cools the windings and the other the electronics/ regulation at the back. I think this is a key to their robustness and Longevity.
I Suppose compared to the under bonnet temps they would experience, any stationary engine use where they are in free, cold air is going to be a doddle for them. The Concealed Twin fan design appears very robust and so far, I have been unable to break one despite all reasonable efforts.
I have seen plenty reverse connected on vehicles and blow main fuses and computers but never take out an alt.
The other thing I have tested a couple of times is trying to explode them through High RPM. I have geared them up to almost 15,000 rpm at which point the make a noise like a turbine engine but they never looked like letting go.
The alts I have tested are rated at 80A and will exceed that output on every one I have metered. On a 12V system, this is slightly better than 1Kw output and these alts deliver very well at low speeds as well.
I'm not sure of their efficiency in standard form. I would guess like most claw pole alts, the design of the alternator itself is good, it's the same design they use on commercial alts and there are only a couple of percent efficiency differences. The real variation lies in the control circuitry.
Car alts live in some pretty horrific conditions and must be built tough even if they aren't built most efficient. With external regulation I have seen other types achieve outstanding efficiency.
I currently have a 6Hp lister with 2 of these alts on it driven off the one serpentine belt and it will pull both of them to full amps although it does seem to be close to it's limit and I have another 6.5 hp China Vertical Diesel also with 2 alts connected and it does them with relative ease. My little China hormonal Diesel was rated at 2.7 KW and it would pull one all day with the greatest of ease but that engine is bit of a power freak and will out pull the 6hp engines on my induction motor generator setup. I never tested it with 2 alts due to difficulty of mounting but I'm pretty sure it would have done them to a pretty high if not full output.
I run the alts with belt drive and a 6" on the engine with the standard 2" on the alt works very well and allows the engine to run at a low revs and still pull a good load easily. If one was only going to use them for battery charging at 30A or so, you could probably run an 8" pulley on the motor to slow it down and reduce wear and noise if you had a 6HP motor. 6 HP on any alt under 100A@12V is way more than they need.
My next experiment is going to be to remove the control circuitry and run the things direct both on the field voltage/ current and the 3 phase output and rectify it externally and see what AC I can get out of them.
I would LOVE to get a Leece Nevile alt, particularly a 300A monster because there are external controllers available that can run them at 12, 24 and 48V output with VERY high efficiency. Unfortunately while the controllers are reasonably priced, The cost of the alts here is very prohibitive so hard to justify for playing around with when I can get all these others for nothing.
As far as running disconnected from a battery, My experience is that this is not a problem most likely on any alt made in the last 20 years especially for a short period of time.