Two alternators on one battery bank?

[BioHazard]

I have a 3kw inverter that I'm not using and I'd like to build a power source for it. Basically my own inverter-generator. Rather than building one large alternator/engine combo, I'd like to use two seperate engines and alternators. This way I have a backup if one doesn't work, and, if I'm only using a partial load I don't need both running.

Anyway, I want to use a pair of Delco 10SI alternators with built in voltage regulators due to how cheap they are. Is it possible to hookup two alternators like this to one battery bank, will they both charge? Or will I need some kind of special regulator setup?

[BruceM]

The problem I see with the standard auto regulators is that they are designed for "hot float" service- just a bit over float voltage, with no temperature compensation.  That isn't going to work well if that's the primary charging system for a deep cycle bank; the batteries won't ever get fully charged, worse at cold temps.  For bulk 12V bank charging, with topping off via a decent 3 stage PV charge controller, they cut out way too soon.  The bulk charge/absorption voltage should be about 14.6V at 68F.  

They don't work out too badly for the typical automotive application since the depth of discharge is so shallow for that application, and the batteries get enough time in the "hot float" stage to get topped off and de-sulfate.  Even for this application they murder batteries operated consistently at hot or cold temperatures.  (Premature plate erosion in hot climates from overcharging, premature sulfation death from undercharging in cold.)

The only thing they have going for them is they are dirt cheap.

As for operating two in parallel- the few older designs I've seen are "bang-bang" controllers that turn the field current either full on or full off based on battery voltage,  there is a good chance for oscillation with two in parallel.  Regulation voltage may be unstable, and or the load will not end up being shared equally.  A regulator which was a true pulse width modulation (PWM) design would fair better with operating in parallel.

I'll bet there may be some guys who have tried this, my thoughts on paralleling are NOT based in experience on this specific application, only the general case of parallel bang-bang regulators.  The stability problem could vary even with wire size and battery charge impedance. 

[BioHazard]

Good info, though I don't expect them to charge a battery completely. I just need it to supply what the inverter is using. Lets say I just had one alternator, if the inverter asks for 50 amps, then that's what the alt will put out, right? I wanted to just use a pair of automotive starting batteries to provide a big surge for the inverter, but only for a short time, no deep discharging.

If the inverter is pulling more than one alternator can output, wouldn't the system voltage drop to 12v or lower, and then maybe the second alt could kick in?

[BruceM]

It might work, if the instability problem doesn't crop up and fry the regulators before the battery voltage drops enough to have them both full on. 

I imagine your efficiency would be so poor that you'll wish you'd used a cheap AC generator head instead...unless your load is sometimes so tiny that you'd just run off the inverter-batteries and then auto start the engine/alternator when needed.  In that case though, you still need a charge regulator that will actually top off the batteries properly, and you'd probably want  deep cycle batteries.

[BioHazard]

I do want to get a fancy regulator at some point, but for now I'd like to just play with the internal regulators. I know it's definately not the most efficient way to make power but I want a 12v generator I can use for a big 12v compressor I have. I also want it to be able to use large loads intermittently with the inverter, such as a circular saw or other small power tools. That way I can do 6KW surge with a tiny little engine.

[BruceM]

If you are talking about two separate small engines with one alternator each, I'd at least use separate dual wire cables (+,-) for each DC connection to the battery.  That would reduce the tendency for oscillation.  Same is true for 2 alternators on one engine, even though the frame would connect the two alternators frames (grounded internally) together.

Worst case is some blown regulators, maybe some diodes.  I'd give this a less than 50% chance.  I did a quick search on the internet and did not find any success stories, only warnings.

You must somehow finish the charge on the battery (check with hygrometer), or it will die young from sulfation.

BobG is an expert on auto alternators here, and he might have a better estimate on the chances of this working with the simple stock regulators.

My auto alternator experience is limited to one custom homebrew low EMF external regulator design (linear, not pulsed) and an MB 300D alternator modified to have an isolated ground, and a massive external passive filter.

[veggie]

Do you have enough HP to run the 2 alternators off the same engine ?

http://www.youtube.com/watch?v=FmCHU5x-oPo&feature=results_main&playnext=1&list=PL128FF9571FD945B1

http://www.youtube.com/watch?v=AZ7_HYtWS0Q

http://www.youtube.com/watch?feature=fvwp&NR=1&v=2pQeeUkPoyk


veggie

[Ronmar]

Spinning those 6 alternators on that suburban isn't a problem.  Finding the 45' serpentine belt to string them all together... Now thats a problem:)

[Henry W]

OK..............................,
Lets get back to the topic.
Why not use one Leece Neville. They are proven and cheap for what you get. Less parts and things to go wrong.

[BioHazard]

Why not use one Leece Neville. They are proven and cheap for what you get. Less parts and things to go wrong.

I think we have two different definitions of cheap.  10SIs are only $29 at Autozone. Part of the reason I like them is because I also have three vehicles with the same alternators, I like parts interchangeability.

I do want to do some serious alternator experimenting with these alts, and it's no big deal if I kill one at 29 bucks...

[Henry W]

Ok...................,

That is cheap. It costs roughly $29 for mushroom boat anchors these days so I guess you will be set if the project fails. Just joking.

On a serious note: This could be a good experiment and good things could be learned so you got my moral support.

Henry

[BruceM]

Chances are the first video's 3 out of 4 dead alternators just had fried regulators, from oscillation, that that was with modern PWM regulators.  Hmmm.

A single external regulator an output beefy enough for both sets of excitation coils would solve the problem if both are on a single engine.  If you blow the stock regulators, it might be worth a look for a suitable one.  Then butcher the stock regulators via Dremel so that only the brush connections remain, so that it can be hooked up to your new single external regulator.

[vdubnut62]

I'm not sure where I heard this or even if it's credible, but there is supposed to be a high chance of blown diodes if the alternators
either happen to get "in" phase or "out" of phase maybe even an old wives tale. At any rate I have seen manuals that tell you to stop the engine or disconnect the alternator on the vehicle you are going to jump start another from....but I never do and I haven't lost an alt either
Just an itch in the back of my memory - YMMV( and probably will)
Ron

[BioHazard]

At any rate I have seen manuals that tell you to stop the engine or disconnect the alternator on the vehicle you are going to jump start another from....but I never do

Interesting, that's what I was thinking about, when you jumpstart you have two alternators connected. I've never heard of disconnecting one before a jump. I should read what the manual for my old truck says to do for jumping.

[Henry W]

I always jump a vehicle with the engine turned off. No exceptions.

Henry