Automotive Alternator Efficiency

[veggie]

Does anyone know the approximate efficiency of automotive alternators when running in the upper end of their amp range?
I have a new Delco 12SI unit rated at 94 amps.
The speed curve for this model shows:

50 amps @ 2000 rpm
65 amps @ 2500 rpm
72 amps @ 3000 rpm
82 amps @ 4000 rpm
90 amps @ 6000 rpm

My plan is to run the unit at 3300 rpm generating about 74 amps @12 volts.
What can I expect as the "mechanical-to-electrical" efficiency at this speed?  70%.?...80%...90%..?

Veggie

[mobile_bob]

you can expect under 50% efficiency from that unit when it has come up to thermal stable operation
which by the way will be hotter than "burnt toast" 

74 amps is likely all it will put out when operating hot, and that is max output.

because of the oem's design need for a unit that can produce 50amps at 2000rpm
there are many compromises made that dramatically affect efficiency at the upper level
amperage output.

also it is my belief the alternator will not live a long life running at near peak output, especially
not at 3300rpm where the airflow is about 1/8 of what it could be if it were allowed to run at 6600rpm

looking at your chart, at 3000rpm it is rated to put out 72amps, but this is likely at room temperature (cold)
and not its hot rating, so it is very unlikely you will get much over 50amps at that speed, which might be beneficial
to the lifespan of the unit anyway.

what is the end purpose of the unit? charging a starting battery? if so you will be fine,

if however your plan is to charge a large battery bank, its not going to work very well or for very long in my opinion
for the reasons mentioned and also because the internal regulator senses alternator heat and derates the voltage
to help to protect the alternator,, lower voltage dramatically increases engine run time so in the not so long term
the added fuel consumed will offset a better option in alternators in my opinion.

i have spent years modifying delco alternators and finally gave up on them because they are just poorly suited for deep
cycle battery charging at higher output.

probably not what you wanted to hear, that is if you wanted to use it to do heavy charging as opposed to starter battery
recharging.

bob g

[veggie]

Hi Bob,

Being that the majority of 6/1's with alternators end up using a Delco (or similar) automotive type charger, your information is valuable.

In my case, the unit would be used to charge two 6VDC Trojan T-105 batteries in series (225 amp rating each) at the rate of 50 to 70 amps.

Lifespan:
The run time for each daily charging would be 2 to 3 hours.
Based on the operating conditions (indoors with 3 hour runs) I would propose that the life span may be quite acceptable. Under the hood of a car in summer there are very high ambient temperatures and these units seem to survive years.
Granted, the industrial alternator of which you speak would last even longer (and at higher efficiencies).

Efficiency:
The efficiency of these alternators is a bit disappointing however. Something up in the 80's would have been nice.
The effy. could become a factor depending on the cost of fuel. (EG: Dino diesel vs. WMO).
In my case, running for only 2-3 hrs. per day (winter only), I doubt that the difference in price between these alternators and the more efficient units would be recovered for many years.
(rebuild kit: brushes, bearings, regulator is $18)

For longer running systems requiring equal or greater output than mine, it appears that an upgrade would be warranted.
I am not familiar with anyone who makes a heavy duty version of automotive alternators.
Perhaps you could share a few in this thread and I can check for availability in my region.

Application:
Good point about "starting batteries" vs. battery banks.
Perhaps other alternator users could pipe in and state the type of alternator you have and what you are charging with it.

Cheers,
veggie

[mobile_bob]

with the size of battery bank being 225amp/hrs and the max charge rate being about 25% or ~56amps, you should be alright
with the delco, the amperage is in a reasonable range and it will fall back fairly quickly anyway because of the relatively small
battery capacity.

as for you point about underhood temps, that is a valid point, except
the underhood application is charging a starting battery of probably not much over 75-90amp/hrs capacity, and
a battery that is generally not heavily depleted before the charge cycle starts again.

the delco cs130 alternator has a warning about being sure to recharge the battery fully before you start up with a new
or rebuilt alternator, otherwise they are prone to going up in smoke. never really understood this engineering shortcoming
but it is reality, in that if you insist on asking it to recharge a heavily depleted starting battery the lifespan of the alternator
is very short,, measured in maybe a hundred or so cycles before it flames out, as opposed to thousands of cycles in an
average car.

i have seen this happen far too often, especially in city delivery vans/trucks where the battery is run down significantly with dome and
interior lights, and 4way flashers, then the engine is started and stuffed to the floor to get to the next stop.  i have seen the varnish
dripping out of the stator cores of these alternators.. they just won't stand up to this sort of abuse.

when delco says 94amps or 120amps or whatever, that is only for a short term load, the duty factor is quite low on most of these small
frame alternators, as designed and as implemented.

you asked about recommendations,  the prestolite/leece neville 110-555jho is a solid performer in my opinion, much easier to work with
albeit probably about twice the cost of a small case delco, they commonly are available new for about 175bucks or so.

for more severe duty, the leece neville 200,250,270 and 320 amp units running at about 60% amperage will run continuously for a very long time.
the duty cycle for full output is probably twice that of a typical delco. the biggest issue with these units is one of first cost, they are a bunch more money than a small frame delco.

anyway, bottom line is you should be ok with your application, just keep in mind when it comes time to increase the battery bank size
appreciably the likely result will be a dramatic reduction in alternator lifespan.

hope that helps,

ymmv

bob g

[veggie]

Bob,

That helps a lot. Thanks.
BTW, $175 for prestolite/leece neville 110-555jho is not so bad.
Compare that to any generator head and it's cheap.
So if the main duty of the engine is to charge a bank, it seems more cost effective to go with a Leece alternator instead of an AC Generator head.
Food for thought!

cheers,
Veggie

[veggie]

Found a few places on the web selling the leece neville 110-555 for about $200.
For anyone interested, here's the spec sheet....

Veggie