Reconciling the outputs of a generator with the C/10 limits of a LA battery bank

[Simtech]

I'm in the design phase and I've bumped into the disparity between the amperage that the generator can generate vs the C/10 limitation for flooded lead-acid batteries.

For example the generator (after feeding through the controller) can output 111 amps at 36V, however the 36V 750AH bank can only take 75 amps without shortening the life of the batteries.

The genny output will be fed through a outback charge controller which I trust will not overcharge the battery bank, however this leaves the genny only running at just below 50%.  From what I understand optimum loading of a generator set is approximately 80% of max load.

There's a lot of potential power not being utilized.

How do you guys resolve this?

Planned specs:  Listeroid 8/1 derated to 6/1 pushing a 5KW ST AC head rectified to DC feeding a Outback power Flexmax80 which charges the battery bank.   I chose to rectify (and heavily filter) the AC output of a ST head because that gives me a failover option of using an 1000 watt inverter/charger that I already own for the bank and passing the AC load directly to the listeroid gen set.
The reason I prefer recified AC power is my understanding that ST heads AC waveform is rather dirty, and my load contains sensitive medical life-support equipment (a home peritoneal dialysis unit).  Thus a separate inverter for my load.

Edit:  Sorry boogered the post, remnants of the marine alternator backup plan that I thought I'd removed.

[Ronmar]

Uuuh, someone correct me if I am wrong, but a 36V battery bank is going to take around 42VDC for charging.  42V @ 75A is 3150Watts...  That is a real good load for a 6/1, pretty much the sustainable electrical load limit for the 6/1-ST-5 combinatoion IMO...

[Tom Reed]

I'll second Ronmar on that. Simtech, what you want to do is work with a common denominator, like watts, when doing these calculations. So lets do the math:
A 6/1 has a rated output of 4.4kw/6hp at best an ST5 is 80% efficient so output will be around 4.4*.8=3.54 kw.

The battery bank is 750ah*.1=75ah @ c10. 75ah*36v=2.7kw

The 2.7/3.54=.762 or 76% generator loading, just about perfect!

[Simtech]

Sorry, I boogered the original post during a rewrite deleting a long section about a marine DC alternator as a backup plan.  

Anyway:

I'd used 4000 watts / 36V giving me 111 amps even using an 80% efficiency factor (which I hadnt considered) still gives me 88.8 amps into the bank, which is still more than I should be feeding the bank. 

This is completely ignoring that the controller can only pump out 80 amps.

Or am I miscalculating the C/10 amp rate?  My understanding was the amp rate remained the same regardless of the voltage level of a string.

Am I missing something?

[Simtech]

Tom at the risk of sounding like an idiot I'm going to step through my reasoning so somebody can point out what I've done wrong and to clarify it to myself.

Maximum safe input to bank is 75A @ 36V = 2700 watts.  This is within the charge controllers capability

Maximum output of listeroid/ST5 4400 watts, after 20% efficiency reduction = 3520 watts = 97A @ 36V

Extra watts genset is capable of but can not use without damaging battery bank:  22A @ 36V = 792 Watts (a decent amount of power but not used to avoid premature wear/burnout)

Now a separate issue, generator loading:  2700W/4400W = 61% of gen max capacity this seems low to me.
If I factor in the 20% loss: 2700w/3520w = 76.7% gen loading which is near optimum.
But do I get to factor in an electrical efficiency loss at the ST head in my planning?

An now something completely different:  is there an advantage to using a larger ST head and just not using the extra capability of it?  If I get a 7.5 or even a 10kw ST head but only use it to generate 3520 watts will the ST head last longer?  I'd think so but I thought I'd ask the question.

I'm trying to size everything based on my planned battery bank size, which is itself sized to give the dialysis unit 2 days worth of use without a bank recharge:

700 watts of power while being used for 9 hours a day = 6300watt-hours per day / 36V = 175 amp-hours per day
2 days of use = 350AH
50% depth of discharge requires 350AH * 2 = 700AH + 50AH for a few lights, etc.

I'd love to do 3 days of reserve power but I dont know if I can financially afford a 1050AH @ 36V battery bank.  A 750AH @ 36V is going to be hard enough.

[Tom Reed]

Simtech,

It looks like you are missing the fact that you don't want to run the engine at maximum output, usually 75-80% is where you will get the maximum economy and equipment life. So using your numbers while the system is capable of generating 97a @ 36v, your 97a*.8=77.6a. Also your rectifier and smoothing xformer will both generate heat and cause associated losses.

[Simtech]

Oh..... crap.  That was stupid.  Thank you.  It just wasnt registering that I can throttle it down....

Any comments on the rest?  Oversized ST head or battery bank size?

[Ronmar]

You will start to loose efficiency by under loading the generator.  It takes more field current to energize a larger field winding.  Typically the larger generator has a larger blower that can cool the larger loads the generator is capable of outputting.  At low load that blower is overkill and moving more air than is needed...  I think you will find the ST-5 a good match and more than adequate at 6HP input. 

[Henry W]

Not all generators, Some Generators from Marathon, Lima-Mac and Mecc-alte are more effecient near half to three quarter thier rating. And these tests have been conducted with power factors of .8 and 1. I read many pages from Marathon that shows this. Mecc-alte USA mentioned this to me also. My 12-ST I had is more effecient at 7.5 KW than an ST-7.5.  I don't know where this myth came about years ago but from what I have found the theory of having to run all generator heads near full capacity to get max efficency out of it is false.

Henry

[Henry W]

Here is one example:

Must be logged on to open PDF

[Ronmar]

Not all generators, Some Generators from Marathon, Lima-Mac and Mecc-alte are more effecient near half to three quarter thier rating. And these tests have been conducted with power factors of .8 and 1. I read many pages from Marathon that shows this. Mecc-alte USA mentioned this to me also. My 12-ST I had is more effecient at 7.5 KW than an ST-7.5.  I don't know where this myth came about years ago but from what I have found the theory of having to run all generator heads near full capacity to get max efficency out of it is false.

Henry

3.5KW of maximum sustainable load on a 5KW head is about 70% load.  If he goes any larger, he will drop below the 50% point on the generator head...  A ST is not a marathon...  I think the 50-75% point on the ST head is a good place to run...

[Henry W]

I agree, running the ST-5 near 50% -70% load and no more than 75% load would be a good guideline to follow.

[Simtech]

Thanks guys, engine powered electrical generation is a whole new world for me and I'm trying to get spun up as fast as I can.
Most of my past tinkering experience is with solar thermal. 

Whats prompted me into this is that we're moving into an area where power outages are frequent and long.

And it just looks like a lot of fun.

[admin]

apart from concerns with available power and generator capacity, if you have excess capability you could certainly
just switch on the inverter to provide power to some scheduled load to use any extra capacity and keep the charge rate
to the batteries under the max recommended .

bob g

[mbryner]

Hello Simtech,

Welcome to the forum.   You are quite optimistic about how much power you can get out of a Listeroid with your setup, and I'll explain.   Your setup will work but my prediction is that you'll get about 2000 watts of charging from a 6/1 measured at the charge controller.   (The guys who have written on this thread have a lot more knowledge than I do, so my opinions can usually be taken w/ a grain of salt.)   

I have quite a bit of experience in exactly the same setup that you have designed (search the forum & you'll find lots of info from when I was building it) and I can give you several hundred hours worth of data.   We live off-grid w/ a 48 V battery bank (L16 batts x 16), w/ Outback inverters and Outback MX80 charge controller.    I also have a Listeroid 6/1 and I went with an oversized ST 7.5 head to be able to start larger loads if necessary.  I charge my battery bank using the AC to DC rectification you're proposing.   Tonight while charging for several hours, the ST case didn't even get warm.   The AC from the ST will have to go through a large isolation transformer, then bridge rectifier, then a large filter choke, then to the charge controller.   You will need to put a *large* 10 Ohm resistor or *large* variable resistor in the excitation coil circuit to decrease the output voltage, or use a Midnight Solar charge controller that can handle high voltage.   i.e. rectified 120 V AC will be way over the 140 V DC input limit to the Outback charge controller.    Overall, I can't get much more than 2000 watts continuous charge with AC blower fan and thermosiphon cooling, and all the other inefficiencies of a transformer, rectifier, choke, ST, and charge controller.   Best guess is that I'm somewhere near 70-75% of max load for my 6/1.   

Here, lets play w/ some efficiency #'s (just ballpark figs, the other guys are better at precise numbers)
     Transformer         80%
     ST                      90%
     Charge controller  90%

     0.8 x 0.9 x 0.9 = 0.65  -->  65%, just accounting for those 3 components

Keep us informed of what you decide and how it works out. 

Marcus