Generator-alternator math problem?

[zero]

Note - this is not a specific RV issue.  Not unless you want to put a bigger alternator on your  Toyota and want to know how much extra power it takes to make X amount of extra amps. This is also about how poorly many AC input battery chargers work when plugged into portable generators.

Situation of mine as thus.  Recently got a cabin on 60 ares in a very remote area of the Michigan UP. No water, no electricity, etc.  So we just stayed there for the first time as sort of a "work mission."  Put up a 30 foot antenna tower for TV and Internet (got that done).  Then installed a battery bank consisting of eight, 12 volt deep-cycle marine batteries from Walmart for $83 each.  All hooked in parallel for a 12 volt system. Wired in a 3000 watt "sine-wave" DC to AC inverter.   I will be installing 800 watts of solar panels on the roof during our next trip up.  This time - I had to use a generator to recharge.   So, once a day for an hour or so, I ran a generator to try to charge them back up.

I built a gas-driven DC generator specific for this place.  Harbor Freight 6 horse engine with a dual-belt drive to a Delco 120 amp alternator.  5" pulley on the engine and a 3 3/4" pulley on the alternator.  That should have the engine at peak power when the alternator is at peak charge of 120 amps.  It did NOT work.  I had tested it at home on a slightly discharged battery and it worked fine.  But when at the camp and hooked to a somewhat discharged battery-bank - the engine would just labor real bad and then die.  NOT enough power to make 120 amps.  So, I am surprised.   The typical portable generator for AC usually makes around 570 watts per horsepower.  Like a Honda 2000i that makes 1600 watts and makes a max HP of 2.8 horsepower.  That is 570 watts per horsepower.  So, it seems my 120 amp setup @ 14.2 volts is 1700 watts and ought to only need  3  horsepower.  But, this 6 horse engine is not enough.  Why, I do not know.    I removed the Delco 120 amp alternator, and replaced it with a Delco 67 amp alternator and now it works fine.  Engine has just enough power to run it when making 67 amps.

So, so much for my "engine-alternator" math. No good.

On that last trip, since my DC generator did not work, I had to revert to using a portable AC generator hooked to my electronic Iota 45 amp battery charger.  Hooked to the portable generator it would only make a max charge of 18 amps instead of 45.   I ran it for four hours and the battery bank came up to 13.5 volts.  Not a very good setup.  Note the generator was a conventional 3500 watt generator and NOT an inverter-generator.   The Iota charger might work better with an Inverter generator  but don't have one up there to try.  I did try it once on my ETQ 1800 watt inverter generator and it just kept tripping the breaker and would not work. Odd again since the 45 amp Iota charger at 14.8 volts should be less then 700 watts.  Yet my 1800 watt inverter generator can not run it.  More math issues? Or a crappy generator?

Here are some old photos of our cabin that I got from the previous Canadian that built it around a camper. Kind of interesting.  You would not know there's a camper in there from the outside.

[Back East Don]

I had an issue with the Yamaha home generator I use when we have storms and loose power.  My furnace had an issue with the ignition transformer that I was unaware of but when I connected it to the generator, it just didn't fire the oil furnace.  I busted out my meter and found the voltage was slightly under spec.  The generator had a trim pot to adjust so I tweaked a bit higher as voltage around here runs a bit hot anyway and the furnace worked.  Later I found out the transformer was on its way out and had it replaced.  Did you check your AC generator's output voltage under load?  Can't help much with the DC generator as you likely know more about it than I other than to say look up both the RPM curve of the Alt and power curve of the motor to see where both should give max performance. 

I considered building one when contemplating adding more batteries but decided I'm not away from AC that long and got more mileage out of making the rig as efficient as possible.  When I researched alternators, some of the marine ones output curves were designed for lower RPM's  Don't know if that along with a larger pulley would get you in your target zone.

[Totem]

Interesting.

At my camp i use an old Schumaker charger powered by the HF predator inverter genny. it (the charger) always works fine on 10 amp setting and will bring the bank to 14.1 volts after a tank of gas. This is 4 interstate golf cart batteries from costco.

Now if i wanted to i could use the generators 12 volt charging port but i never do because I don't have cables to that funky plug that will run all the way to the loft based battery bank in the cabin upstairs.

It seems to me that you must have a faulty alternator; or conversely the battery bank is exceptionally discharged to the point where the alternators dumping current beyond what the motor can handle.

just curious, but you didnt give us any numbers for the "math problem" i.e. whats your voltage when you attempted to start charging? 10 volt? 9 volt? 11.5 volt?

As far as my experience with alternators, and this is just my opine, they are considered "good" at 13.5 -14 volts, however they are not meant to charge a dead battery but provide a maintenance charge. Thats why a schumaker or similar charger hits 14+ volts.

Now I know full well that you know these things; and an alternator charger is a great cheap way to bring a bank back to 13.5 I commend that solution... but unless you know for sure that your alternator is putting out 14+ I would say you have more of a maintenance charger than a charger. This is the exact reason i am such a fan of the predator 2500 inverter. Best $499 I have ever spent. 3 years no issues so far and I wont leave home without it even if i know my buddy is bringing his.

Edited June 15, 2016 by Totem

[zero]

I have a Honeywell 2000 watt inverter generator.  A REAL 2000 watts unlike the Honda at 1600 watts.  I just have been hesitant to leave it up in the UP in case someone breaks in to steal stuff.  I'm willing to lose the cheap Chinese generator I have there now.  We do not want to carry massive amounts of cargo everytime we go up.  We'd like to get to a point where we could go up in a small vehicle like our Chevy Tracker and leave the truck at home.  Thus why I don't want to truck my good inverter generator back and forth.  Same reason why I brought up all my cheap Poulan chainsaws and left my good Dolmar, EFCO, and Stihl saws at home.

The non-inverter Chinese generator I have there now runs at 127 volts AC.   Plenty high enough - at least going by my cheap, non-RMS voltmeter.   It still will only run my 30 amp Schumacher charger at 8 amps max, and my more sophisticated Iota 45 amp electronic charger at 18 amps max.  So either work, just slow. Lower tech battery chargers work at only the highs of the sine-wave peaks.  What we call "120 volt" AC is actually 170 volts if from the grid at the highs and lows, and many chargers only see power that is 145 volts or above.  Portable (conventional) generators are known for having erratic sine-waves that low-tech chargers don't use well.  A portable generator might show 120 VAC on a non-RMS voltmeter, but only have 150 volt peaks. If so, a conventional battery charger has very little to use.

My home-built gas generator with the 120 amp alternator works as it should as far as the alternator goes. I had an amp-meter on it.  It was spiking at a little over 100 amps of charge at 13 volts when the engine ran out of power.   When I first went to charge the battery bank, it was down to 12.5 volts.    For whatever reason - the 67 amp alternator I now have hooked to the 6 horse engine is just right.  Works the engine pretty good when making 60 amps.   Tapers off when voltage reaches 14.2 volts.

My eight batteries have the same amp-hour power as Trojan T-105 six-volt HD batteries.  Mine just are not as rugged or durable.  I bought them at Walmart for two reasons.  One was convenience.  Two was the ability to hook in parallel instead of series and use smaller battery cables.  I am well aware that a set of Trojan or Deka GC2 batteries makes a superior setup. If someone near to me had them for sale, I would of got them instead.   Sams Club was the nearest, in Traverse City and that is a LONG ride from where I live.

[Back East Don]

Mine was at 115 volts when I tested it under load. 127 volts under load should be good.  Only ran into this as my furnace had a yet undetected problem that didn't show up till one day I needed to run it on the generator.

[jjrbus]

My first suspicion would be the rated output of that 6HP HF engine.  

[Maineah]

Well alternator being 50 maybe at the most 60% efficiency and the 6 HP motor only being 6 HP at 3600 that may factor in. The power curves on fractional HP engines are pretty short.

[zero]

HF engine is likely a Honda clone.  Honda equiv. that is near same bore & stroke and rated 5.5 Net horsepower  @ 3600 RPM.  Peak torque is at 2500 RPM.

Hard to figure it all on paper (at least for me).  I tend to trust actual engine size rather then horsepower specs.   Honda 2000i is a 1600 watt generator with a 99 cc engine.  That comes to 16 ccs per watt.   My engine is 212 ccs so using the Honda model, it ought to be good for 3400 watts. One big difference here though.  My engine is not direct drive. My engine pulley is bigger then the alternator pulley so torque is lost. Why that matters, I do not know.

I don't know what alternator inefficiency has to do with any of these comparisons. I assume the alternator in a Honda generator has the same horsepower requirements as an automotive alternator. It takes X amount of power to overcome X amount of magnetic field, and the more amps being made, the stronger the magnetic field to overcome.

So, the answer is -  I have none.  I just know that my 212 cc HF engine works great with the 67 amp alternator making 67 amps @ 14.2 volts.  It cannot make 120 amps @ 14.2 volts. 

[WME]

Just not enough power from engine. WHY ?????

At 3600 alternator rpm you would be making about 100 amps. So select your pulley size to get there at the engines max rated HP.  If you can a 4" seems to be a good starting point

[Totem]

^ great point wme

[zero]

Yes, I could of changed the pulley ratios and therefore lowered the output of the 120 amp alternator.  In my case, it was much easier just to swap in a different alternator.   Those dual-groove, balanced cast-iron pulleys are not cheap.  On the other hand, I already had many good alternators on my shelf.  And the 67 amp Delco 12SI is a direct fit, bolt-in swap for the 120 amp Delco CS-144 I had on it. It took ten minutes to change it.

There also might of been a way to put a trimmer on the exciter wire and make the alternator cut-in slowly. I did not try it.  I have an "on" and "off" switch wired into the exciter circuit and the 120 amp CS-144 would turn on and off just by flipping the switch.   When I swapped in the 67 amp 12SI, once the alternator starts to charge, turning the switch to "off" does nothing.  Maybe slightly different circuitry?  I guess I don't need to know; it works fine.  It does seem though that the 120 amp alternator could of been controlled by limiting the exciter current (which is odd). 

I was originally trying to get the full 120 amp output and that requires the alternator to spin at 6000 RPM.  As I know now, gearing a 3600 RPM engine to spin the alternator at 6000 RPM needs more engine then I have.  If I lowered the alternator speed to 3600 RPM - it would be making 105 amps but the engine-to-alternator ratio would be 1-to-1, so less torque loss.  So maybe that would of worked?  I guess this is why the phrase "trial and error" exists.  Also reminds me of how much power is robbed from a little car or truck engine from the alternator.  Remember that most cars/trucks have a 1 to 3 ratio, from engine to alternator.  I.e. and engine idling at 800 RPM often has the alternator spinning at 2400 RPM or more. Even more torque loss then my setup and more load on the belt.

[Maineah]

Do you need to spin the alt. at close to 6 grand? Most gen sets are 3600 rpm but they are also 60 cycle at least they are in the power band I guess I'm thinking that an OD alt is sucking up too much power and the revs are dropping.

[Totem]

I'm still curious how you will fully charge a bank at 13.5 Volts output.

[WME]

Yea the laws of energy production always confound things.

I always have to laugh when the idea of running the house AC to save energy pops up. Why all you need is a big inverter and a 200 amp alternator, free power, and no load like the engine powered A/C. 

[Maineah]

On 6/16/2016 at 9:12 AM, Totem said:

I'm still curious how you will fully charge a bank at 13.5 Volts output.

If the bank is 12 volt that's sufficient a full charged battery is 12.7 volts does not matter if there is 1 or 20 batteries

[Totem]

13.5 volts output wont usually bring a battery to 12.7 after its disconnected is my experience. usually 14 is needed to do that. Most chargers and alternators that i have seen output 13.5 including the stock converter in my toy home, when disconnected the battery falls to 12.5.. the better chargers i have do 14 volts and will bring a battery to the voltage you speak of even after its disconnected from the charger for a  while.