# generator capacity vs. charger needs

crancreature
Registered Users Posts:

**10**✭
I have 5kw Kohler, 1800 rpm watercooled, 4cylinder, nameplate reads 43.5 amps @ 115 volts, it is a multifuel by design, running propane for fuel, it is in excellent working order and from the early 1960's. I'm using solely to charge battery bank, I currently have a 90 amp Iota, per their specs at 21.8 amps. I would like to add another Iota, if I go with another 90 amp it puts it per spec at 43.6.

If I buy too much charger presumably it will bog down the generator, on the other hand the idea is to maximize the charge for best fuel economy and quickest filling of battery bank.

Does anyone have experience maxing out their generator in this regard to it's capacity, should I play it safe and drop down to the 75 amp charger at 18.2 amps (for total of 40 amps), or should I try for the home run getting the most charging capability?

Batteries are 6 trojan L16re 370 6 volt arranged for 12 volts, total 1110 amphours at 12 volt, chargers at 12 volts of course.

Thanks

Greg

If I buy too much charger presumably it will bog down the generator, on the other hand the idea is to maximize the charge for best fuel economy and quickest filling of battery bank.

Does anyone have experience maxing out their generator in this regard to it's capacity, should I play it safe and drop down to the 75 amp charger at 18.2 amps (for total of 40 amps), or should I try for the home run getting the most charging capability?

Batteries are 6 trojan L16re 370 6 volt arranged for 12 volts, total 1110 amphours at 12 volt, chargers at 12 volts of course.

Thanks

Greg

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## Comments

8,821✭✭✭✭✭I'd tested the PF on my Iota, and I recall it was poor, like about .6

Does your genset have a single 115V winding, or a pair of 115 phase A, and phase B ?

To keep the genset happy, you could connect 1 charger, and after a short while, when the batteries are not sucking the juice as much, connect the 2nd. You may have to rely on the tone of the genset, or watch for black smoke on the exhaust (overload). It's internal breaker should preserve the generator half.

But the power factor of the Iotas will be the kicker, with all the phase weirdness, the breaker may kick out from 2 of them. Do you have a Killawatt to test with ?

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10✭Yes, that's a good idea, check to see actual numbers on the charger draw, particularly on low batts, will try this tomorrow as batts are down today, see if I can get a feel for it. I haven't purchased the second Iota, and only will go with the 90 amp if the gen can for sure handle it, wouldn't want to buy a charger I can't run.

30,642adminRemember there is a difference between Watts and VA measurements.

Watts = VA * Power Factor (PF)

In general, smaller/home type gensets are rated at Watts=VA for maximum output.

A poor Power Factor load should not exceed the VA rating of the genset (even though the Watt rating of the load is way under the "Watt" rating of the genset).

I.e., if your load is drawing 1,100 Watts and has a 0.6 PF:

1,100 Watts / 0.6 PF = 1,833 VA

So--As far as the generator electrical ratings are concerned, the 1,833 VA would be the limiting factor when loading your genset.

-Bill

10✭I called Iota tech support line, the person advised only running the DLS 55 amp charger (coupled with my existing 90 amp). This makes a nameplate total of 35.2 amps max draw on the 5kw generator (43.5 amps on nameplate of the generator). Among the issues as mentioned, the nameplate ratings of all these are not black and white. It's disappointing, but I'll go with the recommendations and not push it. It's also cheaper, and my solar is doing most of the heavy lifting anyway, and after I try it and measure it I will know whether I could upsize someday.

I'll admit I don't understand the Power Factor calculations. As I thought I understood it, a .6 power factor would mean, in this case, the charger only converts a fraction of the input into the charging output--- a waste of fuel, inefficiency, and so on. If I were doing the math backwards from, say, the 90 amps of my charger, then I would have to account for this in determining the wattage required to run the charger. However, my number for the current draw of the charger is coming from the technical specs chart from the Iota company--they would have had to have accounted for the PF already, correct?, or everyone would always have to upsize wires and breakers, and what would be the use of publishing a technical spec like that? I believe the Iota tech derated the generator output when doing his math, and was concerned about the way the charger uses the wave--or maybe the way it is using the electricity IS part of the power factor--but again wouldn't this be accounted for in the nametag rating on the appliance?

30,642adminPower Factor is a "catchall" term. There are two major ways that PF is "affected" by the operation of AC power systems.

One is the "phase angle" between voltage and current. The second is the current profile. A couple of analogies:

Phase angle. Imagine you are pulling a rope. If you stand in front of the car and pull, the angle on the rope to the direction of travile is zero degrees. Cosine zero degrees = 1.0 -- I.e., 100% of your pull is going into moving the car forward.

Now stand 60 degrees off to the side and pull the car forward. Cosine 60 degrees = 0.5 or 50% of the pull is moving the car forward, the other 50% is trying to pull the car to the side. So, to move the car forwards just as fast, you would need to pull 2x harder (1/0.5=2). That is the "phase angle" discussion and usually applies to inductive loads (like AC induction motors). It also applies to capacitive loads, but those are not very common.

The second is the current wave form. In the Iota's case, this is the issue. It only pulls current at the very peak of the sine wave. Like standing in front of the car and pulling the rope at 2x force for 1 second and 0x force for 1 second. The average force of pulling the rope is still 1.0 -- But you need a 2x stronger rope to keep from breaking it.

Here is a scope picture of the voltage sine wave and the current draw... Note that it spikes in the center of the sine wave.

Attachment not found.

And things like heating from wire resistance is P=I

^{2}R -- So if the current during that short time is 2x higher, the self heating is actually 4x higher (square of the current).That is why VA matters and is different than Watts: Power=Watts=V*A*PF=V*A*Cosine (phase angle)

In terms of a generator, the self heating (and magnetic saturation, etc.) only "care" about the current draw (VA rating). The motor and fuel consumption (ignoring other losses) only cares about Watts of the load.

So, you could be running a genset at 100% VA and only 50% fuel flow if your loads had a Power Factor of 0.50 or 50%.

Here is a short Xantrex paper on Power Factor.

While Iotas are a good power supply (rugged, meet specifications, work as you would expect--Not all battery chargers do this). They are not terribly efficient. The Iota is something like a 0.67 PF (probably changes with charging current) and a "modern" Power Factor Corrected (PFC) type battery charger runs ~0.95 or better PF (and probably closer to 90% efficiency vs ~80% efficiency for the Iota). You can run almost 2x more charging currrent with a modern charger (or inverter-charger) vs the Iota.

But, PFC battery chargers are expensive, and still seem to be relatively rare. There are other advantages to PFC power supplies too (one is they can run from 90-264 VAC 47-63 Hz).

A calculation for Iota (vs something else) would look like (note Iota rates at 108 VAC--Your genset may be ~117-125 VAC:

- VA rating = 14.5 volts charging * 90 amps * 1/0.80 eff * 1/108 VAC minimum * 1/0.67 PF = 22.54 Amps (calculated)

A "typical" PFC charge controller may look like (running at 120 VAC nominal:- VA rating = 14.5 volts charging * 90 amps * 1/0.90 eff * 1/120 VAC minimum * 1/0.95 PF = 12.72 Amps (calculated)

Before we get into details too much more--I am pretty sure of my calculations above--However, we have poster "BlackCherry04" who has done measurements on his equipment and even purchased one or two other brands/models of PFC battery chargers. And his experience shows that Iota chargers are not near as bad as I have calculated (Amps/VA is much lower).If you already have the Iota--Get a good quality RMS reading AC Clamp DMM and measure your current loads.

And note that for "typical" non-commercial quality generators, frequently you will want to derate their maximum output by 80% to keep from overheating the engine (some sometimes generator head windings/wiring).

Xantrex makes a TrueCharge2 series of PFC battery chargers... On paper they should be very good, and the TC1 seemed to be a good product. However, the TC2 has been (in years past, late to market, late/no deliveries???) difficult to find in stores. I do not know anything about their quality or how well they run--But they are probably work trying one to see if it can help meet your needs.

-Bill

10✭That's a very understandable explanation, the way the Iota grabs its power out of the sine is not all it could be, resulting in the need to make PF correction. I see in addition you quantify an "efficiency" which is in addition and on top of the PF correction. My RMS clamp meter says the draw at the generator is 14.8ish AC amps, while delivering full 90 amps (88ish), 117+-volts, this is less amps than I expected, batteries make the charging/under load voltage read about 12.8 during measurement-- due to very drawn down 1110 amp hour batt bank.

Without trying to make the equation work, I think my understanding is that if my clamp meter reads 14.8 at the generator, I actually need my generator to be pumping in .6 pf more to account for the ineffective way the Iota will draw off the sine wave, which means my genset must be accounted as producing 24.6 (at time of measurement) or risk running hotter than generator capable of happily doing, in the case of my generator probably more a winding/generator heating issue than a motor issue.

This is handy, if I add another Iota charger and use the .6, I can measure and be sure to stay under genset spec.

A note on my genset, I would characterize as commercial duty with few miles, previous life telecom backup, from the early 60's.

Another note, it seems the tech spec rating for the Iota means... what? Does it account for PF? or not? Power factor would not affect a grid plugged charger, wouldn't change the bill, wouldn't require different wiring or breakers, maybe some effect down at the dam or nuclear plant?

30,642adminIf you use a true RMS reading meter, then the amp reading is the true amp reading of the VA equation.

Also, if you are reading 14.8 amps on the genset, you do not need the 0.6 PF correction, you are measuring the value of current.

Note that the 14.8 amps may increase as battery voltage increases (see equations].

-Bill

10✭I studied up on the true RMS measuring. The summary is that common measurements assume a nice sine wave and come to their AC measurements with that assumption. However, as you have pointed out and posted graph of, the draw of the Iota charger amounts to a different wave, it takes current at uneven places in the graph of the voltage compared to sine wave. What the RMS method does is find a correct average of the current no matter if it matches sine wave or not. So if my true RMS measurement is 14.8, then, from the perspective of the copper inside the generator, it's 14.8 regardless of the shape of it. I assume a true sine wave when measured with RMS are equivalent values, and I assume my generator ratings are based on something close to a sine wave(?), and the basic principle of RMS is it amounts to DC equivalent. So my genset is rated by nameplate at 43.5 amps, and this is a true DC equivalent measure?... and can be used to compare against my 14.8 Iota measurement?

So the world of true RMS makes the power factor discussion go away?

I'm thinking of getting a second Iota--55amp as recommended by the Iota tech--then keeping some good measurements, might be able to add a third small Iota later if I think I can get away with it.

30,642adminYep, you pretty much got it right. RMS is Root Mean Square... With "cheap meters", they simply measure the peak voltage or current and apply a conversion factor based on a sine wave. It is based the area under the curve (and squared value of the measured component).

For a sine wave, it is peak voltage divided by the square root of 2:

https://en.wikipedia.org/wiki/Root_mean_square#RMS_of_common_waveforms

And RMS is the DC equivalent voltage (and current) measurement for power/work.

So, in theory:

43.5 Amps RMS genset / 14.8 amps RMS Iota = 2.94 loading factor for one 90 Amp supply

Or

2.94 loading factor * 90 amps = 264.6 DC charging amps of Iota chargers based on your measurements.

In any case, it would appear that you can easily drive 2x 90 amp @ 12 volt chargers based on your present measurement.

Of course, if the Iota draws more current when the battery voltage rises to 14.x volts and/or you have some drooping on the AC voltage input, the current draw could be higher.

Also note that "non-sine waves" have harmonic content that (typically) odd harmonics of the fundamental frequency of the AC generator (typically 60 Hz in North America). These harmonics can play havoic with windings and field pieces (iron plates) in the generator head and make it run hotter than normal too... Keep an eye on the generator head operating temperatures (and regulator component temperatures) until you are sure everything is running OK.

As you get closer to 100% loading (VA or Watts), the secondary effects can cause more issues.

-Bill

3,738✭✭✭✭As has been explained, the PF makes this untenable. Another consideration is that the Iota's have HUGE inrush currents, 40 amps AC for your charger. It is brief, but some generators don't like it.

Even if the Iota were PF corrected, 180 amps of charging is too much for your battery bank. If the charging current divided equally between your three strings of batteries, that would be 60 amps into a 370 ah battery. That's a bit high, but since the charging current does NOT divide equally, you have the potential to have nearly 180 amps into a single string. That really can happen... especially if a cell shorts out, but in other circumstances also. For example, if a cell in one string becomes high resistance, the current would be diverted into the other two strings (90 amps per string). That 180 amps into two remaining strings will not actually divide equally... The stress may quickly knock out one of the remaining strings and then you have real trouble.

Why do you have so much battery capacity at only 12 volts? A 24 or 48 volt system would be more efficient and you could avoid the many issues that arise with parallel batteries.

If you must stay at 12 volts, there is a better way... you can buy L16 sized 2 volt batteries. Six of them would have the same capacity as your bank with more reliability, longer life, fewer points of failure, etc. If you do go that route some day, I would suggest that you not buyt Trojan's offering... Trojan make very good batteries, but their L16 sized 2 volt battery is actually three 2 volt cells in parallel in one case. That means three times as many cells to water, measure SG, etc.

--vtMaps

3,738✭✭✭✭Sterling makes a line of PFC battery chargers... On paper they should be very good. I do not know anything about their quality or how well they run. I have my eye on one of their 24 volt models, and am waiting for someone to try one and post a review this forum. It could be you!

--vtMaps

10✭I think you are right, I was sort of thinking 15% charge rate, which I think is mentioned quite a lot on this forum, but 180 into 1110 bank is more like 16.1%, but when I do the math on my 90 amp charger+an additional 55amp charger (total 145 amps), it turns out to be almost exactly 13%, Trojan recommends a 10-13% rate.

Greg

5✭