Gen / Xantrex Charging Problem

[CharlieJ]

Captden-I would recommend the Victron Phoenix Multi Plus Inverter/Charger with a power factor of 1.0. Here is the datasheet: http://tinyurl.com/l6d87n

However, you should contact Victron and discuss your genset with them as you have a marginal generator. There is a significant difference in genset frequency control, load management, voltage and frequency stability, and total harmonic distortion. All factors when driving a charger. During the Winter of 2007, Victron did extensive testing of various generators powering their inverter/chargers. A summary report was written by Nigel Calder in Professional Boatbuilder magazine: Professional BoatBuilder - April/May 2008

Hope this helps and good luck.
Charlie

[donradcliffe]

CharlieJ--Nigel Calder has done another well-written article, which has significant design implications for the 'power hog' class of boat. Did he ever publish the promised sequel which compares gensets, or was that suppressed by the advertising revenue department??

[donradcliffe]

Captden--Do you have enough instrumentation on the genset to read voltage and amps output when trying to charge with the inverter and with other loads. Before you toss out the Xantrex, maybe you could check the genset output to see what amperage and voltage it is capable of putting out.

If your genset was reading 40 amps AC at 120v, or about 5 kw output when the charger is putting out 50 amps DC at 13v or about 0.65 kw, you would get a significant improvement from a better inverter/charger. If the genset is only reading say 10 amps, is it still capable of 40 amps or better if you put other loads on it?

Its not an exact science, and sometimes the problem is the instrumentation and waveform wierdness. I used one system where the Mastervolt inverter/charger would cause an 8kw genset voltage to sag to an indicated 98v with a 20 amp current, when the same genset would put out over 30 amps to other loads at 110v.

[captden]

Hi Don thanks for the input. I'm going to check out the current draw when charging this weekend. I would be very suprised if in fact the load is very high when the charger is running as I have run other loads at the same time with no problems, the two biggest 120 volt loads I have on the boat the hot water heater and the ac unit which is a 14,000 btu run with no problems and the gen doesn't even seem like it is loaded up. I do hear a slight drop in rpms when I add these loads but it doesn't appear to be very much. Will let you know after the long weekend.

Captden

[GordMay]

If you just measure the RMS Voltage and RMS Current and multiply them, you get the Apparent power.
As long as the load is purely resistive, the apparent power is equal to the active power.

For any reactive load (usually inductive, such as /w motors), where the Voltage and Current are out of phase, the Apparent power will be greater than the Active power*.

Measuring Active power is a bit tricky (especially /w non-sinosoidal waveforms), and is traditionally done by a watt meter (or more commonly a watt hour meter, like that measuring the electrical energy consumption of your house). Watt meters (and watt hour meters) instantaneously multiply the voltage and current and integrate the result, so they measure true Active power.

* True Active Power = cos (theta)* x Apparent Power
* cosine of the phase angle between voltage and current

[CharlieJ]

Captden-Gord has it right. Your charging problem has nothing to do with the instrumentation or the readings from that instrumentation on your genset (sorry donradcliffe). Look at the table in the Xantrex Technote that I posted earlier. The folks at Xantrex have taken waveform, power factor, peak voltage, etc. and condensed it into a table for the minimum sized genset required to run their inverter/chargers at near maximum output.

If you want to do the math (using Gord's formulae); your 5.5 kW genset (120 VAC @ 45.8 amps resistive load; "real power" noted by the kW units) becomes a: 120 VAC x 45.8 x 0.7 = 3.85 kVA ("apparent power", noted by kVA) genset when supplying the inductive charger function of your inverter/charger. And this does not take into account the debillitating effects caused by poor or marginal genset voltage and frequency sag management and stability, waveform, etc.

Before changing the Freedom, you should try and install the load correction capacitor that the Technote recommends. It should cost you about $35.00 and is probably worth the effort.

This all being said, your genset is too small to drive a Xantrex Freedom Inverter/Charger at anywhere near its rated capacity.

Hope this clarifies.

[captden]

Gord, Charlie
Thanks again for all the info. Yes I see my gen/set is to small to run the charger at max performance. Charlie: I did a quick search for the 50 MFD that is listed in the tech page from xantrex and the only one I found that was close was a 50 MFD 370 VOLT run capacitor. The tech note specified a 50 MFD/270 Vac which I assume the 270 vac means 270 volts a.c. If I remember correctly my genset is only 120 volt ac.
50 MFD, 370 Volt, "Run" Capacitor (2-Pack)

Is this the right one? Also it says to connect it between the hot and neutral wires which means across the line side wires at the genset?

I will try that for a temp fix but I think I may be better off putting a 100-200 amp high output alternater on the genset and charge the bank that way like I said eairler I will be doubling my house bank before I go and I would like to keep my charging time to around two hours a day using around 150 amp hrs a day.

Captden

[CharlieJ]

Originally Posted by captden

The tech note specified a 50 MFD/270 Vac which I assume the 270 vac means 270 volts a.c. If I remember correctly my genset is only 120 volt ac.
50 MFD, 370 Volt, "Run" Capacitor (2-Pack)

Is this the right one? Also it says to connect it between the hot and neutral wires which means across the line side wires at the genset?Captden

The 50 MFD/370 Volt run capacitor will work. You connect it between Hot (Line) and Neutral conductors at the genset, generally inside of the genset's connection box.

Originally Posted by captden

I will try that for a temp fix but I think I may be better off putting a 100-200 amp high output alternater on the genset and charge the bank that way like I said eairler I will be doubling my house bank before I go and I would like to keep my charging time to around two hours a day using around 150 amp hrs a day.

OK, but let's do a little more math. Your 5.5 kW genset probably has an 8-10 hp engine (5,500 watts/746 watts/hp = 7.4 hp). The general rule of thumb is that each 25 amp of alternator output requires 1 hp. 100 amp alternator requires approximately 4 hp to run. That leaves 4 to 6 hp remaining to power the generator end of the genset. More importantly, the side load placed on the engine by the alternator may exceed the design specs of the genset.

Be careful.

Charlie

[GordMay]

Originally Posted by CharlieJ

...Be careful.
Charlie

Indeed.

[donradcliffe]

I've done the math, and something doesn't add up...

I agree with Xantrex's numbers on required input currents for their inverter chargers, as I see 22 amps AC of dockpower into my Freedom 20 when it is putting out a solid 100 amps of DC at 14 volts. Converting over to KvA, my charger is using 22A x 120V or 2.64 KvA to put out 100A x 14V or 1.4 KvA. This translates to an AC to DC conversion efficiency of 53% at peak output, which is miserable by today's standards, and comparable to EPA's minimum requirements of 80%. If I say that the Freedom's power factor is 0.7 based on shorepower, which is more tolerant of capacitance/inductance mismatches, then maybe the inverter is really only using 0.7 x 2.64 or 1.8 Kw, with a resulting efficiency of 76%--not good, but perhaps believeable.

Then I look at their requirements for gensets, and they want a minimum of 6.5 kva from the genset in order to achieve 1.4 kva from the charger. Even if I assume the same AC-DC conversion efficiency of 76%, this means the Freedom's power factor with the genset is only 0.15!!!

The conclusion is that either Xantrex or the genset guys are lying about their specs, and it would be good to get some real life data.

Anyone have more data on how many DC amps their battery charger can put out when running on a genset, and how many AC amps it takes to do it??

[GordMay]

Originally Posted by donradcliffe

... The conclusion is that either Xantrex or the genset guys are lying about their specs, and it would be good to get some real life data...

Perehaps - or perhaps it's just a little more complicated than your calculations admit.
Real understanding, which is beyond my ability to communicate, might be even more illustrative than unexamined (unexplained) observation (real life data).

[captden]

Here's the info I have after testing the charger and gen set. What I did first draw the battery bank down 65 amp hours used and the bank voltage after sitting idle for a hour was 12.55. Charging the batteries the actual current draw on the genset was 8.5 amps with a constant voltage of 115 volts. Then I installed the capacitor across the line side of the gen output and the effect was about the same still 8.2 amps 115 volts. The voltage going to the batteries started at 12.55 and quickly rose up to around 13.75 with the current being around 37-40 amp hrs. When the battery voltage rose up above 13.75 the amp hours started to decline until the charger went into float. This was about the same with or without the cap. installed. I should also mention the these reading stayed about the same even when I increased the load on the generator from just charging batteries up to all the 110 volt loads such as microwave, hot water heater, ac unit, pc.

Now for a couple of questions, if the charger is only drawing 8.5 amps ac I would think running a 8kw genset with just a 8-10 amp load is not good for the genset, no? I do know when I was adding up my total 110v loads it seemed to me that a 5.5 kw gen would of been more than enough power I needed.

While on the subject of charging I have to say I was very suprised with the output of the wind generator I just installed. Over the past weekend it was able to keep up the the reefer after it was down to temp. In around 25 knts of wind I was seeing as high a 5.5 amp hrs being put back into the batteries and with the wind generator up on the mizzen the noise wasn't bad at all .

Captden

[GordMay]

Originally Posted by captden

... if the charger is only drawing 8.5 amps ac I would think running a 8kw genset with just a 8-10 amp load is not good for the genset, no? Captden

Yes, that's too small a load for that generator.

[donradcliffe]

I think your real world data shows that its not a power factor problem, as the 'apparent power' to the inverter/charger is still much less than the proven capacity of the genset. As far as I can tell, the real problem is that the Xantrex battery charger circuit generates harmonics in the system. The higher frequencies then increase its impedance and drop its power consumption and DC output. The shore power supply, with its big generators, long transmission lines, and other loads is much 'stiffer' and the harmonics don't become as big a factor as with the genset.

You could hook a scope to the AC system and look at both the phase angle of the voltage and current and the sine wave distortion which takes place under both shore power and genset power conditions, but these are academic exercises and won't increase the power output of the charger.

Changing out to a bigger genset will not improve the situation, as you are already only using a small fraction of the genset output, and that will drop as you increase genset capacity. If you really want to cut down the genset run time, you probably need to switch to a different inverter/charger, as Charlie suggests, or add a charger which is more compatible with your genset output. Unfortunately, the interactions are so complex that there is no spec you can refer to to assure that you have the right charger ahead of time, and swapping things in and out is really expensive. You might talk to the genset manufacturer and ask for their recommendations.

In the meantime, the wind generator will cut down on your charging time anyway...