Eu2000i-rectifier-MX60, charging

[toothy]

Hello All

I didn't want to hijack the other thread started by jnoble, but I have a sort of related idea. I know that is very dangerous for a fellow who knows less than nothing.

Brief system description:
12 sharp 123 watt panels wired 6 and 6
1350Ah Rolls battery bank @48 volts
10KW diesel generator
2 Outback vf3648's on FW100 panel
1 MX60

The system is still coming together, but I am concerned about winter production from the panels, I'm located in Seward, Alaska.

I am building a 10 foot otherpower turbine, that got me thinking about a way to top off the bank without running the 10KW diesel at it's least efficient.

Ok, here is my idea.

Run the EU2000i, 120 volt output through a rectifier and input it to the MX60 as DC??? Do the bulk charge with the diesel and finish it off with the EU2000i.

I understand that may be the craziest thing you have ever heard, if so please tell me to go back and stand in the corner some more.

Please keep it simple, as I'm way out of my depth.

Thanks
Wade

[icarus]

Why wouldn't you just run the eu into a big conventional charger, like a Xantrex or Iota or something similar? Or how about an inverter/charger?

I am certainly no expert on rectifying current, (only a potentate,,,what ever that is) but it seems like a solution in search of a problem.

Tony

[niel]

interesting idea, but don't do it as the peak voltage of 120vac is about 170v.

[BB.]

Regarding rectifier from the generator to the MX60... First problem is that rectified output of 120 VAC is sqrt of 2 * 120 VAC = 170 volts peak DC -- way over the input limit of the MX 60. So yo would need a ~2:1 step down transformer to bring the voltage down.

The other issue is a simple rectifier has very poor "power factor"... This will waste power, not work very well with the generator, and probably require a large capacitor (and perhaps inductor) to filter the rectified DC for the MX 60 to even hope to begin to function correctly... I would not recommend this process at all.

Your better bet, is to get a real 120 VAC battery charger that work well with your eu2000i (and/or whatever gensets you plan on using).

Using a proper battery charger with a "power factor corrected" front end will be better for the generator (more power, less waste).

The Xantrex line and probably the Iota have power factor correction. I would also like that the Xantrex has a battery temperature sensor option too (cold batteries need higher voltage to properly charge).

In your case, you probably will have to go with the Iota because they support 48 volt charging. Or, you may have to look around for a used forklift battery charger to run on your diesel genset...

You would want to charge your bank at ~5%-13% charge rate or ~68 amps (~4kW at 60 VDC output) to 175 amps (~11 kWatts)...

One eu2000i would probably be closer to a trickle charger range... Will work--but not very fast--and you would not be able to equalize your batter bank.

-Bill

[BB.]

Plus, I should add, that the solar panels themselves are pretty small for that large of battery bank... Ideally, you would want closer to 4kW minimum of solar panels to charge the bank... And/or add in Wind and Generator power (especially when equalizing).

Do you have a good estimate on the amount of power you use during the "sunny months"? Will the panels be enough, or will you need wind+gen to make up for a deficit anyway.

Generally, it is recommended to size the battery to the load (6x your daily load is a good starting point). Then size the charging equipment to properly support the loads+battery requirements.

-Bill

[toothy]

Thank you for you responses.

The dumbest question is the one you don't ask.

I understand my system is a work in progress. I don't have a good estimate of our power use, however I am trying to build with efficiency in mind. We presently use about 10 KWH per day in the less than efficient house we are in now, which is grid connected.

The reason I came up with the idea is because I already have all the stuff and float charging with the 10 KW Northern lights seems like a waste.

I have already started the wind turbine tower but have run out of warm weather. The 10 foot turbine was for practice and back-up I will be building a 17 footer. I have other things that have to be taken care of before freeze up which is soon.

One other question, BB you wrote "First problem is that rectified output of 120 VAC is sqrt of 2 * 120 VAC = 170 volts peak DC". Is there a name for that theorem , equation, relationship or whatever? When I spin my wind turbine by hand , very unscientific, without any load, the voltage seems similar AC or rectified DC. I am not doubting your statement but my test procedure and results.

Thanks again
Wade

[mike90045]

Originally Posted by toothy

One other question, BB you wrote "First problem is that rectified output of 120 VAC is sqrt of 2 * 120 VAC = 170 volts peak DC". Is there a name for that theorem , equation, relationship or whatever? When I spin my wind turbine by hand , very unscientific, without any load, the voltage seems similar AC or rectified DC. I am not doubting your statement but my test procedure and results.

Your meter is likely reading the AVERAGE voltage, not the PEAK voltages seen. You really need an O'scope to visualize this the first time. I don't have any images to post, but some of the equations are listed here: http://www.practicalphysics.org/go/Experiment_675.html
Another page here http://www.bcae1.com/voltages.htm has some sketches, but not really good ones.

[BB.]

Your average AC volt meter reads "Root Mean Square" voltage... Basically, this is the voltage of an AC wave form that has the same "energy potential" as a DC voltage would (sum of the squares, square root). Have to use RMS because power=(V^2)/R (power is equal to the square of the Voltage divided by resistance). If the voltage was linear (not raised to the 2nd power) the math to figure out the "power" would be like figuring out the area under a triangle 1/2 the base times the height.

The AC read the "peak voltage" of the AC wave form. And, using math, it turns out that the area under a sine wave is related to the peak by by the square root of 2.

A full wave rectifier just takes a the wave form that is centered around 0 volts, and makes it into a pulse train that goes from 0 to 170 volts DC--the RMS of which is still 120 Volts.

The other reason that this simple rectifier circuit is bad for using with a generator... When charging your filter capacitor, the current to charge the capacitor will only flow during the "peak" voltage (say from 150 volts to 170 volts)... This makes the peak current several times the "average current" that a normal generator/wiring would see... And because Power=I^2*R --- when you double the current, you get 4x the heating effect... This can cause your generator/wiring to overheat (this is also known as poor "power factor"). Over the last 10-15 years, many electronic power supplies (nearly all?) have been redesigned to have "power factor" corrected input circuits to get rid of this "peak shaving" effect.

-Bill

[crewzer]

12 sharp 123 watt panels wired 6 and 6

Do you mean six modules in series (72 V nominal, ~103 Vmp STC, ~129 Voc STC) with two series strings in parallel?

If so, this is not a good idea in AK. The temperature-corrected array Voc on cold mornings will surely exceed the MX60's 150 VDC maximum input voltage limit spec. It may go high enough to damage the controller, and, since the MX60 records the higherst Voc it ever sees, it may also void the warranty.

The MX60 will suspend operation at ~141 V.

A "60 V" nominal PV array (five 12 V modules in series) is probably the ideal configuration for charging a 48 V flooded-cell battery bank in your location.

HTH,
Jim / crewzer

[toothy]

Thanks for the information, reading them was easy, digesting them fully will be somewhat more difficult for me.

Thanks for the concern about max voltage output. I understand I am taking a small risk. Yes I have 6 modules in series with 2 strings. Seward has a marine climate, the coldest ever recorded was -19F the coldest I have personally witnessed was -6F and that was at night. I can't think of a daylight temp below 0 degrees F. If I have to shut the panels down for part of the day once in a while so be it. If I forget, that's called an idiot penalty and outback has a better quarter.

Thanks
Wade