MEP use in home PV system for topping up batteries

[Wrench Wench]

One use a MEP might be put to is simply as an emergency recharging system when (not if) the solar array fails, the sun and weather aren't cooperating to keep your battery bank charged, or if you simply have larger draws on your system than you scaled your battery bank and solar array for.

To that end, it would be necessary to rectify the AC produced by a MAP to a DC (even if pulsed) current to replace what the charge controller expected to see from the solar array. (We're gonna ignore issues like dual-dual conversion loses, like converting electrical power to battery storage to just convert them back to current, or converting from AC to DC for storage when the end use to which it will be put is AC again.)

My question is, with a 3-phase high current rectifier such as:

150A 800V 3 Phase Bridge Rectifier: Virtual Village - US

What would be the largest MEP unit to which it could be attached without over-current. (I'm presuming a voltage limit of 800 VAC would not be problematic for a family of generators that produces at most a nominal 440 VAC.) There should also be no problem attaching a split-phase MEP to this 3-phase rectifier, but considering that there are many more high power rectifiers at Virtual Village.com that include 2-phase units that cost much less, it would be foolhardy not to properly match the rectifier to the MEP.

I would assume that the DC voltage output by the rectifier would be the RMS voltage from the AC generator, but I'm a bit brain-addled from a month back in college, so I might be remembering that detail wrong. In addition, I can't think about how much power loss this thing could reasonably entail.

So, whattaya think? Would this do the job, and if so, what's the biggest MEP it would work with?

 

[m38inmaine]

I have a 24v 100amp MEP-026A I use as backup for my off grid system, no need to convert anything, the dc generators usually go cheap.

 

[Speddmon]

Oh, where to start....

first off, why???

When you rectify AC to DC the DC output voltage is not going to just be the RMS voltage, although it may be close (speaking 3 phase only here). The calculation is actually V(dc) = 2 x V(p)/ PI, where V(dc) is the DC output voltage, V(p) is the peak value of half wave, and of course PI is PI (3.14........). Power loss wil be minimal, because the voltage drop across a standard silicone diode is only about 0.7 volts.

There are some advantages to using 3 phase rectification to produce DC....if you want a good quality smooth DC signal there are less ripples using 3 phase rectifiers than any other, so less work is involved in smoothing it out. I have to assume that your PV array is feeding into some sort of converter to charge the battery bank simply because there wouldn't be enough voltage and amperage together to get the job done properly. So you actually need to get the DC output voltage in line with the voltage of the battery bank as well as have some sort of charge controller or you would run the risk of seriously boiling your batteries, and making one heck of a huge bomb out of them. The charge rate of most PV arrays is very slow. For the array to produce 150 amps, you would probably have to cover the whole neighborhood ( more likely an even much larger area) with cells to get that kind of power.

On to the meat of your question...to calculate the amperage from a given set use the formula...I = (KW x 1000) / (E x 1.73 x PF).

I = Amperage
KW = Kilowatts (Obviously)
E = Voltage
1.73 = Square root of PI
PF = Power Factor of the set

So...(85 KW x 1000) / (416 x 1.73 x 80%) = I (147.635 amps)
85 Kilowatts would produce the maximum amperage you could feed into your bridge, but an MEP-006 is 60 KW and MEP-007 is 100 KW. We are using 416 Volts here because when you get any bigger then the MEP-003a they are configured for 208/416 3 phase.

Keep in mind also, that unless you have a roughly 150 amp DC load, you are not going to pull 150 amps on the AC side...0 DC amps means 0 AC amps as well (or close to it). So wet stacking and loading issues are going to plague your set-up from the get go.

 

[poppop]

You people make my head hurt!!!!!

 

[Chevytruck]

You people make my head hurt!!!!!

big time

 

[cranetruck]

Use your MV charging system to top off your batteries.

Wet stacking shouldn't be a problem with the air cooled gas engine sets, diesel only...

 

[AN/ARC186]

While complex is nice sometimes I'd keep it on the KISS system on this one. Nothing more than a DC generator coupled to a small gas engine. A simple reverse current relay setup would take care of back feed issues and allow you to use the charge controller you have for the panels. best of all it is a very low buck build.
These days I'm steering away from solid state controlled anything for emergency power. Give me electromechanical any day for tough, reliable and best of all EMP proof.


(If you want to see just how ill prepared most of us are read "One second after")

 

[Isaac-1]

Just a thought here, but could this be a use for those otherwise nearly useless and therefore cheap 400 hz MEP generators.

Ike

 

[Speddmon]

it sure could....if you had a use for that much rectified DC

 

[Isaac-1]

It does not have to be all that much, they even made a 400 hz version of my little 3kw MEP-016B called the MEP-021B as well as a 28V DC version called the MEP-026B (a handful of similar 3 kw sized 28V DC units sold near here a on GL for a lot more than I expected, around $500 each, these were not MEP units, but had another abbreviation and hand/electric start yanmar diesels)

Ike

 

[Speddmon]

even at that, that's still a LOT of DC current available. 200 volts DC isn't going to do anybody much good, so it will need to be at a lower voltage.

3 KW 208 volts 3 phase is about 10.4 amps.

Step down the voltage before rectification to about 28 volts (MV uses) and you're talking about a 7.4:1 ratio. Voltage and current are inversely proportional so 10.4 amps x 7.4 = 76.7 amps available, without losses of course. (even more if you want to use it for 12 volts)...roughly.

You still need a way to control the current draw, or you are going to boil any battery you hook it up to, but that is one heck of a rapid battery charger.

 

[silvrghst]

MEP-002 65hrs +6
Hi
Just got my gen set on line and charging my battery pack of 12 6V at 200 ampere hrs each.
It took 6 hrs of running to charge then to 12.80Volts.
I have hocked it up to give me 120 volts only, But my gages don’t show any thing other then 125 volts
Also the hr meter is running back words.

 

[rogersn67]

On GL, I got a 28v 4.2 KW gasoline genset. It had 7 hours on it. The only thing I had to do was add oil and fuel to start it. I did take off the valve covers and prelube it, as I had no idea how long it had been sitting before I got the unit. IT started on the second pull. It is now charging a battery bank setup in my garage at times when there is not enough sun for the PV system. On the forst day hooked up, it ran for 7 hours and used less than 5 gallons of gasoline.

 

[abh3]

I'v seen the DC gensets sell pretty reasonably on GL, a dedicated DC set would be my choice. And if the right set came up on GL it would be an opportunity to own/operate a gas turbine!

 

[rogersn67]

I'v seen the DC gensets sell pretty reasonably on GL, a dedicated DC set would be my choice. And if the right set came up on GL it would be an opportunity to own/operate a gas turbine!

I had thought about the turbine too, but man do they suck down some fuel! Think about that before you go the route. I think the one I was looking at drank like 8.5 gallons an hour, if memory serves.

 

[abh3]

I'm talking out my behind here, don't have a PV array and can't afford one so DC generation for the battery bank is only a theoretical problem for me, LOL! Maybe one day...

Isn't that 8.5 gph at the rated output? Those puppies can make some power, it might be actually too much for recharging batteries unless you've got a huge battery bank...

 

[rogersn67]

I'm talking out my behind here, don't have a PV array and can't afford one so DC generation for the battery bank is only a theoretical problem for me, LOL! Maybe one day...

Isn't that 8.5 gph at the rated output? Those puppies can make some power, it might be actually too much for recharging batteries unless you've got a huge battery bank...

The turbine drinks almost the same at no load as it does at full load, but yes, unless you have a huge battery bank, they would probably boil off (best case), or explode (worst case) the batteries. I have mine setup with a pretty big load bank (40 gallon Water heater with 24 volt heating elements) to be able to shunt the excess power to as the batteries get topped off.

 

[storeman]

Mind if I ask a stupid question?

Why couldn't one just draw off the 24 volt charging system on the MEP 002 or 003?

Jerry

 

[Keith_J]

Mind if I ask a stupid question?

Why couldn't one just draw off the 24 volt charging system on the MEP 002 or 003?

Jerry

Because it is 7 amperes. Barely keeps batteries charged if you aren't running it but for 15 minutes at a time. And that is without any preheat. Voice of experience here, I've been using mine to power my welder and because I only have a 6 foot power cord plus 15 feet of work lead (but 25 foot TIG torch cable+hose), I shut it off when not actually welding. Batteries started out at full charge and the battery voltage gauge is in the middle green (white hash mark), now it is barely green.

I disconnect the batteries after each day.

If the power off this generator were transformed and rectified to 28 volts DC, it would be over 200 amperes.