Sizing Off-Grid Systems And Using Gen Support

great now I get to relearn everything i thought I knew
Thank you guys (no sarcasm intended)

Good morning Chris,

You said you don't do EQ on your batteries, can you tell me how do you keep your batteries full charge at least once a month?

paulcheung - they actually get fully charged about 10 times a month. The solar panels charge them up with no problem if we get a couple sunny days. What we do different than most folks is that when poor solar conditions set in for a long period of time we do not run our batteries down and then recharge them with a genset. We will only do that if the poor conditions last for 10 days or more and they get down to 80% DoD. When that happens the big genset automatically starts and bulk charges them, but the system stops it when they reach absorb voltage.

We manage how fast they go down during these periods by running our little generator periodically to take the load off them and make them last until the next sunny day.

As I stated originally, this is a very, very hard thing for people used to the "conventional" way of doing it to understand. But it has to do with efficiency and energy conversions. Our loads are AC. We run a little generator at peak efficiency directly powering AC loads. This is much more efficient than running a large generator supplying AC to an inverter that converts it to DC to charge batteries, then the inverter converts the DC back to AC again before it gets to the loads. When you charge batteries with a generator to power loads on a AC system, you lose in the range of 30-40% of the energy you produced with your generator in conversion losses.

The key to making all this work is an inverter with generator support. An inverter with generator support simply kicks in and limits the load on the generator to it's max rating and helps it out while the heavy load is on. So the key to getting peak efficiency from your generator is to run it at 80% rated load or better all the time it is running, and only use it to power AC loads. This requires a generator that is smaller than your base loads, and a regulating system (inverter with generator support) that can fill in the difference.

It was actually Heart Ackerman and Trace Engineering that invented this concept 20+ years ago with the Trace SW. And the Trace SW-series became legendary in off-grid power. Trace was eventually bought by Xantrex in Canada. Xantrex enhanced the SW and it became the SW Plus, which was even better than the SW Series II. Meanwhile, there was lots of contention between the guys at Xantrex and the guys that formed Trace. Those guys left and formed Outback Power. They invented the Outback FX-series, which again became legendary in off-grid power. However, they were competing against Xantrex and the Outback FX was scaled to meet a price point. Generator support made it into the menus in the Mate but it never made it into the hardware because it became a thing where whoever can sell the most inverters wins. The off-grid market was getting pretty slim and the Outback FX was a $1,800 inverter while the SW Plus was a $3,800 inverter. So the Outback is missing things like integrated generator controllers and generator support, but lots of people buy them because they're reliable and much cheaper, and most off-grid systems are quite small.

In the meantime, Xantrex designed and built a completely new split-phase hybrid inverter in 2005 - and the XW-series was born. The market had gone primarily away from off-grid by that point and more to grid-tie. The XW could do both with split-phase power that did not require using transformers or stacking inverters on split-phase grid-tie systems. The XW literally kicked Outback's butt because they didn't have anything that could compete with it. Outback went broke and was eventually bought by the Alpha Group who owns it today.

Meanwhile, Schneider Electric, one of largest manufacturers of industrial control systems and electrical equipment on earth (Schneider is the maker of SquareD equipment) decided to get into the renewable energy business in 2009. They bought Xantrex to get their technology and expertise in inverters and solar power equipment. Today the Xantrex brand is concentrated on the RV, marine and mobile markets. The XW-series is now being rebranded into the Conext brand. The Conext inverters range from the SW-series (small brother to the XW) in battery based, to the 1,250 kW Core XC used in solar power generating plants.

So that is how it has progressed over the years, and to this day there is just a handful of inverters capable of doing generator support for off-grid. You can just about count the inverters that can do it with the fingers on one hand.

Thank you, another question, how long your battery get full charge from absorb start? According to the manual my 4000 series Rolls should take 4 to 4.5 hours to absorb charge. how long yours take? I am asking because it seem mine never get fully charged even I use the grid to charge it for over 6 hours. The SG reading never past 1.260.

Thanks again.

For us, it depends on how deep they've been cycled. If they've been pulled below 50% DoD it can take two days to get a full recharge. On the first day it gets them up to absorb and absorbs for awhile and then the sun runs out for the day. They start the next day at a higher SOC and can usually finish charging. I'd say, sometimes, total 7-8 hours spent in absorb, but not all at the same time, to bring them from 70% DoD to 100% charged.

If they're shallow cycled, like on several nice sunny days in a row, it only takes 1-2 hours usually. The system exits the absorb as soon as the battery current gets to 2% of C/20.

Your 1.260 SG is not low. 1.255-1.275 is the normal full charge SG range for 4000-series.

And just to cover all the bases, that is a temperature adjusted SG value you are measuring, right?

No I just measure it on the hydrometer, I think the temperature is about 90* to 95 inside the battery while absorbing.

So if my SG reading is above 1,255 I should not be worry about sulfate problem?
Thanks

You should not be worried about new sulfate accumulation and you can be fairly confident that any existing sufation has not reduced your capacity much if at all.

No, you're fine at 1.255. I've never seen ours at 1.265 unless they get absorbed many times in a row in summer. Any positive gain in SG means some of the chemistry changed (sulfate going back into solution) to make the electrolyte more dense. But hydrometers are typically not much more accurate than 5-10 points so when you got a reading of 1.260 you're looking at 1.255-1.265 and you're fine. And not applying the temp correction to a non-temp compensating hydrometer throws it off some more.

Have you ever seen those hydrometers that have the red, white and green areas on the scale? Basically if it's in the green she's good because that's about the accuracy limits of a non-laboratory hydrometer that has not been calibrated. I've tested two identical hydrometers bought off the shelf before and get readings 10 points apart on them in the same cell. Which one was right?

Which is a good argument for keeping some standardized electrolyte mix around to calibrate your new hydrometer.

Yeah, I got a Chinese one around here someplace that claims the specific gravity of distilled water is 1.15

Well, I always measure the SG reading when they are absorbing in high voltage at about 61 or 62 volt; they hydrometers I have are 3 different ones, one with 4 plastic coins and one with 3 colors in it and one plain, the plain one is so hard to read as the print are so small and I get up to my age. I mostly use the 3 colors. it is easier to determine the reading and it has temperature gauge too, but I don't know how to apply the temperature compensation, guess have to learn it.

The reading is always in the white near to green, the middle of white is 1.250 and the middle green is 1.275; so it is always in the line of white and green so I would say 1.255 to 1.260 there about and if I shake it a little it might go up to green a bit more.

Thank you guys.

A lot of these cheaper glass float type hydrometers have a float with lead shot in the bottom. Then there's a tube above the lead shot with a scale printed on paper that is rolled up inside the tube. The paper scale ends up at random locations in there when they make it and some of them aren't even glued. The paper scale slides down a bit in the tube and then gives an abnormally high reading. One of those $5.99 plastic Deka hydrometers that has a floating needle that pivots on one side is more accurate than those cheap glass float types that come from China.

This is the one I have.

OTC 4619 Professional Battery Hydrometer