Low battery disconnect + fall back charger?

[wbustraan]

Hello,
I am working on building a battery bank system for my house to run just the essential systems. However, given the winter that we are currently experiencing, I'm wondering if there is a way to set up a back up charger for extended periods of no sun.

I know that the Classic has a Low Bat Disc mode on Aux 1. When the battery voltage hits the low setpoint, it seems like that could be set up to trip a relay that turns on a standard AC battery charger in order to bring it the voltage back up to a "safe" level. As I understand, leaving the batteries at below 2.10V/cell for an extended period of time can cause sulfation damage.

The problem I see with this may be related to my understanding of the way the battery bank voltage is measured. If, for example, a 12V battery bank voltage has depleted to 12.6V (2.10V x 6cells) and that was the setpoint trigger for Aux 1, then the Classic would turn off Aux 1. If that was hooked up to a normally closed relay that turned the power on for a (for sake of example) Schumacher battery charger, it could then begin using mains power to bring the bank back up. However, I would assume that the AC charger would apply a voltage around 14.4V to the terminals of the battery bank. Would the Classic detect this 14.4V and think that the battery bank is now charged and therefore turn Aux 1 back on? Or is it able to distinguish between a voltage from the battery bank and a voltage being applied by an external charger?

If it cannot distinguish, then I considered setting the delay time to something like an hour so that, after an hour, it would turn the external charger back off, measure the voltage of the bank, and then trigger Aux 1 one way or the other depending on the voltage level of the battery bank.

Am I off track on this? How do people keep their battery banks from depleting to damaging levels outside of disconnecting everything and going without power?

[Westbranch]

Your question is not clear.
Are you asking about what happens when your Mains power is down or when it is up?
What PV panels do you have? Number?

From what you describe , if you don't have panels you are in trouble if it is a long outage...
What inverter do you have?

[boB]

Yes, you can certainly do this with Aux 1.

I was thinking the same thing about having Aux 1 stay on for about an hour after the voltage
on the battery rose above 14.x volts, also.

Yep, not a bad idea.  You ~could~ even have it start a generator if wanted.  However, without
feedback from the generator, the system would not know if the generator started and might
keep trying to start it and might not be good for some generators.

boB

PS, I wrote that before seeing Westbranch's posting.

[wbustraan]

I don't have a fully built system yet. Still working on purchasing the components. So far, I have 6 245W panels. I discovered yesterday that the inverter I have won't work; I need 240VAC and it won't do it.

I want to basically have two electrical systems, one for essential systems and one for non-essential systems. I plan on starting small with the essentials systems; probably just the well pump to start. (I know, not really a "small" load, but it's probably the most critical system I have) Then, depending on battery bank capacity, I'll see which systems get moved from one to the other. Probably the furnace fan next. There are a lot of devices that I never plan on migrating. For example, I'll never run a washer, dryer, air conditioner, entertainment system, etc on a battery bank system. For all of those, if I don't have grid power, I'll have to do without.

So, the desire is for the essential systems to run off-grid as much as possible. However, having 6*245W worth of potential solar power isn't worth anything if the sun doesn't shine for a week and my batteries are depleted. I'll need a secondary way, other than solar, to charge the battery bank. So, the thought was, assuming the grid is available, why not use that in a pinch? Hopefully, though, the sun will come back and, at that point, the AC charger would need to step out of the way and let the Classic take control of the battery bank.

[Westbranch]

you should investigate a low cost 240v generator to run all your heavy loads, otherwise this emergency system is going to cost you a lot... for charging in extended outages a smaller inverter type generator (~2000W) will run all day charging and only use ~ 1.5 gal or so of gas. 

Lots of options but the best way to start is to write out a plan based on the measured loads you want to run off solar.  That way you have a target to achieve rather than an idea...

[vtmaps]

I don't have a fully built system yet. Still working on purchasing the components. So far, I have 6 245W panels. I discovered yesterday that the inverter I have won't work; I need 240VAC and it won't do it.

Stop and rethink... You are going to need an inverter for your critical loads.  Why not buy a grid-interactive inverter?  They are designed to provide backup, maintain a battery from grid (if needed), and will get along with your solar controller.  They also have built in transfer switches. 

But actually, I'm not sure what you are trying to do....
Rereading your posts, it looks like you intend to cycle these batteries every day to run your loads, even when the grid is available...  If so, why?  In the long run it will cost you much more than just running the loads off the grid.  If this is your goal, I suggest that a generator may be a more cost effective way to deal with occasional grid outages.

--vtMaps

[wbustraan]

I have to confirm this officially, but the word is that in my area the power company does not allow grid-tied systems. It could be that I've gotten some misinformation and I would have to contact them directly about it. That said, I'm not entirely comfortable with the power company being a stakeholder in what I do. If I'm not connected to them, they have less ability to dictate what I do.

We live in an low population density area where there is not a lot of infrastructure. There is basic rural electric and landline, but no city-type services like natural gas, sewer, internet (we have satellite), etc. We've experienced power outages during the cold months this past year and it's something I want to eliminate.

Sure, I could go the generator route, and I do have one, but this past Christmas there were people in a neighboring county that were out of power for 4-5 days. Thankfully, we were not out that long. That's a significant amount of fuel to maintain.

When I see people building solar systems, I keep seeing three typical systems:
1) Battery bank for backup only - I don't quite understand this. Why go through the expense of building the system only to have it sit there unused 99.9% of the time. And, since you're not using it on a daily basis, you won't know that when the power does go out, the bank is undersized and will only last a couple hours.

2) Whole hog - Put everything in the house on solar, no grid at all. This seems like overkill. These are the people that are are trying to get their air conditioner on their solar system. I'm mainly interested in being off-grid for the systems that need to be off-grid.

3) Grid tied - I know there are more expensive models that can maintain a battery bank, but the typical grid-tied inverter is useless when the power goes out.

I'm looking for somewhere in the middle between backup-only and whole-hog. I would like to size the system so that it's large enough to not drain the bank on a daily basis. However, I think that's going to take some experimentation to get the variables right.

For example, I had the county office pull the records on my well pump yesterday and it's a 3/4 HP, 230V pump. Google tells me that 3/4hp is roughly equivalent to 560W. Theoretically, my 6 245W panels could put out 1470W. Of course, that's purely theoretical; in actuality, I know it will likely be somewhere significantly lower. Predicting that, I feel, is very much like trying to predicting the weather; I figure it's going to be easier to estimate the right numbers, build it, and then scale up or down based on actual measured results. Similar to the sun's unpredictability, while I know the theoretical power draw of my well pump, that's only when it's running; it doesn't run all day long.

I'm basically looking for a safety net such that, if I put my well pump on a battery bank system and I've underestimated the inputs and outputs, I would like a little breathing room, perhaps provided by a grid charger, while I work on scaling things up. When funds allow, I have considered getting a wind turbine and a second Classic, because here, when the sun isn't shining, the wind is usually blowing. The goal, though, isn't to cycle the batteries from full to depleted all the time; that would be hard on them. If that's happening a lot, then it indicates to me that the system isn't keeping up and something needs to be done.

If it goes the other way, and I have way more than I need to run the pump, then the next step is to add another critical system and see if the system can handle it without needing an auxiliary power source.

As far as the inverter goes, I thought I had a 110V pump and then found out otherwise.

[boB]

Weston, are you using one of those Co-Op utility companies ?

[wbustraan]

Nope, just Consumer's Energy in Michigan. It's possible that the information that we got was bogus. The person said that we couldn't do grid-tied because we were in the vicinity of a hydroelectric dam. But they were not an employee of Consumer's.

I'm curious though, what is it about my scenario that's leading people to recommend a grid-tied solution? I mean, the MidNite Classic is designed to charge a battery bank; if I do grid-tied, I don't need a charge controller.

[boB]

Well, for straight grid tie without batteries (backup power) you certainly would not need a Classic or any "charge controller" for that matter...
You would most likely need a PV combiner though.  Maybe that's why they were recommending MidNite ?

I found this web page about your utility...

http://www.consumersenergy.com/content.aspx?id=1800

It ~may~ be that you can only offset your own power usage but I did not read the entire page.

You can also check out net metering info state by state here which is where I started from...

http://www.dsireusa.org/

Not sure about the proximity to a dam but I wouldn't think that would matter, really.  But I'm not positive.

boB

[vtmaps]

I'm curious though, what is it about my scenario that's leading people to recommend a grid-tied solution?

I did not suggest a grid-tied solution (although if it's available you should consider it).  I suggested a grid-interactive inverter.  That means it can use the grid to charge your batteries.  That doesn't necessarily mean that you can sell to the grid. 

I still don't understand why you want to have, and cycle, a large battery system.  The power you produce through your batteries and inverter will be MUCH more expensive than grid power, even if you recharge the batteries for 'free' with solar panels.

Perhaps a small battery system plus a generator would be more cost effective.  Run the generator a few hours per day for well pump, refrigeration, etc, and then use the batteries for 'quiet time'... lights, computer, tv and other small items that don't need the power of your generator.

--vtMaps

[wbustraan]

Ah, I see the disconnect. While a cost efficient system is certainly desireable, it is not the primary motivating factor for me; independence is the primary goal here. Right now, I am dependent on the power company to live my life. That means that if I am unable (or unwilling) to pay my monthly power bill, I will soon experience a crisis. Right now, I have a good job and the electric bill is an acceptable amount, but that may not be true tomorrow; I could lose my job or the price of electricity could go through the roof. I want to adjust the balance to where having a connection to the grid is a convenience, but not an absolute necessity. In fact, I'm very tempted to label the 2 electrical systems, "want" and "need".

Several years ago, at our previous house, we discovered that the power company had been reading the wrong meter for the last 4 years; we were actually paying our neighbor's bill and they were paying ours. My wife figured it out because she had been try to lower the bill by turning down the heat, keeping lights off, etc, but no matter what we did, the bill kept increasing. Meanwhile, our neighbors found that they could use whatever power they wanted and the bill kept going down. After we convinced them that there really was something wrong, it took the power company 6 months to correct the error in their billing system, and in the interim they told us, "We don't really know what you owe us; so just guess and send it in". Then, once the error was resolved, they told us, "Oops, our bad. Yup, you overpaid, but it's going to take too much work to figure out how much we owe you, so we're not going to do that. Have a nice day!" My wife estimates that the amount we overpaid over the 4 years was at least a couple thousand dollars. So, you can see why I'm not too excited about the idea of selling power back to the power company.

From my perspective, when you generate elecricity from solar or wind, the resulting current is too inconsistent and spiky, so you need to dump it into "reservoir" to gain some consistency in output. That reservoir can generally either be a battery bank or the grid and you generally only draw power from one of those sources at a time. I would assume that a "grid-interactive" inverter is mostly the same as a "grid-tie" inverter; you're feeding power and drawing power from the grid and the battery bank only comes into play when the grid goes down. Or, in other words, where a regular grid-tie inverter's anti-islanding ciruitry would shut everything down, the grid-interactive system would start drawing power from the battery bank. The problem I see with that idea is that, since the battery bank is only there for the proverbial "rainy day", it is likely to be undersized and get depleted very quickly. To use my example above, if I lose my job and can't pay the power bill for 4 months, it doesn't feel like I'm going to be able to rely on a system like that.

[vtmaps]

Right now, I am dependent on the power company to live my life. That means that if I am unable (or unwilling) to pay my monthly power bill, I will soon experience a crisis. Right now, I have a good job and the electric bill is an acceptable amount, but that may not be true tomorrow; I could lose my job or the price of electricity could go through the roof.

While you still have a job you should endow a fund to buy yourself replacement batteries and equipment.  Or alternatively, endow a somewhat smaller fund to pay your electric bill in the future.  Try to enjoy being dependent on the battery company to live your life.  (and the oil company for your generator).  Pray that a single lightning strike doesn't take out all your electronics at once.

--vtMaps

[wbustraan]

Since this discussion seems to have devolved into people picking apart my motives for building a battery bank (I thought that of all places, people here would understand), let me attempt to get back to my original question.

If I apply a charge to the battery bank terminals from a charger source other than the Classic, it seems that the Classic will detect that as voltage coming from the battery bank itself and clear the Low Bat Disconnect condition. Assuming the external charger is controlled by the state of Low Bat Disc, unless I add a delay, this would cause it to toggle like the following:

• battery hits V Low setpoint
• Low Bat Disc -> Aux 1 off
• NC relay turns on external charger
• external charger begins charging, bank terminals read above V High setpoint
• Low Bat Disc -> Aux 1 on
• NC relay turns off external charger
• battery has not charged enough, still below V Low setpoint
• GOTO 2

Adding a V High delay, it seems like it would cycle like this:

• battery hits V Low setpoint
• Low Bat Disc -> Aux 1 off
• NC relay turns on external charger
• external charger begins charging, bank terminals read above V High setpoint
• delay for an hour (or so)
• Low Bat Disc -> Aux 1 on
• NC relay turns off external charger
• if battery has not charged enough, still below V Low setpoint, GOTO 2
• otherwise, resume normal operations

[Jumbicat]

Weston,
I know what your asking and why.  I just received a KID for my off grid project and have many questions but hesitate to ask for the very reason stated above.

Have you called tech support?  I would hope a engineer should be able to address this. 

Mark