Newbie looking for advice on his system

adas · March 2008

Aloha, I am in Hawaii with a good 8 hours of sun. and have the following setup but am at a crossroads of where to go next to make the best off grid system for my area. I am trying to run my 2 person residence/office (without A/C) during the day, using the most economical appliances, no stove, no clothes dryer, using solar thermal Hot water, etc). I also want to keep the computers and security system on at night. I am in the most sunny part of Oahu Hawaii.

1: I have 36 x 70watt panels with Voc of 44 and will put out 27.5 v at 9am to peak at 30.5v at 1:30pm charging up the following:

2: 2 forklift 24v 800 amp/hr batteries that are at 1/2 charge. Is this enought voltage to charge these batteries? or do I need a higher voltage that I am getting at 9am (27.5v).???

I have experimented with things beyond this. I have not a charge controller yet and have been turning off the panels that are individually switched by circuit breaker.
Anyway I plan to use 2 MX60's and use a 24v 2400 watt inverter.
Comments on where to go from here. 48v?

Sorry to have to ask this kindergarden stuff.
Frank

n3qik · March 2008

Re: Newbie looking for advice on his system

We will need more info on your loads, like how many watts each device uses. Best bet is to get a device called "Kill-A-Watt" meter.

Here is a link:
http://www.p3international.com/products/special/P4400/P4400-CE.html

When you find how much the load is, then we can tell you if you have enought battery. A note on the battery bank, is is not a good thing to discharge it below 50%

adas · March 2008

Re: Newbie looking for advice on his system

Aloha, load is relatively small.

Constant load during day. 700 watts
Intermitant Load during day 200 watts additional average

Constant load at night. 500 watts

Panel output 70 watts x 36 = 2520 watts x 7 hours = 17640 watts.

I also have 2 windgens putting out on average 16 volts each for an average 3 hours per day. (each feeding into 1/2 of the forklift battery-12v)

FYI even at 7am and 5pm under a light inverter load of 200watts/110 volts the panels still show 26.5 which is higher than the battery level of 25.90v

frank

westbranch · March 2008

Re: Newbie looking for advice on his system

Do you have access to a generator? If so I would suggest that you put a good charge on those batteries... before they suffer some damage from sitting undercharged. There are several good strings that discuss the ins and outs of this topic on this BB.. Good luck in getting a good system up and running. A good charge controller and a true sine wave inverter should be next on your list.

Eric;)

n3qik · March 2008

Re: Newbie looking for advice on his system

The wind gens is a problem. charging 1/2 a battery bank is bad news. One group of batteries will have more charged than the other. You should try to charge/discharge the bank as a hole, AKA 24VDC.

I or someone els will get back with help on the loads cals.

adas · March 2008

Re: Newbie looking for advice on his system

n3qik wrote: »

The wind gens is a problem. charging 1/2 a battery bank is bad news. One group of batteries will have more charged than the other. You should try to charge/discharge the bank as a hole, AKA 24VDC.

Aloha, I am checking out the actual daily load with my Watts-Up meter, but just bought a Kill-a - watt on ebay to make it easier.
I stopped charging the 1/2 bank at a time as I saw problem as some battery cells were 2.0, some 2.05, etc. But all in all the total battery charge is at 25.90 now.

But the question is: Is 29-30 volts enough to charge a 24v battery properly? Or should I series 2 panels and use a MX60 to get the bulk, absorption, float, silent, equalization charge that treats batteries properly?

frank

BB. · March 2008

Re: Newbie looking for advice on his system

It is a little unclear by what you mean by at 1/2 charge (at least to me)...

Using a temperature compensated hydrometer and getting readings at beginning and ending (of sunshine) will be a big help... You can use an accurate and calibrated DVM to measure the resting voltage (no charge/load for 2+ hours) and estimate the state of charge also.

Your best bet would be to get one or two MX60's and let them charge your batteries. Then you can put two panels in series and get Vmp > ~34 VDC (charge controller drop + equalization charge for the batteries).

Getting a battery monitor would also make understanding your loads and charging profiles will help prevent major damage to your very expensive batteries...

A kill-a-watt meter (if you don't have one) allows you to measure cycling loads (like refrigerators, appliances, etc.). It is very difficult to estimate loads from instantaneous readings on cycling appliances.

If you are able, switching to 48 VDC would be handy (can support 2x the wattage of panels with an MX60 vs a 24 volt setup). If you have to buy more batteries and mix old + new--it may not be worth it to change right now (if the original batteries have been abused and/or are over one or more years old)...

Regarding the wind setup--not ideal to setup charging this way... Also, do you have any ideas how much power (watt*hours or amp*at what voltage per day/week/month) they are putting out?

I am a big believer in logging real numbers from wind/sun generation and loads... Many people have way over/under underestimated their actual system parameters and without knowledge of the real numbers from their existing systems--everything becomes an expensive guessing game...

-Bill

adas · March 2008

Re: Newbie looking for advice on his system

BB. wrote: »

It is a little unclear by what you mean by at 1/2 charge (at least to me)...
-Bill

Aloha, so far I have been doing it on the cheap. (except the 36 panels). I got 3 24v forklift batteries from a junkyard for basically nothing. All took a charge and I think I will get 2 to last. One is a gel cell and will not take a deep charge, (will run down quickly), so it may not last. I want to cycle the batteries a few times to see if they will last.

On the 1/2 charge thing, the forklift battery has straps tying the cells together so I tapped off at the 1/2 way point (12v) and charged each with one windgen. but as I am seeing the charge becomes unequal after awhile, so I stopped charging this way.

Batteries seemed fully charged now though at 25.9+volts and are a bit warm to the touch

frank

niel · March 2008

Re: Newbie looking for advice on his system

i would say you do need higher voltages to take full advantage of the sunshine available to you as the battery needs to be around 29-30v to achieve a full charge and an mppt controller would suit that well. i also gather you are not using a controller to regulate the charge for when it goes beyond the bulk charge stage and i don't recommend you going without a controller.
odds are you will be buying batteries soon if the first 2 are any kind of an indicator. also, the 12v wind generators were a waste as you needed for a 24v battery.

Brock · March 2008

Re: Newbie looking for advice on his system

adas, I think you are asking if you should wire your panels up in series for a higher input voltage to the MX-60's? If that is what you’re asking, the answer is definitely yes. Wire them in groups of two in series to put out ~54v volts, then feed half of them to one MX-60 and the other half to the second MX-60 and let them charge your battery bank. I would just leave the wind gennies out of the situation until you get that set.

adas · March 2008

Re: Newbie looking for advice on his system

Brock wrote: »

adas, I think you are asking if you should wire your panels up in series for a higher input voltage to the MX-60's? If that is what you’re asking, the answer is definitely yes. Wire them in groups of two in series to put out ~54v volts, then feed half of them to one MX-60 and the other half to the second MX-60 and let them charge your battery bank. I would just leave the wind gennies out of the situation until you get that set.

Aloha, I have a couple of question to fully understand your comments above.

1: If I have 54 V going into my MX 60 that is charging at say 29V ..no effeciency is lost? I understand the amps will be higher and the voltage will be regulated? (Ohms law) Correct?
2: Would it be better to go to 48v (two series 24v batteries) and series 3 panels to 81v or so and use a 48v inverter? are the only advantages that I would economize on wire gauge size? Comments

thanks
Frank

BB. · March 2008

Re: Newbie looking for advice on his system

The MPPT type controllers are "constant power" devices (less losses)... P=I*V... So if voltage is doubled, current is halved, and the power stays the same. The MPPT controller is, roughly, the DC equivalent of a AC transfomer. So no extra losses here and you actually gain some power because of its ability to match the I*V curve of the solar panels with that of the batteries.

Also, the MPPT will regulate the voltage to the batteries (actually, it is a multi stage regulator so that depending on the charge condition of the battery, it will adjust the voltage and current to optimally charge the battery).

Picking the system voltage--look at the power requirement from your inverters and then see what input voltage/current would be... Trying to keep the input current under 100 amps is probably a worth while goal...

If your power requirement is less than 1,000 watts--a 12 volt system is OK. if your requirement is 4,000 watts--then a 48 volt system would be a better idea.

-Bill

adas · March 2008

Re: Newbie looking for advice on his system

BB. wrote: »

The MPPT type controllers are "constant power" devices (less losses)... P=I*V... So if voltage is doubled, current is halved, and the power stays the same. The MPPT controller is, roughly, the DC equivalent of a AC transfomer. So no extra losses here and you actually gain some power because of its ability to match the I*V curve of the solar panels with that of the batteries.

If your power requirement is less than 1,000 watts--a 12 volt system is OK. if your requirement is 4,000 watts--then a 48 volt system would be a better idea.

-Bill

Aloha, Since I do not have a good grip on my wattage requirements yet ....only an estimate of about 900 watts per hour, (but I know how it can grow as I remember electrical things, (cell phone charger, shredder, etc).

But I think a 24v system would be good for me as 2000 watts should be the max and I think since I am using used 24v batteries, it would not be a good idea to series them, being different voltage readings at full charge since they are older . I also could add another 24v inverter to my parralled 2x24v battery bank and have a redundant system in case of component failure.
And one MX60 for each battery. What do you think?
frank

BB. · March 2008

Re: Newbie looking for advice on his system

Regarding the load... 900 watts for how many hours per day? It makes a big difference...

900 watts * 1 hour = 900 Watt*hours = 0.9 kWhrs per day
900 watts * 10 hours = 9,000 Watt*Hours per day or 9kWhrs per day.
900 watts * 24 hours = 21,600 WH or 21.6 kWhrs per day...

Best bet is to get a Kill-A-Watt meter and measure your loads (or if you have utility power, look at your monthly bill).

Regarding the batteries... I prefer to put all of the batteries into one bank and all of the panels charging the one bank (through one or more charge controllers)... Allows you to maximize the output from your panels into one load to charge--and you don't have to move loads between the banks to balance your charging.

You could put the banks in parallel with a disconnect so that you can run them as an A/B type system for redundancy--if needed.

Regarding paralleling vs connecting the batteries in series... I personally like connecting all of the cells in series. it is easier to find weak cells/batteries and you don't get one weak battery forcing the charge/discharge current through the stronger battery (an issue with paralleling batteries).

Although, some places I have read that series connected banks will last longer (I like this because each cell must carry the full current... Parallel banks don't always share current well because of wiring and cell differences in impedance and voltages)--Wind Sun has said before that they find series or parallel banks both have about the same life.

If your batteries have different voltages--you probably have cells about to fail (or failing now) and no matter what your do, they are not going to last very long or carry your load well...

-Bill

adas · March 2008

Re: Newbie looking for advice on his system

BB. wrote: »

Regarding the load... 900 watts for how many hours per day? It makes a big difference...

If your batteries have different voltages--you probably have cells about to fail (or failing now) and no matter what your do, they are not going to last very long or carry your load well...
-Bill

Aloha, I am looking at 900 watts for 12 hours during the day and 500 watts during the 12 hours of night. Total of 8400+6000 14.5 KWH per day.useage

70W x 40 panels x 7 hours (min) = 19.6 KWH In theory anyway

I don't know what to say about Parallel vs Series batteries, though. It made more sense to me to parallel and if I I had a weak cell in one of the paralleled batteries it would not "take down the whole series cells"?? But you make a good point of having the same current pushing through the cells. I'm confused
frank

BB. · March 2008

Re: Newbie looking for advice on his system

Frank,

That is a good sized system... You probably need 3x the battery capacity you have now (assuming that what you have is working well now) as you are cycling the batteries at a pretty high rate of charge...

But, I have a question, do you need an off-grid system, or can you run grid-tied for your system.

Grid tied is significantly more efficient, effectively up to one year of storage capacity with net metering for a few dollars a month, less costly to install and operate over time, and usually qualifies for solar rebates and tax credits (off grid systems usually do not).

DSIRE info for Hawaii

And, if you need off grid system for emergency backup, a hybrid system, like the new Xantex XW will allow battery backup but support grid tie operation too--you only need to cycle the batteries when the power fails--much lower losses and you should have longer battery life...

Have you done all of the conservation possible at this point (appliances, lighting, using laptops instead of desktop computers, turning off equipment like laser printers instead of letting them run in standby, etc.)? That is a lot of power for an off-grid system (not impossible--but expensive to build and maintain)...

I am also wondering about your assumption of 8 hours of sun per day... Generally a fixed array should harvest about 5-6 hours per day (averaged over 1 year) and upwards of 8 hours per day if these are mounted on two axis trackers...

-Bill

mike95490 · March 2008

Re: Newbie looking for advice on his system

But I think a 24v system would be good for me as 2000 watts should be the max and I think since I am using used 24v batteries, it would not be a good idea to series them, being different voltage readings at full charge since they are older . I also could add another 24v inverter to my parralled 2x24v battery bank and have a redundant system in case of component failure.

You need to do a complete charge cycle for each bank, and equalize (if proper for the battery type) THEN you can see how well balanced they are with respect to each other.

adas · March 2008

Re: Newbie looking for advice on his system

Aloha, here is an update on my refinements.

I have 22x70 watt panels up and running. 2 panels are in series so each panel puts out 30V and in series they are 60v.

Today is the first day with the MX60 and it basically fully charged the batteries (2x 24v forklift 850ah batteries) plus ran the office all day.
Max wattage today is 1800 watts which is 81.8 watts max per panel
Total KWH today is 8.90 so a pretty fair harvest for a somewhat intermittant cloudy day.
Can't wait to get my other MX60 and 22 more panels up and start doing 20KWH per day and start sending some power over to run our assembly shop and maybe turn off the generator.

Frank

Newbie looking for advice on his system

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