First solar setup.

Bad Wolf · November 2013

I’m about to embark on my first foray into solar power.
I have an 1800 Ah battery bank that I was able to get when they changed out the UPS at work. 18 300 Ah 4 volt batteries hooked up into six 12 volt banks. I use it as a UPS for my wood furnace; I have a Xantrex Prosine 600 watt inverter and the relay that they sell to go with it.
What I currently do is throw a charger on it every month or so to bring it back up to 100%. What I’d like to do is put up a solar panel and charger to keep it fully charged. This is only a backup to run things until I can switch on a generator, but this way if I lose power in the middle of the night I don’t have to get up. During moderately cold weather it will use around 3-4KW/24hrs, so I think I could go a couple of days if I needed to.

If I leave the inverter on it draws about 800 ma. So I guess I would need something to overcome that and still charge the batteries. In the past if I put a charger on it while the inverter was on I would get an “over voltage” error on the inverter.

What would be a good starter setup?

Thanks

BB. · November 2013

Re: First solar setup.

Have you looked up the batteries for their specifications? UPS batteries may be designed for "float service" and may not deep cycle very well.

Over voltage error on the inverter is commonly around 15 volts for many units... Depending on the type of batteries, that is either a fairly "hot" voltage for flooded cell (normally around 14.5 to 14.7 volts or so) during normal absorb, or too high for AGM/GEL/Sealed batteries (typically ~14.2 volts or so). If the battery bank is "cold" (towards freezing temperatures or lower, then they do require higher charging voltages). In cold climate, you have to look for inverters with 16 volts or higher (12 volt rated units) for reliable operation.

Another issue with very large battery banks, you need large battery chargers. 5% is usually about the minimum recommended charging current for a cycling battery bank (again, need to know battery type--flooded cell work better with 10% or a bit higher, and most GEL batteries in the US have a ~5% maximum rate of charge).

An 1,800 amp 12 volt battery bank would need a minimum of 90 amps of charging current.

And, you should wire the series/parallel connections so that all the batteries share current equally:

http://www.smartgauge.co.uk/batt_con.html

And don't forget fuses/breakers--An 1,800 AH Lead Acid battery bank can output a scary amount of current into a dead short (many thousands of Amperes).

As always, like to suggest you measure your loads (watts, peak watts, Watt*Hours per day, etc.). Know your load requirements to define the battery bank requirements.

-Bill

niel · November 2013

Re: First solar setup.

a few thoughts on the battery bank before others get into the possible solar for it.

1> 6 parallel banks is going a bit far. many frown upon more than 2 being paralleled. as you state you need 3kwh to 4kwh in 24hrs worst case and at 12v that's 300ah making the bank 600ah for 50% dod. you have 1800ah so i gather you opted for 3 days autonomy. this can lead to failures and would require you to watch for battery inequalities. do add fuses or circuit breakers to each string just in case.

2> it concerns me that you indicated an overvoltage on your inverter when charging your batteries. what are the charging specs for those batteries? what settings if any does the charger have and better yet what charger are you using?

Bad Wolf · November 2013

Re: First solar setup.

The batteries are Liberty Series 1000 Valve-regulated (sealed) Lead-acid.
Recommended float 2.26 Vpc, initial/equalize voltage 2.33 Vpc
So I take that to be 6x2.33=13.98 volts
My charger is a Schumacher XC75 20 amp. The GEL charge voltage was listed at 14.7v which was the lowest setting.
The reason I have 18 is that was the smallest cabinet that we were getting rid of. The others we 24 each (4 racks of 6 each) I think they were wired as 48 volts.
They got rid of 138 of these and I can’t pass up anything that’s free!!

BB, thanks for the link. I found that by moving one connection I can equalize all the banks.

Niel, I wasn't really shooting for three days it just worked out that way. What I was looking for was something that would keep the wood fired furnace circulating till I got home or woke up. After that I have a 2000 watt inverter generator that runs 10 hours on a gallon of gas.

Attachment not found.

westbranch · November 2013

Re: First solar setup.

BW, I assume you may have found this C&D site

http://www.cdtechno.com/resource/support_doc.html

and this document

http://www.cdtechno.com/pdf/ref/41_2128_0212.pdf

hth

Bad Wolf · November 2013

Re: First solar setup.

OK, so I redid the connectors to get equal length leads. I added some fuses to isolate the banks if things go south.
I used a Kill-o-watt meter to find my max loads.
490 watts if every pump is running and the fan. Very unlikely
2-3000 watts over a 24 hr period depending on the weather. In reality it would probably only run for 8 hrs or so until I can fire up a generator.

So… back to my original question. What would be a good starter set up (panels and controller) to keep a float charge on these batteries?

BB. · November 2013

Re: First solar setup.

OK--The math.

1% is minimum for float service, with 5-13% for cycling. and 10+% or higher for daily deep cycling loads:

1,800 AH * 13.8 volts float * 1/0.77 panel+controller deratings * 0.01 rate of charge = 323 Watt array minimum for float
1,800 AH * 14.7 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,718 Watt array minimum
1,800 AH * 14.7 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 3,436 Watt array nominal
1,800 AH * 14.7 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 4,467 Watt array "cost effective" maximum

Based on you energy usage per day--Using PV Watts for Hartford Connecticut, fixed array, tilted to 42 degrees from horizontal:

Month    Solar Radiation (kWh/m 2/day)
1      3.30     
2      4.15     
3      4.47     
4      5.04     
5      4.99     
6      5.41     
7      5.28     
8      5.18     
9      4.77     
10      3.92     
11      2.92     
12      2.67     
Year      4.34

Tossing the bottom three months (assuming you would use generator to support) 3.92 hours of sun for October, based on 3,000 Watt*Hours of AC power per day:

3,000 WH per day * 1/0.52 system efficiency * 1/3.92 hours of sun per day = 1,472 Watt array minimum

An 1,800 AH @ 12 volt battery bank and using 25% of back capacity per day (2 days of use, 50% maximum discharge for longer battery life):

1,800 AH * 12 volts * 0.85 inverter eff * 0.25 discharge per day = 4,590 Watt*Hours per day average recommended draw for 2 days of usage

Or, looking at a 3,000 WH per day load:
1,800 AH * 12 volts * 0.85 inverter eff * 0.50 maximum discharge * 1/3,000 WH per day = 3.06 days of storage

Is this all making sense?

-Bill

BB. · November 2013

Re: First solar setup.

For an array less than 400 watts, and relatively close to the charge controller/battery bank (short PV array cabling), a PWM controller would probably be the most cost effective. For a 323 Watt array (float service):

323 Watt array * 1/17.5 volt Vmp array = 18.5 amps

So, you would be looking at a 20 amp or larger PWM controller

Typically, for arrays over 800 watts, arrays with longer wiring distances, and/or using larger >100 watt panels that have non-17.5 Volt Vmp panels (larger panels, less wiring, cheaper $$$/Watt, but more expensive to ship). Use 1,718 Watt array for 5% minimum rate of charge:

1,718 Watt array * 0.77 panel+controller deratings * 1/14.7 volts charging = 90 Amps

You would need several smaller MPPT controllers--Or one Midnite Classic controller...

http://www.solar-electric.com/chco.html

MorningStar makes a wide range of PWM controllers (and very nice MPPT) controllers. Midnite and Outback make very nice controllers. Schneider makes a nice controller and has a "high voltage" MPPT battery charging controller that is great to use if your array is located very far from the battery bank+charge controller (can use a ~400 volt Vmp array and much smaller gauge wiring from array to battery shed).

Rogue makes a very good 30 amp MPPT controller.

If you are still looking for an AC inverter--If your loads are small, the MorningStar 300 watt (600 watts for 10 minutes) 12 volt TSW AC inverter with "search mode" and a remote on/off input--Very hard to beat.

-Bill

Bad Wolf · November 2013

Re: First solar setup.

I think my head is going to explode.

So you are saying that to maintian the batteries I need a 323 watt array (min) and a 20 amp controller.
That all I'm really looking for, just enough to make up for standby losses and keep them at max. I really don't plan on running extended times on it, just using it as an UPS.
Anything else is way more money for what I'm doing.

The only other thing is that I have a Xantrex Prosine SW600 that draws .6amp on stand by. Would a 400 watt array be enough to overcome this and still keep it charged?

Thanks

BB. · November 2013

Re: First solar setup.

A 1% rate of charge is about right for floating a battery bank in good condition... Remember that you only get ~6 hours of sun per day, so, that 1% rate of charge is spread over 24 hours--Or ~0.25% rate of charge (compared to a "plugged in" AC battery charger/float charger).

Batteries as they age, start drawing more current during float... Forklift batteries can discharge as much as 1-2% per day--Which would be a 4-8% rate of charge (based on a 6 hour out 24 hour day). Sealed/AGM/GEL batteries typically have a much lower rate of self discharge (new and old)--But I try to be "conservative" for system design--You don't want, for example, everything to work well for 3-4 years, and then year 5-8 find out that the battery bank died from under charging (i.e., the discharging during float current exceeded the float charger capability).

So, a 1% rate of charge for an 1,800 AH battery bank would be ~18 amps @ 13.7 volts or so. And I use a 0.77 derating for solar panels and charge controllers (there are different reasons for PWM vs MPPT deratings, but for our needs, the math works out to about the same derating numbers.

Another interesting factoid I ran into--When a battery bank exceeds ~2% self discharge--The bank is near failing, and possibly failing in a flaming mess/explosion (according to a company that builds monitoring equipment for industrial battery banks).

I have heard good things about the Xantrex ProSine inverters... But be careful... The standby/idling current can discharge a battery bank all by itself.

0.6 amps * 12.7 volts = 7.62 Watts

A solar panel with 3 hours of sun minimum would be sized:

7.62 watts * 24 hours per day * 1/0.61 DC system efficiency * 1/3 hours of sun per day = 100 watt solar panel dedicated to AC inverter standby losses

Or, you are looking at 100+323=423 watt panel to now float+inverter standby.

You can use a Battery Switch, or an DC Circuit Breaker to turn off the power to the inverter (if it does not have an "off button").

One of the reasons I really like MorningStar 300 watt inverter is it has a "search mode" (turns on once a second to look for AC loads) and a remote on/off switch--But which use much less power:

http://www.morningstarcorp.com/en/support/library/SureSineENG_R2_1_08.pdf

Self Consumption
Inverter On (no load) 450mA
Inverter Off 25mA (remote On/Off)
Stand-by 55mA (looking for >6 watt 120 VAC load)

-Bill

Bad Wolf · November 2013

Re: First solar setup.

So if I'm reading this right I need 3-4 85 watt panels and a controller just to break even on maintaining the batteries without getting one usable watt. Just glancing at the prices that looks like that's about $1000. That seems like a lot just to maintain the batteries. I would think I could just buy a good float charger/maintain for a lot less.
BTW I don't think I'd turn the inverter on unless there was a storm coming, so I normally wouldn't see an inverter stand by loss.

I see now why off grid systems or on grid with backup cost so much.

BB. · November 2013

Re: First solar setup.

Yep--And why we suggest a much smaller system (say 1/3rd the size) and a nice little backup genset (or even two--A second larger genset to power shop/emergency loads/2nd backup to the first genset).

Run off of batteries over night/middle of day (quiet power for lights, TV, radio, laptop, etc.)--And run the genset morning/evening to top off the batteries, run the fridge/freezer, and recharge the battery battery bank.

Solar only makes sense (economically) if you are using most of the power it is capable of generating (such as running from off grid solar for 9+ months of the year).

For your 1% rate of charge system, a couple different options.

You can get "cheaper" panels say 2 of these Trina Solar TSM-240PA05 for ~$480 (plus shipping--which is not cheap). And a Rogue 30 amp MPPT charge controller (because most larger format panels have Vmp>>17.5 volts needed for PWM on 12 volt bank)--Another $375 (plus shipping).

Another option would be 3x Kyocera KD140GX-LFBS (plus shipping) for $825 plus ProStar 30 Amp 12/24 Volt Solar Charge Controller With Digital Meter for ~$170 (plus shipping).

Otherwise, get one or two Iota DLS-90: 12 volt 90 amp regulated battery charger ($235 each) and a genset... You would need a 3kVA or larger genset to power one of these chargers, or a 6kW or larger for powering two of these chargers (two chargers is 10% for "quicker charging".

To power your loads, you could also use 1/3rd the size battery bank (or just not plan on quickly/fully recharging the bank) with a single Honda eu2000i (1,600 watt) genset (~$1,000) plus a Iota DLS-45 12 volt 45 amp regulated battery charger for $125. And a either a second DLS-45 or DLS-90 for use in recharging the bank when the AC mains comeback.

The eu2000i will run 400 watts for ~9+ hours on 1 gallon of gasoline. You can set up a remote bank and fuel cap for longer run times (the eu2000i has a fuel pump and can draw a vacuum in its fuel tank and pull fuel from a remote 5 gallon jug. Make your own, or buy parts/entire system). 20 gallons of fuel (and fuel stabilizer, replaced once a year) will keep you running for 10 days.

-Bill

Bad Wolf · November 2013

Re: First solar setup.

Thanks, I guess I got carried away with the fact that the batteries were free and more is better.

Gen sets I have, I use a champion 2000 watt inverter that will power the wood fired boiler circuit for 9-10 hours on a gallon of gas. I have a bigger Troy-Bilt 7550 watt for the whole house/large loads.

That's exactly what I did during Irene and Sandy: run the big set couple of hours in the morning to take showers, cook breakfast, flush toilets. Ran the Champion during the day run the fridge, freezer router, computer and TV, then fired up the big one again in the evening to cook dinner watch TV and generally annoy the neighbors. It was warm so we didn't even need the furnace. I charged up the thermal storage that supplies my DHW before the storms and I was good for 6 days.

This battery setup was just to pick up the load if I lost power in the middle of the night and didn't want to get up to fire up a generator. As I mentioned solid fuel boilers don't like to be shut down in the middle of a burn.
After the last two hurricanes, a freak snow storm and a blizzard they have done so tree trimming that we probably won't lose power again in my lifetime.

Sound like I should just cut it down to 1/3 (two banks 600 amp/hr total). That would run me for a day which is more than enough. What would happen to the other 12 batteries if I disconnect them? I know they will eventually lose their charge. Would they be ruined?
I just hate to part with that much power. Sounds like it would have been an ideal setup for a cabin somewhere.

Thanks again. I'm learning a lot. My real goal is to install a GT system when I build my retirement home in a few years.

BB. · November 2013

Re: First solar setup.

As long as you have grid power, you can probably float them on ~0.25% float charger--But realize, that they probably will not last more than a few years like that. UPS batteries, generally, are not intended for decades of float service--And tend to age very quickly when cycled deeply. My own limited experiences with "typical" UPS batteries is they need to be replaced every 1-2 years in a typical computer room application.

You can also use a larger charger and hit them once every 1-3 months for a day or two of charging--The idea being to keep them well above ~75% state of charge to reduce sulfation (which will eventually kill the batteries).

If you let them sit without charging--depending on how fast they self discharge--They can last as long as 3-6 months between charges, or even start dieing sooner.

You can monitor their resting voltage with a volt meter (if they are very cold or very warm, you will need to temperature correct the resting voltage readings) and figure out how long between charges.

Otherwise, find somebody to give the batteries too, or sell (I don't think Craig's List will let you sell batteries), or scrap.

-Bill

First solar setup.

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