New section for dumb questions?

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Solar Expert Posts: 185 ✭✭
I've always been really bad at math and formula's. I already have my new industrial forklift battery on the way and my new panels are already here waiting for the new mounting system. I believe i've got a decent balance in the works, but i'd really be thrilled if those of you good with the math skills to double check me.

The up graded system will be about 3100 watts of panels and the 48 volt industrial battery @ 1372 ah @ the 20 hour rate.

Thanx a bunch and best wishes through the holidays...

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Re: New section for dumb questions?
The up graded system will be about 3100 watts of panels and the 48 volt industrial battery @ 1372 ah @ the 20 hour rate.
• 3,100 watts * 0.77 * 1/59 volts charging / (48v*1,372AH) = 0.0295 = 2.95%
If all the numbers are correct--It would not hurt to at least 2x or even 3x your current solar array.

What is your generator charging capacity?

What is your daily power usage? For now, monitor the cell specific gravity and plan on using the genset to bring the batteries to 90%+ state of charge (perhaps a couple times a week?).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 185 ✭✭
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Re: New section for dumb questions?

Thanx Bill.......I guess (n thats my main problem) I really screwed up this time, but i've got room for another 2000 watts with my current Outback charge controllers.. My generator is a 15 kw Perkins diesel going to a Magnum 4448 inverter with a charging capacity of 48 amps
@ 48 volts...

I've been killing my 8 L16's daily with my existin 700 watts of panels and runng the generator twice a day for several hours each time, but didn't have any choice financialy. I believe i'm using about 8500 watts daily.
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Re: New section for dumb questions?
...charging capacity of 48 amps
@ 48 volts...

I've been killing my 8 L16's daily. I believe i'm using about 8500 watts daily.
• 48 amps / 1,372 AH = 0.035 = 3.5% rate of charge with genset
With full sun and generator running, you are in the ~6% area of charging--probably barely enough for your battery bank's long term health.

• 8,500 Watt*Hours * 1/48 volts * 1 day of no sun * 1/0.50 max discharge = 354 AH @ 48 volts and 1 day capacity
• 8,500 Watt*Hours * 1/48 volts * 3 day of no sun * 1/0.50 max discharge = 1,063 AH @ 48 volts and 3 day capacity
This is the problem with battery banks on the large side--Typically it works out that if you use the 3 day and 50% maximum discharge for daily loads and combine that with the normal amount of sun 9 months of the year--we end up with about a 5% rate of charge from solar (the minimum recommended with the current rules of thumb).

Using PV Watts with Flagstaff AZ, 3.1 kWatt array, 0.52 off grid efficiency:
"Station Identification"
"City:","Flagstaff"
"State:","Arizona"
"Lat (deg N):", 35.13
"Long (deg W):", 111.67
"Elev (m): ", 2135
"PV System Specifications"
"DC Rating:"," 3.1 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 1.6 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 35.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (\$)"
1, 5.19, 260, 22.10
2, 5.92, 265, 22.53
3, 6.27, 307, 26.09
4, 6.44, 298, 25.33
5, 6.56, 306, 26.01
6, 6.61, 286, 24.31
7, 5.95, 263, 22.36
8, 5.54, 246, 20.91
9, 6.59, 292, 24.82
10, 6.19, 288, 24.48
11, 5.47, 258, 21.93
12, 5.07, 253, 21.50
"Year", 5.98, 3323, 282.45
If you are using 8.1 kWH per day, that is ~243 kWH per month

So--with the current power usage and setup--You have just about enough solar panels to keep up with your power usage (average weather day)--But you will have to probably use the genset at least once a week to get the battery charge backup up (you are close to deficit charging with solar PV--It is not reasonable to assume a 250 kWH per month output from PV Watts can keep up with a 243 kWH per month load even with the 0.52 derating).

Normally, we would suggest running the genset in the early morning before the sun hits the panel full on (before 9am?)... Then let the solar array finish off the charge.

In your case, you probably want to run the genset during Mid-Day to help get the 6%+ charging current in there to stir up the battery bank.

Monitor your genset charging current... If it is >~20 amps and the solar is near full current--it is probably worth the fuel... If the genset charging current is less than ~20 amps--it is probably not worth wasting fuel/run time on... (using some engineering rules of thumb; things within ~2x or less of each other are almost the same; things ~10x or greater difference, you can ignore the smaller effect).

Hmmm... 15 kW genset:
• 15,000 watts * 0.50 minimum loading * 80% charge eff * 1/59 volts charging = 102 amps DC charging current
If you could find a used forklift charger in the 50-100 amp range (if 50 amp, use in parallel with the inverter/charger; if 100 amp, use alone) that would be a good match with your genset.

For total bank charging current:
• 1,372 AH capacity * 0.05 = 69 amps minimum
• 1,372 AH capacity * 0.10 = 138 amps nice value
• 1,372 AH capacity * 0.13 = 178 amps--not much reason to go above this
In any case--you certainly will reduce your generator run time/fuel costs with this new setup.

Monitoring your battery bank's state of charge (both with a battery monitor and a hydrometer) as well as watching water usage (say need to refill cells every ~2 months) will tell you how well you are treating your battery bank.

Eventually, if you could get another 3kW of solar panels -- that would probably eliminate your need for the genset except for bad weather... But that is probably for another Christmas.

Anyway--Hopefully some other folks with much more battery experience than I can comment--This is just my engineering back of the envelope guesses.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 185 ✭✭
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Re: New section for dumb questions?

WHEW..............that'll send my little pea brain a reel'n

I know that i'll be running the generator several hours a week for the washer n dryer & for the air compressor and welder when i'm out tinkering in the shop. These were the reasons for the larger than normal generator.

If I could only get my wife into the conservation mode. She spends most of her time with the tv and lights on in the bedroom. With her fibro mialga and ms I can't get myself to push her
too hard...:):):)

I guess I got the battery pretty close and quite a bit short on PV.
• Solar Expert Posts: 720 ✭✭✭
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Re: New section for dumb questions?

I find very few off grid systems that actually have the (Recommended amount of PV) most off grid people just can not afford that many pv or just flat do not need them on a regular basis.

Just how detrimental these issues are to off grid batteries is a good question I have a customer with sams club golf cart batteries that are 7 years old and going strong and they have hardly any pv just could not afford it. They have abused these batteries routinely and they still go? My point being do the best you can do with the batteries and charge them well at least once a week.
• Solar Expert Posts: 123 ✭✭
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Re: New section for dumb questions?
halfcrazy wrote: »
I find very few off grid systems that actually have the (Recommended amount of PV) most off grid people just can not afford that many pv or just flat do not need them on a regular basis.

Just how detrimental these issues are to off grid batteries is a good question I have a customer with sams club golf cart batteries that are 7 years old and going strong and they have hardly any pv just could not afford it. They have abused these batteries routinely and they still go? My point being do the best you can do with the batteries and charge them well at least once a week.

Nothing is for sure. Though, of course, having too little charging power is rolling dice for shortening battery life.

However, I have 1860 a/h of forklift battery. Until three yr. ago, I had only a max of 900 watts worth of 12 v. solar panels. At that point, I more than doubled my array to 2000 watts. People smarter than me say that is still too little pv for the batteries. [Though I don't know what is "best" for them, how much more solar elec. I would need to generate to be "healthy" for the batteries.]

The deal is, I was ignorant of how much pv charging I needed. I do not use a generator except when running a table saw or whatever now and then, and when I do, it is tied in to the inverter which has a charger built in, but only 40 a.

Still, my batteries are right at 20 yr. old and still working, not like new, but well enough to run our house at night.

I have begun thinking about a new battery set, thinking along the lines of NiFe. I have spoken with a number of folks, gathering info', and NONE of them can believe my batteries are still working, what with 17 hr. of only 900 w. of pv coming in. I guess I have been quite lucky in my ignorance.
• Solar Expert Posts: 74 ✭✭
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Re: New section for dumb questions?

What brand of 1860 ah battery do you own? How many amp-hrs do you draw during the night from the Battery?
• Solar Expert Posts: 185 ✭✭
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Re: New section for dumb questions?

UPS freight called and I get to pick up my new GB battery on thurs the 6th. Whether or not i've got enough panels to charge the monster fork lift battery, I really don't know but i'm really excited. GB may have this batter over rated for what the specs show. Comparing it to other companies it may be more like 1140 ah instead of the 1370. This could put me in a little better position......
• Solar Expert Posts: 185 ✭✭
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Re: New section for dumb questions?

Yep, picked up the new Monster Battery last week and a friend of mine came over with his "Stinger Boom Truck" and very gently set it over on it's new home.

I had previously said that my inverter had a 45 ah charging capacity @ 48 volts but got to checking and it's acturally a 60ah charger. It will definitely charge the 1340ah battery and bring it to a nice float state. Now to get the 12 new panels up.