Do I Have Enough Solar For My Battery?

Veteran52

I acquired 4, 100W 12V solar panels, parallel, producing 5A each, 21 volts each with a 40A MPPT controller. Is this enough to fully charge a 200AH, or a 250AH, or a 300AH 12 volt AGM battery without sulfation concerns, given the usual 5 hours of average good sun light each day? I need to know before I buy a new battery so I don't waste money on more than I can properly charge yet get as much storage as I can.

Thanks

InCogKneeToe

I run 4 100w 12v panels with a 40A PWM Controller, hooked to 650AH of 12v Batteries. Usually for Weekend Use only. I get by running a Stereo for 12 hours/day. I can even handle a Bar Fridge 24/7 on holidays for 2 weeks. However I do top up with a Genset hooked to a15A Charger for 3 hours every night then.

mcgivor

Assuming the battery is not being discharged, then yes it will be sufficient to maintain such a battery, if however the battery is discharged on regular basis below the PV's capacity to replenish the withdrawals, then no.

Veteran52

mcgivor said:

if however the battery is discharged on regular basis below the PV's capacity to replenish the withdrawals, then no.

I guess that's my question. If I use 60AH per day will this set-up be able to recharge it the right amount and at the right rate to avoid undercharging and sulfation problems. I'm not worried about cloudy days when recharging can't occur. I don't need to put a load on it every day.

Photowhit

So roughly, 400 watts of array, if angled close to your latitude, and without shade, should produce about 300 watts per hour. For your MPPT type charge controller to make best use of the wattage, they need to be wired at least 2 in strings of 2 (2 in series) and 2 parallel strings. 300 watts charging a 12 volt 200amp battery, charging at 14.5 volts will produce about (300/14.5=) a bit over 20 amps for each hour of direct sunlight. Assuming your AGM battery can take a 10% charge rate, you should be pretty good.  Without knowing where you are, that's roughly what we look for a 10-13% charge rate.

Please note that use is at the battery, so you need to take into account energy used in your inverter and it's idle use.

BB.

Say you use 60 AH per day, and the starting rule(s) of thumb(s) designs would be:

• 60 AH * 12 volts = 720 WH per day of 12 VDC energy
• 720 WH per day * 1/12 volt battery * 2 days of storage * 1/0.50 max discharge planned (longer battery life) = 240 AH @ 12 volts suggested minimum battery size

Standard "golf cart" batteries are around 6 volts @ 220 or so AH--So 2x GC batteries in series would be a 12 volt @ 220 AH battery bank... Probably close enough to 240 AH for now.

Two calculations for solar panels... One based on size of battery bank and 5% to 13% rate of charge. 5% can work for weekend/sunny weather usage. 10%+ suggested for full time daily off grid usage:

• 220 AH * 14.5 voltage charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 207 Watt array minimum
• 220 AH * 14.5 voltage charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 414 Watt array nominal
• 220 AH * 14.5 voltage charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 539 Watt array "typical" cost effective maximum

And there is sizing the array based on hours of sun per day (your location/mounting details) and your loads. Say you are around Chesterfield Missouri, fixed array facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Chesterfield
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 51° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.29	3.73	4.46	4.94	5.03	5.27
Jul	Aug	Sep	Oct	Nov	Dec
5.46	5.35	5.30	4.63	3.38	3.03

And lets "toss" the bottom three months (not use your loads as much and/or use a backup genset when needed). That gives us February @ 3.73 Hours as the "break even" month:

• 720 WH per day * 1/0.61 DC system efficiency * 1/3.73 hours of sun per day (average February) = 316 Watt array "break even" February

Your 4x 100 Watt panels is not a bad fit (assuming I guessed your nearest city)... You can estimate the average harvest by month from teh above data... For February:

• 400 Watt array * 0.61 end to end DC system eff * 3.73 hours of sun per day (Feb) = 910 WH per day

That is not bad give your 60 AH @ 12 volts or 720 WH per day. And 2x 6 volt @ 220 AH golf cart batteries will be a good start (relatively cheap and rugged batteries. If you get Flooded Cell Lead Acid batteries, you can use a hydrometer to measure the State of Charge, and add distilled water when needed.

AGM batteries will be fine too... Just monitor the charging/loaded battery bus voltage to ensure all is going OK (batteries get fully charge several times a week, and don't get discharged below ~12.0 or 11.5 volts under heavy loads).

The above calculations are relatively conservative and intended for an off grid cabin or other usage that needs relatively reliable power. It is a "balanced" system design (battery banked sized to loads, solar array sized to battery bank and hours of sun per day).

One thing you did not mention is what the peak current/power will be... More or less, ideally a C/20 Discharge rate is optimal. C/8 max continuous. C/5 max for an hour or two, and C/2.5 maximum surge current (numbers are for Flooded Cell, AGM batteries can discharge faster/surge more current--But the C/X numbers given should keep the AGM or FLA Batteries happy:

• 220 AH * C/20 = 11 Amp discharge (say 11 amps for 5 hours a night, for 2 nights to 50% discharge)
• 220 AH * C/8 = 27.5 Amps for 8 hours (really a bit less) until battery bank is dead (taking bank "dead" is not good--Batteries will not last long)
• 220 AH * C/5 = 44 Am;s for an hour or few hours (5 hours to "dead").. Heavy draw for shorter time frames
• 220 AH * C/2.5 = 88 Amps for a few seconds to a few minutes (starting surge for well pump, refrigerator compressor, etc.).

-Bill

mcgivor

Veteran52 said:

mcgivor said:

if however the battery is discharged on regular basis below the PV's capacity to replenish the withdrawals, then no.

I guess that's my question. If I use 60AH per day will this set-up be able to recharge it the right amount and at the right rate to avoid undercharging and sulfation problems. I'm not worried about cloudy days when recharging can't occur. I don't need to put a load on it every day.

My mistake I misread and thought it was a single 100W panel, 400W would allow some discharge, Bill has furnished you with enough information to get some idea, but what I will say is, many make the mistake of overestimating the production potential versus load demands, in general it's better to have a surplus of PV potential.

Veteran52

BB. said:

 Probably close enough to 240 AH for now.

Thanks Bill. That was all very informative. I have a lot to digest but it looks like based on all you said that a 225AH battery will be about best for me. I'll throw the higher ones off the table given my current solar set up and some shading on my panels for a few hours per day. I actually should drop to 200AH now that I think about it.  I don't want to under estimate the shading effect.

Dave Angelini

Good Luck to you and thanks ! 

Remember there can't be any shading 9am to 3pm for what you are doing. NEVER
Also keep in mind there are 2 kinds of people, those who want solar, and those who want more!

BB.

The "solar numbers" for energy harvest--Anything within 10% of the above numbers, is pretty much "exactly the same" (it is surprisingly difficult to accurately measure solar power within 10% accuracy without laboratory instruments).

I.e., ~220 AH +/- 22 AH -- Not really different from 220 AH itself.

And as the folks above said--Almost any shade on a panel/array will cut harvest by 50% to 100%.

If you have shading, look at what it takes to get rid of the shade (move panels, move roof vent pipes, cut trees, etc.). If you have things you cannot move (nearby house, AC power lines overhead, etc.)--Solar electric may not a good fit...

-Bill