These Batteries Suitable For Solar Battery Bank?

System · July 2009

Hello, all! I am new to the forum and have been interested in exploring solar energy options for several years. I think I have gotten to the point where I want to officially jump into it. I currently have a small cabin in northern Minnesota that currently hash no electricity and thought it would be a perfect place to set up a system and learn.

Based on some information I have read the past couple of years, I have decided to start with AGM batteries. I have read about pros and cons. First of all, I'm not planning to run a lot of appliances...no A/C, electrical heat, blowdryers, clothes dryer, or anything else with electric heating elements. We would like to get some power to run a couple laptops, an LCD TV, a microwave, and a few fluorescent lights. My calculations show that anticipated usage is between 40-80 AH tops per day.

I have an opportunity to get a couple different battery banks. I can get six of these batteries (http://www.aiconsol.com/svr80-12.html) and 0 gauge cables for $500 total. The batteries were used only once at a car show and have been sitting around for about a year. My question is, would these be suitable for such a project? Would I need to be worried if they haven't been charged for close to a year (I was told they all currently read 12+ V)?

Otherwise, I have the option to purchase six Group 24 AGM batteries that have been pulled from medical equipment (such as wheelchairs) for $180 total. These would have seen regular usage for about one year.

Any advice?

Thanks in advance,
Lance

icarus · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

Lance,

One huge question regarding used AGMs that have been sitting is how they have been taken care of during the idle period. I am not an expert on AGMs, so I suggest that you look here:http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries
http://www.batteryfaq.org/

I WOULD worry about not being charged for a year. Like I said,, I don't know much about AGMs. Remember that battery voltage is not a very reliable test of battery condition. The at rest voltage of a "12 volt" flooded battery is ~12.6-12-7 vdc. 12.0 is essentially dead and if it has sat that way for a while it is likely toast. (read the above links) As for you other battery option, I think the biggest issue with used batteries is how they have been treated. Have they routinely been drawn down 80%? Have they been drawn down every day? Have they been fully charged each time? $30 a battery is pretty cheap,, but cheap isn't always a bargain.

On the issue of solarizing your cabin. It is a laudable idea. My first suggestion is read all you can here and else where to get as educated as you can.

The first thing I would suggest is that you make a realistic assessment of your expected loads in watts and then in watt hours. Buy a Kill-a-watt meter to find out what your stuff really draws. Add up all the loads and all the expected run time. Then you will know where to begin. You suggest that you will use ~40 ah (I assume 12 volts) which would be ~480 wh. (My guess is that number is going to be substantially low)

Once you know how much you are going to burn, you can then design a system to charge the batteries in a timely way, with a reserve so that you don't over draw the batteries.

Just fyi,, we live off grid, we have ~300 watts of panels, and we use 4-600 wh/day for all uses. (We have no microwave, and no tv, no resistance heat appliances except for a soldering iron). We run off of 4 T-105 golf cart batteries as they are cheap and if taken care of will last ~7-10 years with our loads. The one truth we have learned is that loads WILL grow over time. Once you have 24/7 power, the uses multiply. More lights, more laptop charging, more TV time, Sat receiver etc. It is very easy to under estimate your loads. I suggest that you design a system that will allow you to grow into it,, especially charge controllers.

It is also quite common that people new to the off grid/Pv world will "kill" a set of batteries prematurely due to the learning curve, hence I personally would shy away from expensive AGMs (unless they were real cheap) and go with flooded batteries instead.

Good luck and welcome to the forum,,

Tony

niel · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

on the $500 deal the voltage may indicate condition somewhat in this case as they haven't been used or on supposition, charged either. this will indicate the charge condition, but can vary from manufacturer to manufacturer to some degree. agms do hold their charge better than fla types, but to sit for a year i can't say they won't have sulfation damage either. i would assume they have lost at least 25% of capacity from sulfation damages and who knows how long they will last.
the other set has probably been cycled deeply which also degrades capacity and lifespan. unfortunately you take a chance with either set and will be your call if at all on either deal.
make sure you will have the proper means to charge them in any case be it with solar or a utility charger.

Cariboocoot · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

My advice is: buy new batteries.

The trouble with used ones is that no matter what sort of testing you do, they're still an unknown quantity. You don't know what they've been subjected to during their life cycle and there's no telling if they've been stored properly (including periodic re-charging). AGM's will suffer worse from mistreatment than flooded cell, and you can't check the specific gravity in a sealed battery. So the battery you buy may be working today, but tomorrow?

In my opinion buying used batteries is a false economy unless you get them for scrap price. Determine what your system requirements actually are, and look at the cost of good new batteries (such as Trojan - very good value there usually) before you spend your money on a pig in a poke.

RandomJoe · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

I wonder about those AGM batteries. Looking at the link you provide, the combination of relatively low AH rating for the size, and the prominent mention of CCA and reserve capacity makes me think these are intended for car starting batteries. At least for flooded cells, car starting batteries make a very poor choice for solar / off-grid systems as they are designed for rapid shallow discharge then immediate recharge, instead of longer slower discharges. The difference is in how the plates are made, not sure just how equally that translates to AGMs.

I agree with the others, the other batteries are likely to have been used hard and coming from a variety of devices could be difficult to get a balanced / equally worn set thus increasing likelihood of problems. That said, if you know the history of those batteries and they were replaced as part of a regular maintenance cycle - "replace at 1 year regardless of condition" - the price is quite tempting for six batteries.

But that's only if you are willing to take the risk of bad batteries (thus no power). Depending on circumstances, I might do that if I could get a 480AH AGM bank for $180! (I'm not familiar with "Group 24", but a quick Google found some around 80AH capacity.) Certainly they'd be nice for learning on at that price. Four Trojan T-105s - roughly 440AH at 12V - would run around $400 brand new. New AGMs with that capacity will be considerably more.

If you intend or need to rely on the power though, I'd also suggest going for brand new, and selecting a battery that is definitely intended for "deep cycle" use.

System2 · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

I appreciate the feedback, though I'm still not sure which direction I will ultimately go. Part of the reason for AGM is simply the fact that they are nonspillable. We just purchased the property and it is out on an island. There will be a lot of loading and unloading of vans and boats with kids, cats, belongings, etc. and I wouldn't need to worry about accidental spillage or contact with acid.

As someone here speculated, I forgot to mention that it will be a 12 volt system. I have a Vector 800-watt modified sine wave inverter and 45-watt Harbor Freight solar 3-panel set I used for about 3 years with 5th wheel camper I just sold, and another solar panel (I am guessing about 30 watts) that came with the cabin. If I understand correctly, a small panel may be used during the winters to keep the charge topped off. What sort of wattage would be appropriate for this task, considering an average January temp of about 5 deg F and lows that can approach -40 deg F or even colder on occasion (assuming I wind up with a 480 AH bank at 12 V)? Also, as someone here speculated, doesn't this make AGM suited to this climate?

I see that there is a small charge controller (still learning and don't know much about charge controllers yet), but am wondering if anyone has recommendations if I am around 75 total watts with panels right now and I doubt I will ever go above 150 watts. A generator was left behind in the event I get in a bind with a string of cloudy days and I am on an extended stay.

Thanks,
Lance

niel · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

"I doubt I will ever go above 150 watts."

well that settles the issue with both offers as you would need far more to charge them than 150w.
as to the keeping a trickle charge on a battery, this is about 1% to 2% of the battery ah capacity (not the cca). so what is that in watts you might ask? the same ratio is present as the watts = volts x amps and so it is still the same percentage and would still be 1% to 2% of the battery wattage. you may want to add another 10% or more over the pv's stc wattage values though due to higher temps that normally exist on pvs while in operation.

BB. · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

Rough rule of thumb is 5-13% for charging current and 1-2% for trickle charging. Assuming 480AH at 12 volts (panel Vmp=17 volts):

480*5%=24 amp charging current
480*13%=62.4 amps charging current

24 Amp (x17v) = 408 watts minimum (5%) solar panels
62.4 Amp (x17v) = 1,061 watts maximum (13%) solar panels

480*1%=4.8 amps of trickle charging current
480*2%=9.6 amps of trickle charging current

4.8 Amps * 17v = 82 watts min. for trickle charging
9.6 Amps * 17v = 164 watts max. for trickle charging

Fully charged flooded cell batteries will probably never freeze for you (~-92F, 40% State of Charge ~-16F). Discharged (dead) batteries freeze at 32F.

AGM's, since the electrolyte is in a fiberglass mat (with air)--even if they freeze, the should not cause any internal damage.

So--here you are with a lot of battery storage and not much solar panel to drive it...

AGM's would be a better choice as they, typically, have lower losses (lower self discharge, no/very little equalization required) than Flooded Cell batteries.

But--you still have to charge them back up, somehow, after you have used power from them.

If you electric power needs are low--then you might squeak by... But if you electric power needs are low--then you don't really need such a huge battery bank.

And, if you do need more charging current--then you will need more panels and/or lots of generator run time.

By the way, the Harbor Freight 45 watt panels--you should measure how much power you get out of them yourself. Some people have not found them to be at name plate rating (lot less than 45 watts).

A rough estimate for summer power would be:

5 hours of sun * 50% sys efficiency * 150 watts = 375 Watt*Hours of AC power

So--it is up to you how you divide that out... Take the number of watts * the number of hours you run your loads per day, and divide that into 375 Watt*Hours:

I.e., 800 watt inverter running at 1/4 load (200 watts):

375 Watt*Hours / 200 watts = 1.875 hours = 1 hour 52.5 minutes

At some point, you will probably have to get some "real" solar panels for your cabin.

Highly recommend that you workout your power needs first (lighting, radio, computer, dvd player, etc.) first before you buy anything.

Plan your system out or you may be in for some unpleasant surprises.

-Bill

icarus · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

I too live on an island. I have used flooded batteries exclusively. A bit of care and you are fine. (A big safety issue with any battery is to cover the terminals in transit to avoid dropping a wrench or something across the poles!)

T-105's are pretty manageable, L-16's get to be a bit much to lug through the bush!

I go back to my original suggestion, if you are serious about solarizing your place,, do a good look at your loads,, then work backwards. As for keeping the batteries in shape over the winter, consider the minimum charging currents suggested here and by the manufacturer(s). I keep one small battery bank in float over the winter with a 63 watt panel.

Good luck,

Tony

niel · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

hey bill, the battery voltage is 12v and not 17v. remember we are determining battery wattage to find a pv's wattage. you may want to revamp your post.
example is 100ah battery at 12v. this is a 1200w battery and 5% of 1200w is 60w while 13% of 1200w is 156w. trickle may be 1% of 1200w or 12w.

BB. · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

Niel,

Actually, I did 17 volts on purpose to calculate the rough solar PV panel wattage (~15 volts battery charging + wiring losses + 2 volts controller drop = 17 Vmp at panel).

I just left his battery stuff in Amp*Hours and skipped the whole Watt*Hour/kWHr storage calculation since we did not know any of his loads for the 3day + 50% maximum discharge rule of thumb.

And went for the min/max rule of thumb for charging current/solar panel array size to fit his proposed bank.

-Bill

niel · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

determining loss factors and other such items do not enter into this as this is merely an extension of the 5-13% of the ah rule of thumb. that applies to the ah of the battery and the wattage is determined by the multiplication of the voltage with the ah. the rule of thumb does not apply to loss factors as they are variable and are considered separately in spite of your effort to wrap it up into one.

BB. · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

Niel,

I am not understanding your statement... They are "rough" rules of thumb for a guideline to what will work OK for an "average system" that charges during the day and uses power during the night.

I did not use any losses with the 5-13% rule...

And the average output of a 150 watt array + charge controller + flooded cell battery + inverter with variable load; was just that--an average or rough rule of thumb.

The fact that it is supported by the math and experiences of other here does sort of support its use. If I used 50%, 52% or 60%--those are all pretty much with the margin of error for any generically operated system with batteries and and an inverter.

If it was a specific designed system (radio repeater) with known components and operational profile--one could try and get a closer set of numbers (satellite/space systems are known for their consistent "day/night" power profiles)... But those numbers down to the 5% range or less would be blown away by the 10-20% variation in average daily output for those people in coastal areas with significant marine layers and other areas... So--again, to argue anything to a 5% accuracy seems to be a waste of effort.

-Bill

niel · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

ok, rereading it i misunderstood part of his question. my bad.

System2 · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

So...if I am understanding this right,

Rough rule of thumb is 5-13% for charging current and 1-2% for trickle charging. Assuming 480AH at 12 volts (panel Vmp=17 volts):

480*5%=24 amp charging current
480*13%=62.4 amps charging current

24 Amp (x17v) = 408 watts minimum (5%) solar panels
62.4 Amp (x17v) = 1,061 watts maximum (13%) solar panels

480*1%=4.8 amps of trickle charging current
480*2%=9.6 amps of trickle charging current

4.8 Amps * 17v = 82 watts min. for trickle charging
9.6 Amps * 17v = 164 watts max. for trickle charging

and I currently only have 75 Watts of panels, this wouldn't be enough for trickle charging the batteries?

I guess what I am trying to wrap my head around is this, if you add more batteries (or AH capacity) to a system, one MUST add more solar panel wattage? Is this because of the cumulative self-discharge of the batteries? And if this is the case and AGM batteries suffer less from self discharge, couldn't one add more AH capacity in AGM than in flooded cell?

Remember, I'm a newbie. Be gentle on me.

Thanks in advance,
Lance

niel · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

that panel is on the low side for a trickle charge, but may squeak by to maintain the batteries. it would be better than nothing in keeping the batteries going even if it proves to not be enough in the long term. how old or efficient the batteries are can have a impact on the trickle requirements and may vary somewhat.

BB. · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

And, you are correct that AGM's have less self discharge than Flooded Cell.

However, as all batteries age, their self discharge goes up... At least for Flooded Cell, that can reach as high as 1-2% self discharge per day. Although, the usual number is probably a few percent per week.

But remember that you only have about 2-6 hours per day of "full sun" (depending on season and local weather patterns)... So, even 1% per day only works out to a 24 hour charge rate of ~1%/4 hours ave sun per day = 0.25% per day charging.

Also, the 1-2% maximum charge rate is, roughly, the maximum charge rate from a solar panel that can be used without having a Charge Controller connected to prevent damage from over charging (you still may need an external blocking diode to prevent discharge through the panels at night).

-Bill

icarus · July 2009

Re: These Batteries Suitable For Solar Battery Bank?

Lance,

"I guess what I am trying to wrap my head around is this, if you add more batteries (or AH capacity) to a system, one MUST add more solar panel wattage? Is this because of the cumulative self-discharge of the batteries? And if this is the case and AGM batteries suffer less from self discharge, couldn't one add more AH capacity in AGM than in flooded cell?"

The KISS answer is more batteries=more self discharge=more charging capacity. There are probably math differences between different battery chemistries.

The other KISS answer is more batteries=more loads=more charging capacity.

As we have debated before,, it is probably best to start with your loads (including self discharge as a load!) using a kill-a-watt and work backwards (forwards if you wish) to design a Pv system,, from batteries to inverters to Charge controllers to Pv array.

Under charging and over draw is probably the single biggest killer of battery banks. Keeping the batteries in float to keep up with self discharge is probably much easier to cover with minimal charging capacity than other loads,, obviously!

Good luck, keep in touch and listen to these guys,, they have forgotten more than most of us will ever know about Re and Pv.

Tony

These Batteries Suitable For Solar Battery Bank?

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