Unique battery situation - how to use them safely?

javid

Solar newbie here!  I am looking for some help designing my battery wiring for my solar system.

I plan to do a small 48v system to service my small cabin.  Daily loads will be around 500Wh.  I have the MidNite Solar Kid 30a Charge Controller so far.

I also have a bunch of used batteries.  They are SLA 12v 18AH each.  They are used for around 3-4 years in a wind turbine pitch system before I get them and they have been treated well.  I have 25 of these batteries.  I normally wouldn't deal with used batteries since that seems like a headache from what I have read, but I will have access to 25 more batteries every year going forward, so I thought maybe I should try to figure something out to make good use of these.

As I said, I plan to do a 48V system - what would be the best way to string these batteries?  From my calculations, I need to use at least 16 of these batteries in my system (if I go to 50% DOD), but if there was a safe way to use more, I would prefer to use up to 24.  

I know it isn't ideal, but all I could come up with so far is (4) strings of (4) batteries wired in series.  Any and all help much appreciated!

westbranch

I think there is a error in your calcs...
As I said, I plan to do a 48V system - what would be the best way to string these batteries?  From my calculations, I need to use at least 16 of these batteries in my system (if I go to 50% DOD),

I get 48V x 18 Ah x 50% = 432Wh

So you need 1more string to get  cells to get to ~860Wh, which is a healthy reserve and still  stay > 50% dod.  However you only have about 1 1/2 days of autonomy.  To get  to 3 days autonomy you would need 3 parallel strings of 4 batteries and you are approaching a potential problem of uneven charging so a close eye will be needed on the charge state of each string...  doable though not optimal, you just need some slightly higher rated batteries...
hth

Marc Kurth

javid said:

Solar newbie here!  I am looking for some help designing my battery wiring for my solar system.

I plan to do a small 48v system to service my small cabin.  Daily loads will be around 500Wh.  I have the MidNite Solar Kid 30a Charge Controller so far.

I also have a bunch of used batteries.  They are SLA 12v 18AH each.  They are used for around 3-4 years in a wind turbine pitch system before I get them and they have been treated well.  I have 25 of these batteries.  I normally wouldn't deal with used batteries since that seems like a headache from what I have read, but I will have access to 25 more batteries every year going forward, so I thought maybe I should try to figure something out to make good use of these.

As I said, I plan to do a 48V system - what would be the best way to string these batteries?  From my calculations, I need to use at least 16 of these batteries in my system (if I go to 50% DOD), but if there was a safe way to use more, I would prefer to use up to 24.  

I know it isn't ideal, but all I could come up with so far is (4) strings of (4) batteries wired in series.  Any and all help much appreciated!

Are you getting these free, or close to it?

Since you obviously are not afraid of an experiment - I would likely do what you said: Four strings of four for a total capacity of 72 ah at 48v = 3456 watt hours. With a daily load of 500 watt hours, 3 days with with no sun would consume 1500 watt hours out of your 3456.

If you want to add more strings, go for it since you have them. But you might be better off keeping some spares as replacements. Regardless of how many strings, be sure to fuse each string, and the battery bank as a whole. With a new supply of batteries available each year, who cares if the system is less than optimal........

Do yourself a favor and weed out the weaker batteries first. Then monitor voltages and currents closely so how they balance out.

javid

Thanks for both replies!

WestBranch - you are correct that I could make do with less strings, but I was hoping to use these in winter with low battery temps and have at least 3 days reserve (we get a lot of cloudy days in winter here).  Also thinking I might add more consumption in years to come and I was looking to figure out a system now that would allow for a little growth.

Marc - yes - getting these for scrap price, which is basically free and likely will get a fresh set of used each year (occasionally every other year).

I did a rudimentary load test on each battery - 100watt lightbulb on for 15 seconds using cheap 12v inverter.  At 40 degrees F ambient, I got voltages ranging from 12.0v-12.3v.  I have 20 batteries that were >12.15v and planned on using those.

How would I go about monitoring voltages and currents closely when the batteries are all connected?  

Also, if I did 4 strings of four batteries, how do I figure out what size fuses to stay safe?

PNjunction

javid said:

<snip>
I did a rudimentary load test on each battery - 100watt lightbulb on for 15 seconds using cheap 12v inverter.  At 40 degrees F ambient, I got voltages ranging from 12.0v-12.3v.  I have 20 batteries that were >12.15v and planned on using those.
<snip>

Javid - an agm in GOOD shape will read 12.2v when it is at 50% DOD when allowed to rest for at least 8 hours. (no load, no charge).

Your initial ups-style test is not the type you'll be using in your long-draw application.  Here's a better test.  We'll draw current at the C/20 rate, but only for 10 hours.  This will represent nearly 50% DOD.  If the battery is in good shape, then after 8 hours rest, your terminal voltage should be 12.2v or higher.

Instead of using a 100w bulb with the inverter, drop that back to only TEN watts, and run for ten hours.  The C/20 rate for your 18ah batteries would actually be 900 milliamps.  Essentially, you can find an LED bulb at the grocery store that will provide this.  A typical 60-75 "equivalent" led bulb actually runs about 8 to 12 watts.  See the box ratings.  Your msw inverter, with it's losses means you divide by 10 (not 12v like one would think).

You can fudge it a bit by running only 9 hours, or maybe 11 hours if your bulb wattage isn't exactly on target at 9 watts)

Dischage the light, and let it rest for 8 hours and take a reading.

But there is something MUCH more important than this!

PNjunction

What is more important right now is that you have just become the trash-man, and as such this is merely a technical, and not really a practical, nor safe application.

I get it - you have a yearly delivery of aged batteries, and you want to eek some use out of them, maybe a year tops before you recycle, and start over with your new delivery.

Nobody in their right mind would suggest a 4P4S bank of 3-4 year old batteries is starting out on the right foot, even if they were free.

Keeping these calendrically-aged batteries all well balanced will be like herding cats.  And, I'll be that these are generic offshore general purpose 18ah agm's, and not something like a more costly industrial Enersys/Genesis pure-lead.

Your own time, if you value it at all, is worth something.  Not to mention you'll learn NOTHING from decrepid 4 year old cheap batteries other than how to be the trash recycler.

Start out on the right foot.  Since your time is worth something, start FRESH.  New, inexpensive batteries that have been under your control since day 1 is the right way to go, like a set of GC2 flooded or similar.
 

mcgivor

Welcome to the forum javid, by the way.

Trash-man is a little harsh, more like e recycling broker .....if anything you might learn a thing or two before you get some batteries that are designed for cyclical use on a solar system. SLA batteries are generally used for standby, held on float for long periods and called on intermittently during a power outage. The first thing you will learn is the won't work well, or for very long but we all start somewhere. Wouldn't plan on long term with them because the lessons learned will unlikely translate directly to what you will eventually end up with.
Please don't be discouraged by the coments, they are constructive attempts to point you in the right direction, nobody got it right the first time, we all made mistakes, wish I had discovered this forum before I started, could have avoided a few problems, PNjunction is right though, start fresh with flooded cells to learn about batteries, get the right tools (hydrometer clamp-on DC ammeter etc.), perhaps even start with a 12v system  to minimize the losses on the learning curve, as most trash their first set of batteries.

Marc Kurth

mcgivor, SLA batteries aren't used in off grid homes and other cyclic applications?

mcgivor

Yes Mark that is exactly what I said, they are generally used for standby, held on float for long periods and called on intermittently during a power outage (UPS, fire alarm,emergency lighting etc.). What was meant was, something may be learned with SLA prior to the eventual outcome , some batteries designed for the purpose, deep cycle.No offense by your comment and hope none taken by the response. 

Marc Kurth

Well, perhaps there is a difference of terminology. Sealed Lead Acid batteries is broad class of batteries which include AGM's and Gel. Both of which are of course available in deep cycle versions and are applied in off grid solar and deep cycle industrial applications daily.

Here is a 40 KWH ,7 year old 4S x 4P battery bank in an off grid home, being opened up for load testing last year. The last time the hatch was pulled was 5 years prior.

Fourteen of them were in the 86% to 88% range, but two were lower. Optimal layout? Heck no, but it is his first battery bank so he didn't know that.




I appreciate you saying it, but of course no offense meant or taken - just a discussion and sharing of experiences.

Marc

mcgivor

Would be great if javid could post a picture of the batteries aquired, 12V 18Ah is a common battery used in fire alarms.

Marc Kurth

mcgivor said:

Would be great if javid could post a picture of the batteries aquired, 12V 18Ah is a common battery used in fire alarms.

That would be a non-cyclic battery type indeed!

PNjunction

Javid - just know I certainly didn't mean to offend you, but more like grab your attention.

What it kind of represents is something many newcomers to solar do - looking to take shortcuts or the cheapest route.  Trust me, I know how tempting it is to see what appears to be a great battery deal go to waste.

At 3-4 years old out of the gate, and with a large 4S4P bank, you'll be chasing your tail as these aged batteries drift around in balance - usually behind your back since unlike flooded, you can't check the health of individual cells to spot trends.

That's why for me, who like's AGM by preference, despite their lower cycle life and other issues, always recommends to strive for the least amount of batteries you can get away with.  Personally, that is only TWO in either series or parallel, tops for agm.

javid

Thanks so much for all the feedback!  I don't take offense to any of it, and I appreciate everyone who took the time to help me out and offer their opinions and experience.  

Sorry for being out of touch - was away for work and forgot to check in on this forum.  I will post a pic of batteries tomorrow and probably have some more questions.  Thanks!

softdown

Bah humbug on the naysayers. This is a delightful experiment and a cheap way to learn the ropes. Scrap value really is *FREE* since they will always be worth scrap value.

Since most people murder their first battery bank, it makes sense to make the first one a cheap one. This board is full of solar pros who have a 180 degree different perspective on how to do solar.

Off grid homes are usually in the boondocks and America currently offers very few jobs in the boondocks. Saving money is as critical as anything.

After javid has dealt last rites to enough of these batteries, he may be ready for a more professional set of batteries.

The best time to check battery voltage is very early morning. Before the charge controller has kicked on.

As for deciding which batteries to use, that one is also easy. Roughly 95% of the time, the "best" batteries will deliver the highest voltage reading.

We once had a poster who successfully macgivored a huge number of very small nickel cadmium batteries. Shiver me timbers.

Do use fuses or circuit breakers. Batteries have a lot of energy.

javid

Again, thanks for all the feedback.  I have attached a picture of the batteries I have.  

Just like McGivor said, these batteries were held on float for long periods and called on when there was a power outage in a large wind turbine - they would feather the blades out of the wind if there was power outage to keep turbine from destroying itself...

I know there was a better way to load test, but I needed to test 40+ batteries, so figured the 100w light would at least tell me something about the state of batteries.  I should probably do a proper load test on a few of them to get better idea of their possibilities.

I hear what many of you are saying because I have made this same mistake a few times with other projects - getting by with sub-optimal equipment, only to eventually purchase what I should have at the beginning to save myself headaches...

But there are more reasons I would like to try to use these.  Nothing these batteries will be used for is critical at all and I have a ton of flexibility about timing loads.  Last summer we got by with headlamps and one deep cycle battery I charged 3 times all summer.  The bigger issue is we really don't know our eventual loads or system needs fully.  To simplify, I said this is for our cabin, but our cabin isn't built yet and in the mean time we will be using mini-motorhome for living quarters.  Sometimes it is hard to know what you need until you try and we need to spend more time living on our land to figure that out.

If I was sure of our system needs, I would buy new batteries, and I thought in the meantime these batteries would give me some experience, while providing some power needs.  

I also wanted to stick with 48v because we have two electric bikes with very high quality 48v 24AH batteries in them (lithium-ion) with less than 100 charge cycles each.  Eventually, I hope to get these connected to my system to provide a large backup battery supply.  So, I thought I would stick with 48v to keep from having to replace things in the future.  When I get my system needs figured out, I may even use just lithium-ion batteries for my system.

So all that being said, I am a little unclear about drawbacks to using these batteries in a 4S4P bank - particularly the safety concerns.  I am okay with batteries being abused and there not being equal charging, etc (at least I think I am okay with that), but I don't want to burn my shed down.  

So - let's assume I will be using these batteries in a 4S4P bank - what is safest/best way to do that?

Thanks!

mcgivor

Well it seems you are willing to experiment and personally I'm all for that, I did the same and learned the hard way, I used similar batteries to start and had a small 12V system using LED strip lights which drew very little current for a few hours per night. The battery lasted about a year before the capacity tapered down to virtually nothing and that was with a new battery  and a 10A controller. The reason we suggest getting deep cycle is to save your time and skip that phase of the learning curve. 
One thing I must caution you on is don't  combine your high quality lithium bike batteries with the lead acid as they need a different charge algorithm, keep them seperate.
The 4 strings of 4 will complicate things and most likely increase the chances of failure through unbalanced charging and discharging. Be sure to fuse each string to reduce the risk of fire should one battery dry up and become a dead short, I have seen them in emergency lighting systems swell up and actually catch fire.

javid

Thanks mcgivor!  Yes - I was planning on another charge controller if I ever add the Lithium batts to my system.

I guess when reading all previous comments, I didn't completely understand the part about how bad these particular SLA batteries really are for cyclical applications.  If I can't reasonably expect them to last 1 year, then there is indeed very little point in trying to work with these...

Marc Kurth

What are the approximate dimensions of those batteries?

softdown

"I said this is for our cabin, but our cabin isn't built yet" Don't buy new batteries. Took me three years to get my solar system installed...after purchase.

Construction is *always* behind schedule and over budget.

While batteries may pose a fire risk, keep in mind the board is populated mostly with solar pros. A game warden sees more grizzlies than an ordinary camper....if you get my drift.

BTW...you can study our sponsors longish article on batteries away from this discussion board. The writer mentions some telephone float type batteries that lasted 30 years... On the other hand.....these batteries are really small. That is the only real problem that I have with trying them.

mcgivor

Marc Kurth said:

What are the approximate dimensions of those batteries?

The approximate dimensions are around  7.1" ×" 3 × 6.6" typically.

javid

mcgivor said:

Marc Kurth said:

What are the approximate dimensions of those batteries?

The approximate dimensions are around  7.1" ×" 3 × 6.6" typically.

Yes - That is the exact size of these batteries.

mvas

I would put a fuse in each series string, then combine into parallel.

1) Fully recharge each battery independently (Some AGM batteries can and should be equalized)
2) Rest each battery for 24 hours
3) Measure the voltage of each battery after resting
Report your results.
Do they all hold 12.8+ volts ?
A battery that cannot hold 12.7 volts is questionable.

I would arrange the batteries as follows ...
one @ 12.90v battery in each string
one @ 12.85v battery in each string
one @ 12.80v battery in each string
one @ 12.75v battery in each string
==============================
Same 51.3 volts in each string

Equal length wires and symmetrical connections from combined (+) to fuses to each battery to combined (-).
A monthly voltage comparison will indicate if an individual battery is going south.

mcgivor

mvas said:

I would put a fuse in each series string, then combine into parallel.

1) Fully recharge each battery independently (Some AGM batteries can and should be equalized)
2) Rest each battery for 24 hours
3) Measure the voltage of each battery after resting
Report your results.
Do they all hold 12.8+ volts ?
A battery that cannot hold 12.7 volts is questionable.

I would arrange the batteries as follows ...
one @ 12.90v battery in each string
one @ 12.85v battery in each string
one @ 12.80v battery in each string
one @ 12.75v battery in each string
==============================
Same 51.3 volts in each string

Equal length 

These batteries are not AGM  and should never be equalized as they will be ruined, they also do not like a high initial current, so a custom algorithm may need to be programmed into your controller, see attached PDF. 

Marc Kurth

I did not realize that gel batteries were being deployed in this type of application. Even looking at the picture, I would have bet that they were standard VRLA AGM's. Glad I didn't bet!  

I assumed that they were something like these: http://www.vision-batt.com/site/product_files/CP12170H-X.pdf

(Included the link to show why I thought that!)

mvas

mcgivor said:

mvas said:

I would put a fuse in each series string, then combine into parallel.

1) Fully recharge each battery independently (Some AGM batteries can and should be equalized)
2) Rest each battery for 24 hours
3) Measure the voltage of each battery after resting
Report your results.
Do they all hold 12.8+ volts ?
A battery that cannot hold 12.7 volts is questionable.

I would arrange the batteries as follows ...
one @ 12.90v battery in each string
one @ 12.85v battery in each string
one @ 12.80v battery in each string
one @ 12.75v battery in each string
==============================
Same 51.3 volts in each string

Equal length 

These batteries are not AGM(*1)  and should never be equalized(*2) as they will be ruined, they also do not like a high initial current(*3), so a custom algorithm(*4) may need to be programmed into your controller, see attached PDF. 

(*1) Windurance states that you are wrong ... these are AGM Batteries.
(*2) Lifeline states that you are wrong ... their AGM batteries can and should be equalized.
(*3) PowerStream states that you are wrong ... AGM batteries do like a high initial recharge current (ie fast recharge).
(*4) Since these are AGM batteries ... A Standard AGM algorithm will work, a custom algorithm is not needed.
The AGM Battery Industry disagrees with everything that you wrote ...

Always follow the manufacturer's recharge instructions for their AGM battery.
AGM = Absorbent Glass Mat

mcgivor

@ mvas Interesting, the battery has no AGM  markings, and on the Windurance site I can't seem to find any information on their batteries other than they state a 1 year warranty, do you have a link as you now have me curious, we can all be wrong at times. Have sent them an email with the question as ro the type of battery, will await a reply. 

Marc Kurth

Slow down just a bit MVAS. I don't know mcgivor and will not speak for him, be I felt that he was simply sharing a whole lot of experience and offering guidance. His comments were based on the batteries being gel, and would be totally valid in that case.

A few things to remember:
- Lifeline didn't make those batteries - that I can promise you.  Most AGM's will suffer greatly when equalized. Only a few brands can take it. Yes, if they were mine I would equalize them at 15.5v for 2 hours at 75-80 degree F, instead of two x 8 hour runs.

- High charging current is a relative term. All batteries have upper limits, but I am intimately familiar with these:
Fullriver doesn't want to see more than 35% to 40% of battery C/20 rating. Exceeding this by an appreciable amount is damaging.

Lifeline will readily accept 5 times the C/20 rating for a short duration and 2.5 times C/20 continuously. (Yes, that means 500A on a 100A battery for 2-4 minutes. And yes, I have been there and done that) 

- Be careful with "standard" AGM algorithms. Different manufacturers have different requirements. There is no single charge/discharge that works equally with all AGM's. I do see batteries slaughtered yearly because of this concept.

I'm still curious to see how this experiment goes regardless of battery type!

Marc

javid

OP here - I am not sure what kind of battery this is, but from what I know about AGM (which is from 15 minutes of reading just now), it seems likely.  

These are used in the hub of a utility scale wind turbine.  They are in a metal box (18 to a box, with about 30K of circuit boards), spinning 360 degrees 15x/minute 24/7 if the wind is blowing.  They would need to be very leakproof and stable.

Will these fail within a year with my intended use?

westbranch

To add to Marc's comments (as I have AGM's) My maker's manual talks about an Refreshing /EQ charge but it is only a few 1/10's over the max Absorb voltage. And they stress caution in doing so, hence I have not done one as yet but have used the max Absorb V setting in the winter   and since we get minimal sunlight from mid Nov I set the Absorb TIME to longer than the sun is up. ( also I have a Classic CC and use a WBjr to monitor the End Amps setting and I raise that a bit)   This I do till Late February. 
So please proceed with caution in the Voltage you choose to use in Absorb and EQ.

mcgivor

javid said:

OP here - I am not sure what kind of battery this is, but from what I know about AGM (which is from 15 minutes of reading just now), it seems likely.  

These are used in the hub of a utility scale wind turbine.  They are in a metal box (18 to a box, with about 30K of circuit boards), spinning 360 degrees 15x/minute 24/7 if the wind is blowing.  They would need to be very leakproof and stable.

Will these fail within a year with my intended use?

It is impossible to predict how long they will last, they are used, so the best bet would be as other have said, weed out the weakest and hope for the best.

Could you elaborate a little on the, 30 K is that 30K $? And the spinning  360 degrees..........., just curious as this would give some indication of the type of battery. 

Generally speaking a battery that is a deep cycle AGM would state that on the side, these batteries  are most likely  not manufactured by Windurance, so for now the jury is out on that one as doubt has been cast, personally I can't find any information on their site orelsewhere, to indicate exactly what they are.