two batteries connected in series going out of whack

[ClassicCrazy]

Where I work we have AGM batteries in the fire alarm system that floats them pretty much all the time. They are 24v with two 12v in series. One of the systems gets new batteries every year because the annual inspector says capacity is bad. After only one year . I can never figure that out , the charger always seems to be working but it just floats them all the time .

I would never get AGM batteries for my system just because of the problems you are experiencing. At least with flooded lead acid you can take the specific gravity of each cell and figure out what is going on and check for imbalances.

At work the testers use a capacity tester on the AGM . I don't really understand how they work or if they are truly accurate . Not sure they figure out the internal resistance of the battery or if they do some kind of load test . Usually no AGM battery ever shows their rated capacity even when new. 

[vtmaps]

BTW, if it is the case that one of the battery has a bad cell and the has higher internal resistance, is there a way for me to test the internal resistance of each battery?

If I were to hook up each battery individually to the solar or AC charger to charge, would I be able to detect a bad cell?

Unfortunately you cannot get to the individual cells to check their voltages.  You can do capacity tests and impedance tests that can tell you something. 

You ask about detecting a 'bad cell'.  As I mentioned earlier, cells are not all the same... they can vary in their capacity, charging voltage, charging time, lifespans, etc. 

It is normal that you should overcharge most cells in order to fully charge the weakest member of the bank.  The question is do you have a truly bad cell (warranty issue) or do you just have a weak cell.  Sometimes a cell is clearly defective (either open or shorted), but there is also a range of weak cells, some of which may be so weak as to be a warranty issue. 

If you can charge the batteries individually, that is best... for two reasons:
1) since you have to overcharge some cells in order to charge the others, you might as well limit the stress to one battery at a time.... one of the batteries probably does not need to be stressed as hard as the other.
2) You can compare the the two battery's electrical behavior, both under charge and under load.

Regarding that last point:
It would be good if you had a shunt based ammeter... can you get a whizbangjr and shunt?  If so, you can watch the current into each battery as it charges.  Normally, during absorb the current tapers off to an 'end amps' value.  It would be good to compare the end amps of the two batteries. 

Another thing you can do is watch how the batteries perform under load.  Get them as charged up as you can (long, long absorb).  Then put a constant load on them (maybe run 200 watts of incandescent bulbs).  See how the voltage holds up for an hour or two.   

Another load test that can give you some useful information is to watch how the voltage sags under a larger load.   Let the battery rest for a couple of hours, then with the voltmeter on the battery, connect a large load and immediately record the voltage.  If you know the voltage drop and current, you can estimate the battery's internal resistance.  example: you know that your large load draws 40 amps DC from the battery.  You observe that the voltage drops from 12.50 volts resting to 12.38 volts under the large load.

0.12 voltage drop ÷ 40 amps = 0.0033 ohms

If you can do this sort of testing on both batteries, you will have something concrete to discuss with us on the forum, and with the battery manufacturer if you want to submit a warranty claim.

--vtMaps

[New KID On The Block]

BTW, if it is the case that one of the battery has a bad cell and the has higher internal resistance, is there a way for me to test the internal resistance of each battery?

If I were to hook up each battery individually to the solar or AC charger to charge, would I be able to detect a bad cell?

Unfortunately you cannot get to the individual cells to check their voltages.  You can do capacity tests and impedance tests that can tell you something. 

You ask about detecting a 'bad cell'.  As I mentioned earlier, cells are not all the same... they can vary in their capacity, charging voltage, charging time, lifespans, etc. 

It is normal that you should overcharge most cells in order to fully charge the weakest member of the bank.  The question is do you have a truly bad cell (warranty issue) or do you just have a weak cell.  Sometimes a cell is clearly defective (either open or shorted), but there is also a range of weak cells, some of which may be so weak as to be a warranty issue. 

If you can charge the batteries individually, that is best... for two reasons:
1) since you have to overcharge some cells in order to charge the others, you might as well limit the stress to one battery at a time.... one of the batteries probably does not need to be stressed as hard as the other.
2) You can compare the the two battery's electrical behavior, both under charge and under load.

Regarding that last point:
It would be good if you had a shunt based ammeter... can you get a whizbangjr and shunt?  If so, you can watch the current into each battery as it charges.  Normally, during absorb the current tapers off to an 'end amps' value.  It would be good to compare the end amps of the two batteries. 

Another thing you can do is watch how the batteries perform under load.  Get them as charged up as you can (long, long absorb).  Then put a constant load on them (maybe run 200 watts of incandescent bulbs).  See how the voltage holds up for an hour or two.   

Another load test that can give you some useful information is to watch how the voltage sags under a larger load.   Let the battery rest for a couple of hours, then with the voltmeter on the battery, connect a large load and immediately record the voltage.  If you know the voltage drop and current, you can estimate the battery's internal resistance.  example: you know that your large load draws 40 amps DC from the battery.  You observe that the voltage drops from 12.50 volts resting to 12.38 volts under the large load.

0.12 voltage drop ÷ 40 amps = 0.0033 ohms

If you can do this sort of testing on both batteries, you will have something concrete to discuss with us on the forum, and with the battery manufacturer if you want to submit a warranty claim.

--vtMaps

vtMaps, thanks! Some great info there I'm sure -- not all of which I fully understand but I'll slowly try to digest it all.

I do have a Whiz Bang Jr with shunt. I purchased it with the KID. Your suggestion above is to monitor each battery individually with the WBJ, correct? I already monitor the battery bank with the WBJ. Then I monitor each battery with individual Fluke meters.

I will try out some of your load testing suggestions. The only minor problem is that each passing day the sun is giving me less and less total Ahr as winter is coming. I have less leeway to test and play around but I'll try to do some testing.

[New KID On The Block]

I would never get AGM batteries for my system just because of the problems you are experiencing. At least with flooded lead acid you can take the specific gravity of each cell and figure out what is going on and check for imbalances.

At work the testers use a capacity tester on the AGM . I don't really understand how they work or if they are truly accurate . Not sure they figure out the internal resistance of the battery or if they do some kind of load test . Usually no AGM battery ever shows their rated capacity even when new.  [/size][/font]

ClassicCrazy, yes, I agree somewhat with what you're saying. I now have my own beef with AGM batteries in general but there are pros with AGM batteries. While I like the ability to equalize flood batteries, my batteries are placed/stored in an environment that would NOT be suitable for much gas to be released.

At any rate, I don't want to this thread to get into a wet vs dry battery war. There are pros and cons to each type of battery. Thanks for your comments.

[New KID On The Block]

I have added two new 100w panels for a total of 600 potential watts. 

What I find interesting is that I'm able to ABSORB and FLOAT much earlier in the day. 

What I dislike is that I have 6 of these panels in series and it's getting knocked out very early into the afternoon @ ~ 3:05 pm. There is a tree to the side of the house and its shade is hitting the panels starting as early as 3 pm. 

during the summer, it didn't hit until much, much later, but now it's hitting it around 3pm, especially with the two new added panels.

I may be forced to separate the 6 panels into two strings of 3 parallel. 

[vtmaps]

I may be forced to separate the 6 panels into two strings of 3 parallel. 

Huh?  That doesn't make any sense.  If you have two strings, each string would be three panels in series.  The two strings would be parallel to each other. 

If you have three strings, then each string would be two panels in series, and the three strings would be parallel to each other.

Putting all six panels in series is the worst of your options, unless the panels are very far from the controller.  The controller will be more efficient if the string voltage is lower. 

I believe (correct me if I'm wrong) that your panels are "12 volt panels" with a Vmp about 17 or 18 volts.  If so, your optimum string length for a 24 volt system is three panels in series (two strings). 

It will probably work OK with a string length of two panels, but there is not much headroom for the controller to work with.  If your batteries need higher voltage absorb or EQ, and the panels are hot, two panels in series may not have high enough voltage. 

When you reconfigure your panels, try to make sure that all the panels that get shading are in the same string.

--vtMaps

[New KID On The Block]

I may be forced to separate the 6 panels into two strings of 3 parallel. 

Huh?  That doesn't make any sense.  If you have two strings, each string would be three panels in series.  The two strings would be parallel to each other. 

If you have three strings, then each string would be two panels in series, and the three strings would be parallel to each other.

Putting all six panels in series is the worst of your options, unless the panels are very far from the controller.  The controller will be more efficient if the string voltage is lower. 

I believe (correct me if I'm wrong) that your panels are "12 volt panels" with a Vmp about 17 or 18 volts.  If so, your optimum string length for a 24 volt system is three panels in series (two strings). 

It will probably work OK with a string length of two panels, but there is not much headroom for the controller to work with.  If your batteries need higher voltage absorb or EQ, and the panels are hot, two panels in series may not have high enough voltage. 

When you reconfigure your panels, try to make sure that all the panels that get shading are in the same string.

--vtMaps

Hi vtMaps. Still being new to this, my terminology/phrasing my not be correct. My next configuration will be 2 strings in parallel, each consisting of 3 x 100w panels in series. However you want to phrase it, that's my next config if I change my current setup.

I understand about the theoretical efficiency loss with 6 panel in one string. I want to see some empirical numbers of my current 6 panel in one string before potentially changing my config to compare the results of the two configurations.

All 6 of my panels are Renogy RNG-100D. They are advertised to work optimally for 12v systems.

Vmp: 18.9V
Imp: 5.29A
Voc: 22.5V
Isc: 5.75A

RE: Grouping 3 panels into shade region. Yes, I definitely thought of that. Will be doing just that if/when I reconfigure. Thanks!

Next thing up is probably ordering two additional 125Ah batteries. I'm still conflicted, trying to figure out the exact problem with my current batteries. If I get two more batteries, I'll have more data points to see which battery is behaving anomalously. But I'm concerned about buying two additional batteries before having my current, possibly bad, battery replaced under warranty.

[Westbranch]

I understand about the theoretical efficiency loss with 6 panel in one string.

to be clear, if you have a shading problem, you have trouble and if you stick with a single string of 6, you risk the entire strings output  when the shade comes into play, depending on the amount of shade on how many (?)  panels...

you need a layout that allows for thee 'normal max output' from some of the strings ie 2p X 3sor possibly 3p x 2s as Vtmaps has said.  Your choice as you are the one that can evaluate the amount of shade and number of panels affected.

FYI the shade of a single branch on a leafless Aspen can (did) cut the production of panels by > 50% at mid day for me... that tree did not last long...

hth

[New KID On The Block]

I understand about the theoretical efficiency loss with 6 panel in one string.

to be clear, if you have a shading problem, you have trouble and if you stick with a single string of 6, you risk the entire strings output  when the shade comes into play, depending on the amount of shade on how many (?)  panels...

you need a layout that allows for thee 'normal max output' from some of the strings ie 2p X 3sor possibly 3p x 2s as Vtmaps has said.  Your choice as you are the one that can evaluate the amount of shade and number of panels affected.

FYI the shade of a single branch on a leafless Aspen can (did) cut the production of panels by > 50% at mid day for me... that tree did not last long...

hth

I did not like what I had to do today, but I did it. I cut off a lot of branches from the lemon tree next to the house.  I am a big time tree lover/hugger. I do not like cutting down trees. I try to comfort myself by saying it was just some minor trimming but honestly it was some major branch cutting. It was quite a bit of work but I got all the branches that was immediately affecting my panels.

That gave me about another hour of 100% sun in the later afternoon -- albeit, that 100% sun is at quite an angle.

I also just contacted via email the seller of my batteries. I am still going to purchase two more 125Ah batteries from them but told them I wanted to work on getting one of my current two batteries replaced at the same time. The email was quite long. We'll see how they respond. I guess it's a good thing that I get to test out their warranty process before forking over another $500 for two more batteries. 

[Westbranch]

I'm a forester.

One of the problems with shading trees is that they grow each year...  this might be an ongoing process, look into how to prune that tree to keep it in check... for at least a few years.
I use a bit more draconian method, a chain saw, end of problem.

[New KID On The Block]

Also be aware that a new battery takes a while for the plates to 'form'. I would put a couple dozen cycles through them and review the situation. If you find the high battery going over 15 or 16V then start to worry. Thats about where thermal runaway will commence, but even that will take hours. Thermal runaway is more acute problem with AGMs and something to watch out for. And certainly if the battery gets hot, (feel all around it), then turn it off. AGMs dont get hot in normal use.

Hi zoneblue, it's been over a dozen, closer to two dozen, cycles. It's still the same.    At the end of absorb, I see one battery at 13.8v and the other one at 14.9v. I have my ABSORB set at 28.8v. When I've set absorb to 29v, the bad battery will be closer to 15.1v at the end -- that is with a load; if there's no load at all, it'll get above 15.1v. I've seen it go to 15.2v at which point I added a HIGH load to bring down the voltage. Now a days I set ABSORB at 28.8v and constantly keep at least a light load so the bad battery doesn't go  above 14.9v.

Thermal runaway is of course worse than the battery boiling/gasing itself. Even at 14.9v, I can start to hear bubbling sound inside the battery. At 15.1v, it's definitely bubbling inside.

I purchased a infrared thermometer. I do monitor the temperature of the batteries, among other equipment of this entire solar setup. As you allude to, the batteries are not generally hot at all, even when it's bubbling inside. I've never seen the batteries above 80F. Most of the time they're exactly room temperature. I also have a Midnite Solar temperature probe connected to my KID that monitors the batteries. (Hmm.. wonder if Midnite can program the KID to fire off a blinking LED if the temperature probe detects temps above a certain threshold. That would be a helpful.) The temp of the batteries doesn't deviate too much from 25C.

[New KID On The Block]

I'm a forester.

One of the problems with shading trees is that they grow each year...  this might be an ongoing process, look into how to prune that tree to keep it in check... for at least a few years.
I use a bit more draconian method, a chain saw, end of problem.

A forester for the US Forest Service I hope. 

I'm headed off to camp @ Stanislaus National Forest tomorrow through Sunday. Won't be able to tinker with/monitor my setup for the weekend.

The shade has previously been great at shading the sun from the house so it gets less hot during summers. Now it's going to be more sun hitting the roof during summer.  But I guess it can't be helped. I need sun for the panels.

It's kind of sad. I live in CA. We desperately need rain. We're in EXTREME drought conditions -- probably worse than what the Aussies were enduring several years back -- but I hate the rain now because any clouds, let alone rain, drastically reduces the output of my panels. I was monitoring the KID one day when it did rain all day here and I was probably getting 5% --if that-- of the max output of my panels. 

[Westbranch]

A forester for the US Forest Service I hope. 

Nope, I am in BC Canada

One may not like to do it but it is  a choice of what takes precedence,  shade or solar power...  chioces choices...

[New KID On The Block]

Another thing you can do is watch how the batteries perform under load.  Get them as charged up as you can (long, long absorb).  Then put a constant load on them (maybe run 200 watts of incandescent bulbs).  See how the voltage holds up for an hour or two.   



If you can do this sort of testing on both batteries, you will have something concrete to discuss with us on the forum, and with the battery manufacturer if you want to submit a warranty claim.

--vtMaps

Hello vtMaps.

I am working with the seller of the batteries -- who in turn is supposedly working with the manufacturer -- on a warranty replacement. They're having me do some testing on my end.

Last Friday late afternoon, after finally going into FLOAT, I disconnected the KID from my batteries and let the batteries rest for 3 hours -- as instructed by manufacturer to see what the voltage would be after resting.

---------------------------------------
At 4:41 pm PST Friday I disconnected.
At 7:43 pm I went and checked voltage for the two batteries. They both read 13.04V. 

I then proceeded to test using my desktop computer setup connected to the inverter and the load was around 150 - 200 watts. I subsequently checked the voltage thereafter each hour for the next 5 hours.

At 8:44 pm, the two batteries showed A) 12.64v and B) 12.61v.
At 9:44 pm , the two batteries showed A) 12.54v and B) 12.51v.
At 10:44 pm, the two batteries showed A) 12.43v and B) 12.39v.
At 12:33 am, the two batteries showed A) 12.21v and B) 12.18v.
At 1:44 am, the two batteries showed A) 12.07v and B) 12.03v.
----------------------------------------

That is part of what I sent them. They were suppose to call me today but did not. I'll see if they call me tomorrow.

On another related matter, I have reconfigured my PV panels. Just now, as the sun was about to set, I split my 6 x 100w single series connected panels string into TWO parallel strings, each consisting of 3 x 100w series connected panels.

That should be most efficient into the KID for my 24v battery bank, yes?

Tomorrow I'll see how this new config performs. The best I achieved out of the previous config was about 435W out of a total theoretical 600W.

Also, with further testing, I've been able to determine that if I set ABSORB to 28.5V, then the highest voltage the 'bad' battery will reach is about ~14.75v. So I've decided to leave ABSORB to 28.5V so as not to 'equalize' my AGM batteries daily. But in doing that, the 'good' battery is charging at only 13.7v or 13.8v during ABSORB. :-(

The interesting thing is -- as can be seen in my results above -- even with lower voltage during ABSORB, the 'good' battery holds voltage better than the 'bad' battery under load.

[dgd]

I am working with the seller of the batteries -- who in turn is supposedly working with the manufacturer -- on a warranty replacement. They're having me do some testing on my end.

At 8:44 pm, the two batteries showed A) 12.64v and B) 12.61v.
At 9:44 pm , the two batteries showed A) 12.54v and B) 12.51v.
At 10:44 pm, the two batteries showed A) 12.43v and B) 12.39v.
At 12:33 am, the two batteries showed A) 12.21v and B) 12.18v.
At 1:44 am, the two batteries showed A) 12.07v and B) 12.03v.

That is part of what I sent them. They were suppose to call me today but did not. I'll see if they call me tomorrow.....

The interesting thing is -- as can be seen in my results above -- even with lower voltage during ABSORB, the 'good' battery holds voltage better than the 'bad' battery under load.

Two batteries within 0.3volt of each other after several hours loading 

I think you will be darn lucky if the battery supplier entertains a warranty replacement as both these batteries look good to me.
If you are expecting dead on identical performance from two same model batteries from the same manufacturer (and even from the same manufactured batch) then I fear you are always going to be disappointed.

I recommend you forget this issue and only make  a warranty claim if one of the batteries fails

dgd