two batteries connected in series going out of whack

[New KID On The Block]

Hi all. The title of this post may not be the most descriptive but it's late/early and I can't think of a better way to describe my problem.

I have (2) 12v 125Ah VMAXTANKS AGM batteries connected in series to form a 24v 125Ah battery bank. It's connected to the KID. I thought the batteries would charge up in synch but I have noticed that when the batteries are nearing the end of the absorb stage, their voltage start to diverge. That is to say, battery A will get very high in voltage (15.1v or even 15.2v) while battery B backs down in voltage (e.g. 12.9 or 13.1); then in a little while, battery A will decrease in voltage and battery B will increase in voltage. Eventually both batteries will decrease to around 13.5v individually and they will enter FLOAT. I am able to see these voltage difference because I individually measure voltage on each battery with separate Fluke DMs. (They are calibrated and measure/read the same.) If I measured the battery bank as a 24v battery, then the combined voltage of the two 12v batteries are inline with the ABSORB and FLOAT voltage. That is to say that one would only notice this divergence of individual 12v battery voltage if one were actually checking them individually. One would not notice anything wrong with the 24v series battery bank if one were solely monitoring the 24v bank itself.

Is this normal? I didn't see this behavior when I had the battery connected up in parallel. When connected in parallel, both batteries basically measured the same voltage all throughout all the charge stages. I switched between a parallel and series battery setup to see if this divergence in voltage showed up in a parallel setup. It did not, only in my series setup. 

What I am most concerned about is the spike in voltage at the latter stages of absorb. Each of the batteries -- when connected in series -- are going up to 15V+. It's not for a very long time -- probably 20 to 30 minutes -- before gradually coming down in voltage. I have my ABSORB set to 28.8v (normally it's set to 29v) so as to try to reduce the HIGH voltage each of the battery reaches at the end of ABSORB.

Is this normal? If it's not normal, what might be causing this behavior? The KID? Or the batteries?

Batteries were purchased brand new (Sept 2014).

Thanks in advance for any helpful insights!

NKOTB

[zoneblue]

What this means is that the batterys are not the same. No suprise there. To be precise they are either at a different SOC or possess differnent internal resistance.  Most often youd see something like this is you took two random batterys and combined them for the first time. To mitigate this you would usually precharge both cells individually, prior to connecting them. Also theres a rule of thumb around here that batterys should only be combined that are identical in terms of brand, rated capacity and whose difference in age is "insignificant".

On one hand youre right enough to watch out for this, because one battery could overcharge. If your cells are otherwise identical in both capacity and history, then one cell may be deteriating faster than the other. How old are these batterys? Of course your average multi cell battery endures this everyday. Offsetting this is the fact that lead acid batterys are fairly tolerant to overcharge. And the fact that the cells have always operated together and were tightly binned to start with.

In general terms if the bank gets properly absorbed, then the pack becomes more or less top balanced so capacity differences show up at lower SOC. In that case you should notice this less as time goes on. If it gets worse then it may be a sign that one of the cells is dieing.

[New KID On The Block]

What this means is that the batterys are not the same. No suprise there. To be precise they are either at a different SOC or possess differnent internal resistance.  Most often youd see something like this is you took two random batterys and combined them for the first time. To mitigate this you would usually precharge both cells individually, prior to connecting them. Also theres a rule of thumb around here that batterys should only be combined that are identical in terms of brand, rated capacity and whose difference in age is "insignificant".

On one hand youre right enough to watch out for this, because one battery could overcharge. If your cells are otherwise identical in both capacity and history, then one cell may be deteriating faster than the other. How old are these batterys? Of course your average multi cell battery endures this everyday. Offsetting this is the fact that lead acid batterys are fairly tolerant to overcharge. And the fact that the cells have always operated together and were tightly binned to start with.

In general terms if the bank gets properly absorbed, then the pack becomes more or less top balanced so capacity differences show up at lower SOC. In that case you should notice this less as time goes on. If it gets worse then it may be a sign that one of the cells is dieing.

hi zoneblue. Thanks for replying.

These batteries were brand new when I purchased them (mid Sept 2014). They were purchased within 2 weeks of each other in Sept. They have a manufacturing date of June 2014. They are the same make/model. (Vtank Vmax 125Ah Solar batteries) ( http://www.vmaxtanks.com/servlet/the-43/vmax-tanks-AGM-battery/Detail ) It's possible (but I suspect highly unlikely) one of the batteries is prematurely dying. These are very robust AGM batteries ($250/each) and they were brand new when purchased. They came precharged at ~13.04 to 13.09v -- I measured as soon as I got them.

What I don't understand is why this out of synch voltage happens only when connected in series. It does NOT happen when connected in parallel. I've monitored the batteries as they are charged when connected in parallel and they both go up in voltage in synch as they undergo ABSORB and FLOAT. It's only when I connect these same batteries in series that they go out of 'whack' during the ABSORB stage.

There is ONLY one cable that connects the series changed batteries together. I've tried re-wiring my setup so that the one cable connects to the other two terminals but that doesn't change anything. Same thing happens.

If one of the battery were truly bad, wouldn't this out of synch phenomenon happen when the batteries are hooked up in parallel? It does not happen in parallel.

Because it doesn't happen in parallel hookup, I was thinking perhaps it might be the KID malfunctioning?

One thing that is kind of frustrating is the fact that I can't seem to 'equalize' these batteries as they are AGM and completely sealed. If I were able to equalize them, I think I could probably help this situation a bit.

Would it be very bad to try 'equalizing' these AGM batteries at a lower voltage than normal equalization voltage of 15.4v? say 15.0v or 15.1v for an hour or so? I now wish I went with the Trojans lead acid batteries so I could equalize, but the AGMs have a lot of positive qualities in their own right. When these batteries are connected in parallel, they work great. No problems. The only thing that really prevents me from changing to a 12v parallel battery bank is that my nice 1500w pure sine wave inverter is 24v and I don't want to get another 12v pure sine wave inverter. Otherwise I'd just connect the batteries in parallel and be done with it.

This out of synch voltage at end of ABSORB really caused a lot of anxiety for me. Everyday I feel like my batteries are prematurely being overcharged to death when it gets to 14.9v or 15v at the end of the ABSORB stage.

I've ordered (2) more 100w mono panels to my existing (4) 100w mono panels for a total of 600w.
I plan to order (2) more of these 125Ah batteries for a total of 250Ah (24v) series/parallel connected battery bank. But I really wanted to work out this out of synch phenomenon before I ordered the additional batteries.

I understand it's not ideal to have batteries that are not matched up perfectly/exactly but I think a two month difference should be acceptable given that these batteries are designed for 8-10 years of lifetime use.

[TomW]

Dc circuits 101:

ANY 2 batteries (or other circuit components) in parallel will always read the same voltage when connected together in parallel .

The nature of parallel connections is that they all see the same voltage. Period.

So that begs the question how do  you know they only get out of whack in series?

In a perfect world matching batteries should be very close on voltage in series. In the real world connection imperfections could easily skew readings, especially under charge or load.

Just from here.

Tom

[dgd]

Because it doesn't happen in parallel hookup, I was thinking perhaps it might be the KID malfunctioning?
...
This out of synch voltage at end of ABSORB really caused a lot of anxiety for me. Everyday I feel like my batteries are prematurely being overcharged to death when it gets to 14.9v or 15v at the end of the ABSORB stage.

This is definitely an issue with battery cell internal resistance.
The KID cannot see each battery separately but just one nominal 24volt bank so it cannot adjust charging to account for the SOC of individual cells in the bank. Therefore its very unlikely the KID is malfunctioning.

There is probably nothing you can do about this without changing to different batteries.
In parallel then each battery is probably charging at  different rate but the parallel connection to the controller means voltage will appear identical on both batteries. Hence your comparison will be meaningless.

I would suggest ignoring this and if  battery fails within warranty period then replace it (them)

dgd

[New KID On The Block]

Dc circuits 101:

ANY 2 batteries (or other circuit components) in parallel will always read the same voltage when connected together in parallel .

The nature of parallel connections is that they all see the same voltage. Period.

So that begs the question how do  you know they only get out of whack in series?

In a perfect world matching batteries should be very close on voltage in series. In the real world connection imperfections could easily skew readings, especially under charge or load.

Just from here.

Tom

Tom, thanks for your comments.

I fully admit I'm a neophyte when it comes to electricity. I don't have a EE degree. I studied Philosophy and Economics. :-)

I measure/monitor each battery individually -- when connected in series or parallel -- with separate Fluke DMs. I was under the impression that is how I could tell if each battery was charging at appropriate voltages. I stand corrected if my impression was incorrect.

Are you saying that even when each battery is reading, for example 13.15v, during charge that they may actually be charging at a different voltage? That doesn't sound right to me. I thought a voltage reading at the battery terminals would reveal the actual, instantaneous charging voltage to the battery. That is why I assume the batteries were properly charging at the same voltage when in parallel because I can see the voltage from each battery from separate Fluke meters. 

[New KID On The Block]

This is definitely an issue with battery cell internal resistance.
The KID cannot see each battery separately but just one nominal 24volt bank so it cannot adjust charging to account for the SOC of individual cells in the bank. Therefore its very unlikely the KID is malfunctioning.

There is probably nothing you can do about this without changing to different batteries.
In parallel then each battery is probably charging at  different rate but the parallel connection to the controller means voltage will appear identical on both batteries. Hence your comparison will be meaningless.

I would suggest ignoring this and if  battery fails within warranty period then replace it (them)

dgd

Thanks dgd for your comments.

I monitor each battery in parallel with separate DMMs. They show very similar voltage. I'm confused about this issue. Am I to understand that even though a battery shows a certain voltage when connected in parallel that they actually may be charging at a different voltage than what my DMMs read? That just doesn't seem right given my understanding -- albeit, I am a neophyte to all this stuff.

Regarding your last sentence of ignoring this, I am starting to accept that I may have to. 

[dgd]

I monitor each battery in parallel with separate DMMs. They show very similar voltage. I'm confused about this issue. Am I to understand that even though a battery shows a certain voltage when connected in parallel that they actually may be charging at a different voltage than what mhttp://midniteforum.com/index.php?action=post;quote=20202;topic=2153.0;last_msg=20202#y DMMs read? That just doesn't seem right given my understanding -- albeit, I am a neophyte to all this stuff.

The voltage over each battery when parallel connected will be identical, its the charging current to each battery that will be different, the lower resistance cells taking the higher currents.
Keep the 24volt configuration. When you connect the next two batteries in series to each other then in parallel to the exiting series pair remember to take +ve from one pair and -ve from other pair -
see the link Vic posted previously:
http://www.smartguage.co.uk/batt_con.html

dgd

[New KID On The Block]

I monitor each battery in parallel with separate DMMs. They show very similar voltage. I'm confused about this issue. Am I to understand that even though a battery shows a certain voltage when connected in parallel that they actually may be charging at a different voltage than what mhttp://midniteforum.com/index.php?action=post;quote=20202;topic=2153.0;last_msg=20202#y DMMs read? That just doesn't seem right given my understanding -- albeit, I am a neophyte to all this stuff.

The voltage over each battery when parallel connected will be identical, its the charging current to each battery that will be different, the lower resistance cells taking the higher currents.
Keep the 24volt configuration. When you connect the next two batteries in series to each other then in parallel to the exiting series pair remember to take +ve from one pair and -ve from other pair -
see the link Vic posted previously:
http://www.smartguage.co.uk/batt_con.html

dgd

THANKS dgd! Current: that's what I was missing! You're right. Obviously it's not only voltage that matters but the current as well when charging. I totally forgot about current into each battery! 

Yes, I am aware of smartguage's diagrams. I will be sure to follow their suggestion. 

[New KID On The Block]

So in sum, this "phenomenon" of seeing different voltage in series connected batteries is actually normal. It may be the case that my batteries are getting out of whack to a greater degree than other batteries connected in series -- due to internal resistance of each battery -- but any batteries connected in series would show some divergence in voltage as they accept different charging current. Is that correct? I think I'm seeing the bigger picture now.

 

[mike90045]

You can try manual balance on them, by charging the lower voltage battery a bit with a standard charger, say 5 or 10 AH at a time. Since AGM are sealed, it's hard to equalize them safely.  So charging the lower battery a bit on it's own should help even them out.

[New KID On The Block]

You can try manual balance on them, by charging the lower voltage battery a bit with a standard charger, say 5 or 10 AH at a time. Since AGM are sealed, it's hard to equalize them safely.  So charging the lower battery a bit on it's own should help even them out.

Thanks mike90045. I have been doing just that to the one battery that is lower by ~0.05 volts.

I will admit that I may have initially made the situation worse -- when I first noticed the batteries going out of whack -- by connecting a 12v DC fan directly to the 12v battery I saw going above 15v during ABSORB. I kept the fan running on that battery to lower the charging voltage. It did lower the charging voltage but it probably added to the problem of the two batteries now being out of balance.

I figured that this would be resolved (balancing out) whenever the batteries are fully charged and floats. But that has not been the case.

[vtmaps]

So in sum, this "phenomenon" of seeing different voltage in series connected batteries is actually normal. It may be the case that my batteries are getting out of whack to a greater degree than other batteries connected in series -- due to internal resistance of each battery -- but any batteries connected in series would show some divergence in voltage as they accept different charging current. Is that correct? I think I'm seeing the bigger picture now.

Not correct.  In series they have the same current.

The problem you are having is probably a bad cell in one battery.  Realize that you have 12 batteries in series.... but because 6 batteries are in one block (12 volts) with only one set of terminals, you can't see what is happening at the level of one battery cell.   

If you had a 24 volt system made up of individual 2 volt batteries, you would find that one of the 12 cells was the outlier.

One of the dirty little secrets of battery charging is that the manufacturer's recommend charging protocols are designed to routinely overcharge almost all their cells... explanation:

No two cells are ever exactly the same... and about half of them are below average 

Suppose a manufacturer produces cells and finds that 99.90% of them will fully charge up at 2.4 VPC (volts per cell -- that's 14.4 volts for a 12 volt battery).   If they recommend 2.4 VPC and sell only 2 volt batteries, they will have warranty issues with 0.10% of their batteries. 

Now, what if they sell 12 volt batteries?  Their warranty claims will be almost 0.60% (six times as many chances to get one of those outlier cells in the battery).

The battery manufacturer also knows that if they recommend 2.45 VPC (14.7 volts on a 12 volt battery), then 99.99% of their cells will fully charge up.   That means their warranty claims on a 12 volt battery will be 0.06% (rather than 0.6% with 2.4 VPC). 

Naturally, battery manufacturers recommend the higher voltage... and that's why the vast majority of battery cells are routinely overcharged.  Fortunately, lead acid batteries are fairly tolerant of overcharging (unlike some lithium batteries which need BMS (battery management systems) to prevent overcharging the first cells that reach full charge.

In your circumstance, I suspect you have an outlier cell in one battery.... possibly far enough out that it should be a warranty issue. 

--vtMaps

[zoneblue]

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.

[New KID On The Block]

vtmaps and zoneblue, thanks for your input.

lol. So in parallel they have the same voltage but potentially different current, whereas in series they have the same current but potentially different voltage. haha. Confusing, but I'm starting to get it. Thanks vtmaps.

I think once I get the additional two batteries of the same make/model, it will clarify a few things. I'll test out all four batteries and see which one behaves the most differently.

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?

Thanks all for your input/comments/feedback. It's much appreciated.