Author Topic: Monitoring Battery Cells  (Read 7779 times)

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SparWeb

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Monitoring Battery Cells
« on: December 28, 2013, 08:42:10 PM »
My battery pack is old, but still ticking away.  They are sealed AGM types, 24 cells at 2 volts each, designed for commercial building backup power supplies.  I got them at the "end" of their life, but the standard of reliability for a national ISP service is much higher than my own, so I've benefited from 5 years of service from them.  When I got them they were in a very sorry state.  Broken bus bars, dropped in an alleyway, on a rainy day.  After cleaning them up and keeping them in an insulated (but not heated) building for years, they have delivered the juice consistently ever since.  No leaks, no off-gassing, no bulges on the front (though they are swelled enough inside their steel containers that I can't slide the individual cells out).

That said, they DO show signs of deterioration.  It's probably worth posting about because I've come up with a method of monitoring my batteries that I haven't seen written about.  For any forum users that have a dozen or more cells, this might help to spot a bad trend.  Before it becomes a disaster. 

It's likely that many owners of batteries monitor them with voltage measurements, and I expect they measure cell by cell, looking for cells with voltage lower than the average.  I've been doing that fairly regularly, too (if every 4 months counts as "regularly").  Since I write the results down as I go, I've started entering the numbers into an Excel spreadsheet.  Now the trends are obvious.  Cells 6 and 8 are consistently the worst in the pack, while cells 10 and 16 are usually low, but not always.  Other cells tend to be very good, such as 7, 13, and 18. 

Then I tried a "standard deviation" on the 24 cells in my stack.  That's when another overall trend showed up.  I was able to go back through the measurements I recorded years ago, and get a standard deviation for the cells on each date.  Plotted on a chart, the trend is clear: the variation between the cells is growing, meaning the bad are getting worse.



On the graph, the variability between the cells was very high when I got them; off the chart.  Considering the mess they were it, that's expected.  After repeated re-charging from a 2-cycle (bulk/float) battery charger the standard deviation went way down, and I was able to keep it down after installing them and keeping them with wind power.  The variability still went up and down from time to time, and then in 2010 I put solar panels on the system.  From then on the variability has followed a continuous trend, creeping upward ever so slowly and inexorably.  The bad cells are getting badder.

To make sense of the numbers, it means that my 4 worst cells are usually 0.0065 volts below the average or lower.  Actually, my worst cell (#8) is 0.018V below the average today.  On cells whose standard voltage reference (2V) should be 2.25 volts at float, this cell is undercharging by 7%.  It's like a fully-charged 12V battery resting at 12.55V rather than 12.6V.  Showing its age.

On the whole, the group seems to be in OK health, but the bad cells are going to kill the batch someday.
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Bruce S

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Re: Monitoring Battery Cells
« Reply #1 on: December 29, 2013, 11:45:40 AM »
Is there any possible way of sorting the bank as a whole into banks of good and bad ? the "bad" ones into their own bank so they don't bring further harm to good ones?
2V 24 cells is either 48V or lower , so if there are enough of the bad ones ( counted 4) so banks of 8Vdc units then same for good ones ?
I know the rule of thumb is lower count of connections the better, but when you're in crisis management mode, then triage comes into play.
Funny story about the excel spreadsheet: Back in 1996 I wrote my former employers very first "electronic" inventory using Access ! To this day; it remains the only inventory program not to have crashed and lot ALL data  ;). granted Access has a 65K line limit , but it was a fun text from an employee who still remembers that program :-) .   
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SparWeb

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Re: Monitoring Battery Cells
« Reply #2 on: December 29, 2013, 12:21:00 PM »
Oh, I wish I could.  I came up with a plan like that 2 years ago.  Had a plan to regroup them all into two equivalent strings, starting with the two best at the head of each string, and work my way down the ranking to the worst to at the two ends of the two strings.  When the time came to pull the batteries out of their cases....  none of them would budge.  I built a cute little tool to screw into both terminals with a wooden bridge between them, and pulled as much as I dared.  When I realized that if I ever got one out this way I would never push it back in again, I just gave up.

The photo shows how they are encased.  It prevents me from casually re-ordering them without extracting them from the containers (and then having to shove them back in again).



Anyhow, just a thought I wanted to pass along to others.  Perhaps someone else with a few dozen batteries has done the same measurements and recordings.  If so, I'd greatly appreciate seeing their statistics - it would improve my understanding of what's bad/ what's good, and help me judge when the end is really near.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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DamonHD

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Re: Monitoring Battery Cells
« Reply #3 on: December 29, 2013, 02:41:27 PM »
Thanks for the share.

And well done on keeping the appropriate data points...

Rgds

Damon
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Mary B

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Re: Monitoring Battery Cells
« Reply #4 on: December 29, 2013, 03:54:12 PM »
If those are screw terminals on top you could use all equal length cables and have a spider web over the batteries to interconnect them

SparWeb

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Re: Monitoring Battery Cells
« Reply #5 on: December 29, 2013, 08:04:00 PM »
If those are screw terminals on top you could use all equal length cables and have a spider web over the batteries to interconnect them

Would Bruce make me change my alias if I did?  ;)

That's an interesting idea, though.  Currently it's a group of copper bars going from cell to cell in a regular pattern.  Maybe I could jump cables from cell to cell in an apparently random pattern, but in actual fact be hooking up the corresponding good cells and bad cells in order, to share the load better.  I fear that the resulting puzzle of wires would be quite a tangle.  Maybe it could be optimized...
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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SparWeb

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Re: Monitoring Battery Cells
« Reply #6 on: December 29, 2013, 09:38:34 PM »
The arrangement would go from this:



to this:



No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
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SparWeb

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Re: Monitoring Battery Cells
« Reply #7 on: December 29, 2013, 10:05:40 PM »
There seemed to be a lot of ways to do it, but this one kept the wire lengths down.

The existing stack is divided into 4 groups of 6.  Each group gives 12V (nominal).  I've put the groups in pairs, with a fat bus cable between them.
All the cells are numbered on the diagram, and then I copied the diagram, and wrote in their rank beside, in green.  Cell 24 is ranked 20th, and so on.

To re-wire them going best-to-worst, without jumping all over the place, I kept them in the same order and made 2 parallel strings again. 
It seemed easier to find the positive bus near the bottom, so I picked the lower pair to start, and jumped from the positive bus to cells 5 & 11, which are the two best.
From there I continued the two strings in parallel, until I arrived at the worst pair, cells 6 & 8.  They join the carry-over bus which goes to the upper pair of batteries.  Repeated from there.

Now the strings are organized like this:

+| 5 - 12 - 2 - 1 - 4 - 6  | c | 17 - 18 - 14 - 23 - 20 - 16 | -
+|                         | c |                             | -
+| 11 - 9 - 7 - 3 - 10 - 8 | c | 22 - 19 - 13 - 21 - 15 - 24 | -


Which builds up a ranking like this:

+| 1 - 8 - 11 - 13 - 16 - 23  | c | 4 - 6 - 9 - 17 - 19 - 22 | -
+|                            | c |                          | -
+| 2 - 10 - 12 - 14 - 21 - 24 | c | 3 - 5 - 7 - 15 - 18 - 20 | -


As a simple way to guess if the strings are now balanced, add the ranks of the cells in each sub-string:

1 + 8 + 11 + 13 + 16 + 23 =  72
2 + 10 + 12 + 14 + 21 + 24 = 83
4 + 6 + 9 + 17 + 19 + 22 =  77
3 + 5 + 7 + 15 + 18 + 20 =  68

Nope.
Gotta tweak the arrangement so that all of the sub-strings are roughly equal.

I'm still a bit dubious about re-wiring them like that, though.  I'll think about it some more.
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OperaHouse

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Re: Monitoring Battery Cells
« Reply #8 on: December 30, 2013, 09:28:53 AM »
That is an interesting chart.  We all should  have some battery monitoring program.   I implemented a low voltage  battery monitor im my ARDUINO fridge program.   It records the lowest voltage at the inverter when the fridge starts, 120A draw.  Everyone has some high load which is repeated daily that can serve as a system benchmark. It has made me aware of poor connections and battery condition.  A couple years back it helped me sort out the good from the sulfated batteries since they were identical.  I used one at a time and recorded the results on tags.  I really should get a USB memory card adapter to record the real life of the bank in daily operation.  Of course this only helps if you actually look at the data.  At the end of the season I brought home the best two batteries based on the tests of the previous year.  One of them I put back in my truck and the other in the back just in case I got stuck.   I was doing a little job and left the radio on for twenty minutes.  The radio started acting funny, but I was ready to leave.  Dead battery.  No problem I had a spare.  That was also dead and would not take a charge.

kitestrings

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Re: Monitoring Battery Cells
« Reply #9 on: December 30, 2013, 12:18:04 PM »
SparW,

Thanks for sharing this.  Very interesting.  I've been trying to better tracker variations in ours for some time.  I went to using a hand-held refractometer for cell readings in an attempt to take the temperature, and potential for cross-contamination out of the equation.  I've been using mini eye-droppers, and one of those plastic pill organizers that I hope to never need.  With these I can take all the readings from the basement and then sit at the table to view the readings.

I'd be interested in viewing your file if you care to share it.

Regards,

~ks

SparWeb

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Re: Monitoring Battery Cells
« Reply #10 on: December 30, 2013, 12:33:59 PM »
Hi Kitestrings,

It's an Excel file with lots of worksheets.  I make a new sheet for every day I take a reading, then update a group of sheets at the end which summarize the results on the daily sheets.  Not all of those are actually up to date.  Not organized for show-and-tell, either, but if you can manage, I can send it to you.  Watch your PM's.
I've never cracked the cells open so there are no SG readings - it's all cell voltage measurements.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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SparWeb

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Re: Monitoring Battery Cells
« Reply #11 on: December 30, 2013, 01:30:27 PM »
I just had another idea-
Rather than try to find the ideal way to re-wire the entire set-up from best to worst, I could just try to re-wire the 6 worst cells for a period of time, and equalize charge them, with a 12-V charger, or maybe by solar (during the summer of course).

Currently, the total voltage of the pack comes up to 28V during bulk charge, but the best cells are at a higher voltage than the worst cells.  Now I realize how much they are preventing the worst cells from seeing a proper bulk charge voltage - allowing them to fall farther behind as time goes by...

For instance, I have a bulk charge record which shows cell 17 at 2.334v while cell 8 is 2.314v.  It may not look like a big difference, but if I multiply by 12 to illustrate what it would look like for a full 24V system pack:

Best Cell 17:  2.334v * 12 = 28.008v
Worst Cell 8: 2.314v * 12 = 27.768v

Which shows how the best cell benefits from a full bulk charge every day, but the worst cell doesn't quite get into the ideal bulk charge 27.8v - 28.0v range, ever.

So now I think the best thing I should do (this summer) is temporarily take off all the bus bars, wire up the worst six cells as a 12V system, and run off a 12V-120VAC inverter for a while, when there's lots of sun for the solar to top them off every day.  The other 18 cells can rest for a month - the self-discharge rate is so slow for these guys that it doesn't worry me.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
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Mary B

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Re: Monitoring Battery Cells
« Reply #12 on: December 30, 2013, 03:39:07 PM »
All cables would have to be the same length or you could introduce an unbalance

Bruce S

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Re: Monitoring Battery Cells
« Reply #13 on: December 30, 2013, 04:31:02 PM »
If those are screw terminals on top you could use all equal length cables and have a spider web over the batteries to interconnect them

Would Bruce make me change my alias if I did?  ;)
Not this year  :).
I had the same idea, as MA, but thought it wouldn't be worth the cable hassle, but it would work if for nothing other than segmenting the good to good and b-t-b.
 
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SparWeb

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Re: Monitoring Battery Cells
« Reply #14 on: December 30, 2013, 06:19:14 PM »
All cables would have to be the same length or you could introduce an unbalance

Yes, if I went through with the 6-cell "rehab" pack wire-up, I would make a batch of 5 equal-length cables for the inter-cell connections, and a longer pair to reach out to the bus bars.
There would be no parallel string to compete with the invalids.  The 24V inverter wouldn't talk to the temporary 12V bus, but I have a 12V inverter that I could plumb in temporarily.  Or just ignore the 120VAC entirely.  Put no loads on the rehab cells, just charge them up all day every day.  In the summer there's not much need of power out in the barn and sheds that are served by my RE system.

This is starting to sound too easy.   :)
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dnix71

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Re: Monitoring Battery Cells
« Reply #15 on: December 30, 2013, 06:58:11 PM »
SparWeb if you want to remove a cell, the electrolyte has to be dumped/pumped out first. Without the liquid pushing on the cell walls, they will lift out with some effort. You can't do that with a sealed AGM.

Depending on how the tops are sealed on an AGM, the tops might be unsealed and the top and core lifted out as well. Then the electrolyte and bottom sludge can be removed. If you have sludge on the bottom shorting out a cell, that's the fix.

Outside of a steel case the cells must not be set on their bottom or they will be damaged by sagging under their own weight.
Laying a cell on it's side without electrolyte is acceptable, but risky when they are old. You might break the plates off that way.

That's a lot of work you would not normally undertake unless your life depended on a fix rather than a repair.

birdhouse

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Re: Monitoring Battery Cells
« Reply #16 on: December 30, 2013, 08:16:05 PM »
spar-
nice logging!  cool to see it in charts! 

any chance you could flip the whole bank upside down??   :o

i swear i read somewhere about agm batts having the electrolyte dry out on the tops of the cells, and by the user putting the batts (in the steel cases) in various positions, the electrolyte was able to re-wet the top portions of the cells to improve performance. 

there is probably a risk of shorting the cells by having the bottom gunk drop onto the plates, but hey, it may work. 

maybe it was tom w that did that.  i think he has a bank similar to yours.  IIRC he put them on their sides and flipped them every few days so all sides got some "ground time". 

adam

kitestrings

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Re: Monitoring Battery Cells
« Reply #17 on: December 30, 2013, 10:47:03 PM »
Since you have the 12V inverter maybe you could just rewire then at that voltage, but split the pairs, good and bad.  If you had the ability to isolate them - via a storage/starter isolation switch - you could also control the level, depth and frequency of charge cycles in the better summer weather.  You'd still have the benefit of drawing them down for something useful, rather than individual cell cycling. 

If things improve you go back to normal.  If not, you let the losers die a quiet death without taking hostages.

~kitestrings

Bruce S

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Re: Monitoring Battery Cells
« Reply #18 on: December 31, 2013, 09:18:53 AM »
birdhouse;
Tomw's are wet NiCd batteries and must be on their sides, otherwise the tops dry out.
They are actually made to be on their sides.
With winter beating up on Iowa, he's deep into keeping things warm, but he may chime in to let us know how they are doing.

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SparWeb

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Re: Monitoring Battery Cells
« Reply #19 on: December 31, 2013, 01:18:22 PM »
Sorry!  I assumed enough of you guys had seen my battery stack, but the discussion about draining cells won't help me much.
Here's a photo to clear up some misunderstanding:



When re-stacking the cells in the new shed this fall, I did a pretty good job of jiggling, rocking, rolling, and generally shaking them around.  I'm quite sure the electrolyte has been stirred about in that process.

As for removing a cell, each cell can be seen in the steel case, with two terminals and a valve on each.  The number sticker is on the valve.  The cells are plastic so the sides are flexible.  I remember the first day I picked them up with a forklift, two of the cells started sliding out of the case when I tipped them face-down.  That won't happen today because all of the cells are tightly swelled together.

They MUST be operated lying on their sides, not facing up.  I could turn them upside-down, but I think there wouldn't be much point.  I don't suspect any drying out at this point.  So no need to "re-wet" plates that are on top.  If Tom did that once or twice, when he was re-habilitating is bank (I remember that discussion too), then I can see the goal of agitating them, but I doubt he would (or could) flip the cells on a regular basis.

If I suspected them of being low on electrolyte, there is a way to top them up.  I haven't tried, but a member of WindSun forum went into a lot of detail about it once (Peter Akko IIRC).  So far, I'm being optimistic and assuming they haven't vented any.  A little electrical top-up, so to speak, is all they need right now, I hope.  I have discharge-tested them and I got about 75% of the rated charge out of them that day (April 2011).  It was not 25 degrees C that day.  On the few days where I have been able to test them, and it actually was 25 degrees C (rare in Calgary) I found the resting voltages were pretty close to spec.  That was several years ago, so by now I think that the bad cells need a little TLC to bring them back to spec.

Birdhouse, if you want to see the spreadsheet, too, I can e-mail you a copy. 
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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birdhouse

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Re: Monitoring Battery Cells
« Reply #20 on: December 31, 2013, 07:56:36 PM »
Quote
Birdhouse, if you want to see the spreadsheet, too, I can e-mail you a copy.

thanks for the offer, but i think i'm okay.   :)

looks like i'm not the only one who uses blue tape to mark DC neg. 

it gets awefully confusing working on an off grid system with DC and AC and their stupid IMO color choices.  especially if like me, you run some lighting direct off the bank. 

my colors for designating wires is:

black- AC hot
white- AC neutral
red- DC hot
blue- DC neg
and as always, green or bare for ground. 

adam

SparWeb

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Re: Monitoring Battery Cells
« Reply #21 on: January 01, 2014, 01:04:49 AM »
I should have used purple for my cross-over cable between the two groups, but I had to settle for green instead!

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kitestrings

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Re: Monitoring Battery Cells
« Reply #22 on: January 01, 2014, 09:28:36 PM »
SparW,

Thanks for the file.  I look forward to spending more time on it, but you've got a knack for distilling a lot of data into something revealing.  I remember the work you did summarizing the spec's, features and behavior of - what was it - 20 or more home-built's a few years back.  I thought that was very well done.  I tend to be a bit analytical, and I like measuring/monitoring things, but I can't always organize my results in the most concise format.  Always more to learn.

Best, ~ks

rossw

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Re: Monitoring Battery Cells
« Reply #23 on: January 02, 2014, 03:23:02 AM »
I don't post here much (some of you know why).
Over here is a thread about a project you might find interesting.

Some "Before" and "After" (manual balancing) plots are more illustrative.



And yes, of course each cell can be seen individually. Also the "delta" plots which show each cells difference from the entire batterys average. Strong and weak cells really stand out.


SparWeb

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Re: Monitoring Battery Cells
« Reply #24 on: January 03, 2014, 07:06:02 PM »
Ross,
Thanks for the link to your project.  Seems you were thinking of the answer to my question, before I asked it.  :)
Small isolated boards monitoring only 3 of the cells at a time - I never would have thought of that, but now, how obvious it seems!

One of your cells went to 2.5 Volts, while another wasn't even passing 2.2 at the same time - Yikes!
Any story I may want to tell about nursing along my old batteries will be overshadowed by your efforts.  I will watch your progress with interest.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
www.sparweb.ca