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Author Topic: Why are the 2 volt cells in the center of my string not getting charged fully?  (Read 10494 times)
magnus
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« Reply #15 on: October 11, 2013, 03:17:05 PM »

@Eleanor, i finished reading your epic post from 2012.   Luckily i have clear SAN containers so i can watch the little bubbles form on the plates, also i have the same stamp on all my batteries.

In general, this is my question to all of you.

When the amps from the batteries in the middle of the string make their way to the final end terminals, is it correct that each amp must pass through the plate/electrolyte/plate of the adjacent battery on its way to the terminal?  that has to be true, right?   or am i an idiot?  or both?  so internal resistance within the cells will always make the middle cells lag slightly and get under-worked in general?

@Paul, i switched the position of cells 3 and 4 in the bank and within 2 days saw improvement on cell 4, which had previously had a lower than average charge.   this makes me think that periodically rewiring the cells so that the terminal ends are sometimes on batteries 6 and 7 would be worthwhile and fairly easy; that would make cells 1 and 2 the new middle cells.   although there must be an easier way if this is a common problem.

i read an article in Issue 95 of Back Woods Home by Jeff Yago , who said " I have noticed the cells closest to the end of string load connection posts seen to loose the most water." ( http://www.backwoodshome.com/articles2/yago95.html )
 
this makes me think this is a common occurrence, and leads me to wonder if there was some good reason why most batteries were 12 volts usually.

last night i turned off the turbine, and now i am allowing the batteries to discharge to 1.8Volts/cell.   i am curious to see how long my house can continue to function and also to 'test' my new batteries.  after this, the turbine goes back online and i will check the stats.

the battery interconnects are given by the manufacturer and are 70mm and identical, so i don't think that is the issue.

i use power tools that draw heavy for under a minute and then recharge with the hydro immediately, so i am thinking it is all the quick movement that is taxing the cells near the posts the most. 


@billi, yes the MS is dumping surplus the whole time i am running the EQ.  the white paper from TAB states to run 28.8 for 72 hours.  and indeed, the plates bubble like mad from bottom to top the entire time they are kept above 2.35 volts.
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nowty
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« Reply #16 on: October 11, 2013, 03:33:23 PM »

Sounds like you have a mixture of strong and weak cells which is normal if you donít do periodic EQ charging. The EQ voltage you are using is too low to bring the weak cells up enough.

I found this datasheet for OPzS batteries, maybe not exactly the same as yours, but it shows a range of voltages for absorption and float and I note your settings are at the very bottom end of the ranges which may allow your cells to drift out even faster and further.

http://www.victronenergy.com/upload/documents/Datasheet%20-%20OPzS%20batteries%20-%20rev%2005%20-%20EN.pdf

In comparison my Sunny Island default settings for generic flooded lead acid batteries (charged with PV) are 2.5v for absorption charging. So your EQ voltage is not even as high as my absorption rate. I set my EQ rate at 2.65v.

If it was me, I would try a 6 hour EQ at 2.65v, monitor the temperature (donít let them go over 45 degrees) and measure the SG after each hour. If the SG is still rising after 6 hours, keep going until it stops rising.
« Last Edit: October 11, 2013, 03:51:17 PM by nowty » Logged

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« Reply #17 on: October 11, 2013, 04:27:35 PM »

Whenever I've joined batteries up to make a bank, I've always charged them individually first so they're at the same state of charge, then wired them together..........

If you are disconnecting the cells, it might be worth doing as Martin suggested prior to reconnecting them.

At leaset that way you are starting from a level position.

John
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magnus
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« Reply #18 on: October 11, 2013, 04:33:03 PM »

2.4 Equalizing charge
Equalizing charges are required after exhaustive
discharges and after inadequate charges; they
can be carried out as follows:
- Up to 72 hours at constant voltage of max.
2.4 V/ cell.
- With the I or W characteristic as under 2.6.
If during equalizing charging permitted load
voltages are exceeded, appropriate measures
must be taken, e. g. disconnection of the load.
If exceeding the maximum temperature of
55įC the charging must either be stopped
proceed with reduced current, or be switched
to float charge to allow the temperature to drop.
The equalizing charge is completed when the
electrolyte densities no longer increases within
a period of 2 hours.

2.6 Charging currents
The charging currents are not limited up to
2.4 V/ cell. When exceeding the charging volt
age of 2.4 V/cell, greater water decomposition
occurs. The charging currents per 100 Ah nominal capacity shown in Table 1 must not be
exceeded.


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« Reply #19 on: October 11, 2013, 04:55:37 PM »

sorry for the fragmented posting, the internet on our island is quirky as hell.

the last post i made is from the PDF that TAB provided that details the charging characteristics. 

i am not clear how to interpret the table.  help please.

i am in touch with the company, but no info yet.

i hope to NOT remove all the cells, as the odds of me dropping one on the concrete increases every time i muck about with them.

if i just switch the terminal batteries that may do the trick.  but i want to get their input first for warranty purposes.  these have only been online for 60 days. i think they are OK, but i want to be sure.


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« Reply #20 on: October 11, 2013, 05:04:07 PM »

at any rate, i had expected for more people to pipe up and say that, like Jeff, this is a normal occurrence.

in fact, do most of you find that your deep cycle cells release water and hold charge equally in 24 and 48 volt systems?

how do those little electrons find their way?  i am loving the trip down chemistry lane.  fascinating stuff all this.
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« Reply #21 on: October 11, 2013, 05:15:13 PM »

Hi
Is it worth checking the voltage across each cell during an equalisation charge. I would imagine there might be quite a difference. Although you are putting in 2.4 V/cell it would be interesting to see what each cell is actually getting across it individually. I would think that if the low ones could be charged up separately and then put into the battery again there would be an improvement in the balance between the cells.
Iain
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« Reply #22 on: October 11, 2013, 05:31:19 PM »

at the beginning of EQ i was getting 2.65 and 2.13, after 72 hours i got 2.53 and 2.26.  always the average is 2.4 of course.

i have no way to charge individually. 
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DaveSnafu
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« Reply #23 on: October 11, 2013, 07:07:23 PM »

It may be worth checking that the interconnecting cables are actually bolted down properly and are making a decent connection.
I recently fitted 2 sets of 12 cells with the bag of brand new interconnects that came with the cells, the new injection moulded interconnecting cables had moulding "flash" over the metal boss where the bolt goes through.
This was preventing a good connection and was giving me a bit of a headache for a minute or two.
The excess rubber can be removed with a knife, check all of the connectors for this, especially if they are new.
My old set of cells go up and down together, and use the same amount of water, but then they are all equally knackered.
I have not had the new cells long enough to notice anything weird yet.............

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« Reply #24 on: October 11, 2013, 07:12:03 PM »

at the beginning of EQ i was getting 2.65 and 2.13, after 72 hours i got 2.53 and 2.26.  always the average is 2.4 of course.

i have no way to charge individually. 

Therefore you need to increase your EQ charging voltage as you need the weak cells up to circa 2.5v, i.e. an average of 2.5 - 2.7v or so.

The battery bible (batteryuniversity) many off gridders swear by :
http://batteryuniversity.com/learn/article/equalizing_charge

In fact google equalizing charge lead acid and you will see time and time again charging voltages way above what you are using.
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« Reply #25 on: October 11, 2013, 07:20:33 PM »

Have you any possibility of charging at 12v  - 15v

If so pair a well charged with a poorly charged cell and give the  (6 x 2 cell) banks a steady charge for 2 or 3 days.

let them rest for 12 hours   and if necessary  re-couple   highest with lowest  and continue the process.
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Eleanor
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« Reply #26 on: October 12, 2013, 11:42:14 AM »

Hi Magnus, are you able to link the whole TAB document? I think it will turn out to be the same as the Enersys one I posted earlier. In that case the table means that you can charge at up to 5A per 100Ah of battery capacity at 2.6-2.75V/cell. My equalisation current is about 2A per 100Ah at 2.67V which is the highest voltage the charger will do. All the cells in my 24V bank charge at very similar voltages and I haven't noticed any difference in the amount of water they use.

It seems that some have just started to lag behind. If you're getting 2.53V and 2.26V when charging at 2.4V/cell you may be able to get the lower ones up above the gassing voltage. If you were to start making smaller banks which hopefully you won't have to you would need to match the closest cells together.
« Last Edit: October 12, 2013, 12:06:01 PM by Eleanor » Logged

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« Reply #27 on: October 13, 2013, 07:12:29 PM »

May be I should have explained the logic behind my recommendation

I will try and do my best 

Any string of cells will have an identical amount of current passing through every cell in the string .

So cells with a higher voltage and Specific gravity ( which will go hand in hand if they were setup correctly when leaving the factory )


will inevitably gas excessively while those with the lower SG will never catch up.

By coupling cells with differing voltages  there will be a slight transfer of charge from one to the other over time .  But more importantly when on charge  the current passing through the paired cells will be in relation to the voltage difference of the cells,   the one in need of extra charge   getting a greater proportion of the current than it's partner .

It may take a while and will not sort out problems such as those encountered by  Eleanor.

But it can reduce the amount of in-balance within the bank of cells.



as an aside   Eleanor .
   How has you Anti  Stratification  system worked out  I was hoping you would have given us an update and if possible details of precisely how it works.
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« Reply #28 on: October 13, 2013, 08:03:58 PM »

Magnus , i do not understand this Pic from you

is it only the Morningstar connected ? . where is the turbine and the inverter ?  I might be wrong but the cables are  thin ....
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« Reply #29 on: October 13, 2013, 08:40:12 PM »

Magnus,
We used to have a problem with lead acid cell voltage balance when a cell was removed for teat down testing. It replacement would be a new cell, cell whose plates were not conditioned to a similar state to that of the others. Basically, the problemyou have. Plates have to be exercised to get the activesurface into an proper working state and this takes time.

You say that 6 of the middle cells are lower than the 3 at each end, i.e. the other 6.
Simple answer, charge these cells in parallel with the existing string. We used to do this on our station batteries  be it one of the 240v 1900Ah or one of the 110 1100Ah or one of myriad of 55V or 24V schemes on the other electrical or diesel schemes on site. We used a portable adjustable Davanset charger with an isolation transformer on the mains input forextra safety, so not to induce an earth  fault on the system.

For your application, use a car battery charger connected across the six low cells. These will receive extra charge whilst the string charge (voltage) controller will controll the overall string voltage by reducing the charge on the remaining six cells. Monitor the sg of the cells as well as the cell voltage.
If you do not like that idea, turn the battery off and charge the said cells with a 12 volt car battery charger.

We maintained a float of 2.25 vpc, not 2.23vpc. The latter discharge them over time or certainly not bring them back to balance after a discharge.
Philip R

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