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Author Topic: Why are the 2 volt cells in the center of my string not getting charged fully?  (Read 10332 times)
Eleanor
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« Reply #30 on: October 14, 2013, 01:16:44 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.


Bxman, I agree that all the cells in a string will see the same current but we'll have to agree to disagree on connecting the cells with the greatest difference in charging voltages together. What we, ie Magnus in this case, want is for all the cells to charge at the same voltage and have the same SGs. The way I see it if the whole problem is caused by the cells drifting apart and charging at different voltages all you are doing is making a load of smaller banks with the same problem. If there was transfer of charge between unequal cells resulting in them sorting themselves out wouldn't all banks just have cells that were all at the same state of charge? We want all the cells to charge at the same voltage and as far as I can see the best way to do this is to start with cells which are already as close together as possible.

The anti-stratification system is quite simple and has an external pump which sends air through tubes inserted into one corner of each cell. It seems to work well - I think I have posted a picture somewhere but not in a thread specific to the subject. If it stays sunny over the next day or so I'll do an equalisation charge and report the results. Previous measurements have been well within acceptable limits  Cool
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bxman
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« Reply #31 on: October 14, 2013, 03:36:14 PM »

Eleanor 
   On the assumption  that Magnus 's   batteries were all filled with identical concentrations of acid when filled from dry .

which I know can not be guaranteed .

I would like to think he has reasonable chance of being able to get hold of a 12volt charger where he is.


He has cells at the moment  in the range between   2.08 v  and 2.60 v 

Were he to couple the  2.08 to the 2.60  they would overtime move closer to each other in there resting voltages .


And were they to be given a charge at say 2.75 volts the bulk of the current would pass through the cell with the lower voltage .

which I believe would bring the state of charge  and the S G 's   into  closer relationship. without gassing off the electrolyte from the higher voltage  cell.

Ideally were all 12 cells coupled together and charged from the same source it  would stop any cells from getting more voltage than is good for them.

Little chance of finding that in the Azores I suspect.   I have been wrong in the past and certainly will be in the future  I am sure.
 

But I think it might be worth a try.
 


Thanks for the information on your system  so it is like an aquarium air pump introducing bubbles of air at the bottom of the cell to move the acid around.


Am I right in thinking there are systems with an external reservoir of acid which gets circulated ?
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JohnS
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« Reply #32 on: October 14, 2013, 04:05:57 PM »

I am puzzled as to how Magnus will power a 12 volt charger as he is off grid.
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Eleanor
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« Reply #33 on: October 14, 2013, 07:57:06 PM »

Eleanor
   On the assumption  that Magnus 's   batteries were all filled with identical concentrations of acid when filled from dry .

which I know can not be guaranteed .

I would like to think he has reasonable chance of being able to get hold of a 12volt charger where he is.


He has cells at the moment  in the range between   2.08 v  and 2.60 v

Were he to couple the  2.08 to the 2.60  they would overtime move closer to each other in there resting voltages .


And were they to be given a charge at say 2.75 volts the bulk of the current would pass through the cell with the lower voltage .

which I believe would bring the state of charge  and the S G 's   into  closer relationship. without gassing off the electrolyte from the higher voltage  cell.

Ideally were all 12 cells coupled together and charged from the same source it  would stop any cells from getting more voltage than is good for them.

Little chance of finding that in the Azores I suspect.   I have been wrong in the past and certainly will be in the future  I am sure.
 

But I think it might be worth a try.
  


Thanks for the information on your system  so it is like an aquarium air pump introducing bubbles of air at the bottom of the cell to move the acid around.


Am I right in thinking there are systems with an external reservoir of acid which gets circulated ?



"He has cells at the moment  in the range between   2.08 v  and 2.60 v

Were he to couple the  2.08 to the 2.60  they would overtime move closer to each other in there resting voltages .


And were they to be given a charge at say 2.75 volts the bulk of the current would pass through the cell with the lower voltage .
"



I've never seen a cell with a rest voltage of 2.6V - 2.16V perhaps. Two cells in series will be subject to the same current - more current will not pass through one cell than the other. The cell with the higher resistance will charge at the same current (I) and higher voltage (V) and the result is that the cell with the higher resistance has a higher input of Power (W) = V x I which is what really counts. They can't move closer together as the cell with the higher resistance just keeps charging at a higher voltage. The voltage of a 2V cell relates to the difference in potential between it's own electrodes and doesn't mean that it will "transfer charge" to another cell which has a lower voltage.

Edit : Brain completely frazzled! The very different cells should move closer together eventually when charged with a constant current but it is still better to charge similar cells as a group if the bank was to be split which shouldn't be necessary anyway
« Last Edit: October 15, 2013, 07:23:17 AM by Eleanor » Logged

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bxman
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« Reply #34 on: October 15, 2013, 09:18:06 AM »

Eleanor 

You are right those readings Marcus took must have been while it was charging stupid me for missing that .

" Two cells in series will be subject to the same current "      agreed

In the method I am proposing the cells  are temporally placed  in parallel not series .

When charged like this both cell in the pair will be subjected to the same voltage  I am sure.

Thereby preventing the fully charged cell from over-charging .

 And hopefully bringing the other in the pair up to a similar level of charge and voltage.

  Yes I did appreciate Marcus was of grid and nothing was going to be easy for him .

They are clearly brand new and his supplier should be replacing the Entire Bank for him    IMO.
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Eleanor
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« Reply #35 on: October 15, 2013, 10:36:08 AM »

Bxman, if you're going to charge two cells in parallel at 2.75V which is a 2V bank I don't understand why this would be better than just charging one cell at 2.75V?

Edit : I guess you mean charge two strings of 6 cells in parallel in a 12V bank?  Huh
« Last Edit: October 15, 2013, 11:15:49 AM by Eleanor » Logged

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bxman
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« Reply #36 on: October 15, 2013, 11:20:32 AM »

Eleanor
Sorry
Clearly I did not explain my self that well

I was thinking Marcus might be able get hold of and run a 12v charger .

He needs to balance the ones has got as best he can if he just charges the 6 lowest cells they will all be getting identical current through each and while hopefully increasing their state of charge will not themselves become balanced with each other or those that are not in the system.

By pairing High state of charge with low state of charge  and making  6 pairs  into a 12v string  some degree of balancing I believe would be achieved.
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Nickel2
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« Reply #37 on: October 15, 2013, 11:41:56 AM »

On a historical note; years ago when I was a GPO apprentice, all large telephone exchanges had a battery-room. The main negative batteries (2) at Aldershot were 36,000 Ah at 50 V. (yes, 36,000@50). The cells were glass tanks 4ftx4ftx3ft high, open topped.
One of my weekly tasks was battery maintenance; check the SG of each cell, top up levels, scrape the plates to remove local growths, and measure each cell voltage.
If a cell had started to go down, the exchange was switched to the other battery. The low voltage cell was attended to as necessary, by use of an individual cell-charger on a trolley, (battery doctor), that could charge at up to 3000A.
When all cells were balanced, the 'A' battery was put back on to the exchange, and the 'B' battery put on float/maintenance.
I never found any particular cell went down, it was all random. When we did an equalisation charge, no-one was allowed in the room and the extractor-fans were switched to boost. Nowadays all that has gone, each suite of racks of equipment has it's own power/battery rack with AGM batteries.
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« Reply #38 on: October 15, 2013, 07:59:18 PM »

Nickel2,    "each suite of racks of equipment has it's own power/battery rack with AGM batteries."

And what jolly good batteries those AGM's are. Most are still good after 3 years of service.

I re-cycle them you see.! extrahappy 
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Philip R
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« Reply #39 on: October 15, 2013, 11:05:13 PM »

Nickel 2

Did your clothes fall to pieces after a wash? Mine did. I was preparing a new battery (240V 1910Ah Plante type) for discharges test by the supplier. I had the battery on boost charge with the portable charger ( weighed about 3/4 tonne) up at about 2.5 vpc  after a few hours and bubbling nicely. I went in wearing jeans as it was a sunday afternoon on overtime. Got home, girlfriend (now Mrs PR) washed them and they mysteriously turned holy! angel

The old cells on the A station were open, much smaller than the GPO ones you worked on. Needed to be carefull not to induce a cell short by a bit of judicious maintenance! Submarine Lead acid batteries were an impressive sight.
Philip R
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Philip R
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« Reply #40 on: October 15, 2013, 11:12:14 PM »

bxman #36,

I assumed that there was mains voltage availible to run a car battery charger to charge the middle 6 cells in tandem with the string, either on line or offline.

If mains not availible, is there access to a car, specifically the output from its alternator from the battery terminals. Connected to the 6 middle cells by a short length of some decent highish cross section copper wire. The alternator should raise the voltage to about 2.4vpc (14.4V on a 12 volt system) however the revs will need maintaing for quite a while so the engine will get hot and burn a bit of fuel.
Philip R
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Nickel2
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« Reply #41 on: October 16, 2013, 10:36:04 AM »

Philip R
Everything in the GPO was run from the TI's or EI's. (technical instructions even told you how to boil a kettle). They supplied an over-suit a bit like a sou-wester, gloves, goggles etc. Plate cleaning was the worst job: leaning over umpteen hundred gallons of sulphuric acid and sliding a thin wooden stick down the plates. This job was given to smart r's apprentices by way of punishment, I suspect. When the main batteries were on boost, I swear you could hear the hissing from the other side of the door.
I was curious as to why there was no fuse on the bus-bars, until it was pointed out to me 'who would be brave enough to change it?'. The big bars were solid copper flats about 18" deep, 1" thick, interleaved in 6 or more layers. Only the section bus-bars were fused, 400-1000A with jockey fuses for the alarms. They were happy days, and every time I smell old 3000 type relays or varnish etc. I'm gone, back to a lost world.
The positive batteries (used for metering purposes) were a lot smaller, about 6" square, 18" deep. but maintenance was limited to top-up only.
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EpEver 4210A at 24v
24V 400 Ah battery. (4x200Ah FLA)
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« Reply #42 on: October 16, 2013, 05:10:35 PM »

i am dling  this thread now to read tonight.  still no lines to my house, so a quick trip to the city to get online.

i think i found my problem and have resolved it.

i am embarrassed to post it online, but of course i will, so other newbies can learn from my stupidity.

at this point the batts are EQing nicely with bubbles on ALL cells.  extrahappy  2.35 to 2.46 Volts max difference on all cells now.

i can actually watch the battery positive plate go from a dull black with splotchy grey patches turn into a dark charcoal color as the splotches disappear.  very cool process to observe.
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« Reply #43 on: October 16, 2013, 10:54:37 PM »

Please let us know what happened.

The suspense is killing us, if the battery fumes don't get there first.
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« Reply #44 on: October 22, 2013, 03:43:44 PM »

over the center half of the battery bank i had attached a 9 watt car stereo that runs on 12 volts.   i figured how could such  a small draw have any effect on such a large system.

this little device draws between .5-1 amp continuously, as we use the stereo as a clock in our house. 

as it turns out the PWM generally charges at under 1 amp about 98% of the time, meaning that the center 6 were never getting there full share of juice.   facepalm

i removed the load and ran the EQ program again, now they are all coming back into sync as the PWM works its magic and my wife has the go ahead to run the clothes washer, crock pot and blender to her hearts content.  After cycling down to 2 volts and back up a few times the cells are coming closer and closer together.

all the numbers for charging and the times given were verified as correct by the manufacturer.  so i am doing the right thing for these OPzS batteries.

thanks for all the help.  learn from my stupidity! 
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