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Author Topic: Measuring Stratification in 2V Cells  (Read 48128 times)
djh
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« Reply #105 on: May 05, 2011, 04:37:23 PM »

I'm going to have to get someone in to look at it

Advise the supplier of this before you do it and tell them that you'll be seeking to recover that cost in court as well. It might prompt them to some action.
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« Reply #106 on: May 05, 2011, 06:18:16 PM »

You are right.  The type of separators will affect the total internal resistance of the cell.  This has a low impact at low charge current but has an impact at higher charge current, as cells with higher internal resistance will develop more Voltage across that resistance, for a given current.

This could be significant, because a bank can sit at 20 Amps at the absorption Voltage.  5 milli-Ohms increase in internal resistance will make the cell Voltage increase by 0.1V at 20 Amps charge current.  As the charger just maintains the total string Voltage at whatever it wants (28.2V in my case), having some cells that sit at 0.1V higher than others under charge means that the others must be sitting 0.1V below their ideal Voltage, so that the total adds up to 28.2V.  That's why mixing cells of different internal construction in a series string is "a bad thing"... Even if two cells are made with the same materials and are nominally the same capacity but a different shape (e.g. tall & skinny vs. short & fat), the geometry will be different, and so the internal resistance will be different.  Age of the cell (in terms of cycle age and calendar age) also affects internal resistance; older cells increasing their resistance as they degrade (gel cells especially get very high internal resistance as they get old / worn out).

The bolts that connect the cells together can have a much bigger effect, as contact resistance can vary widely.  That's why newer fork lift packs come with welded on interconnects, rather than bolted on - it reduces the contact resistance and ensures that the interconnect resistance is at least repeatable and consistent from cell to cell.

But I don't think the debate should revolve around the detail of the cell construction (especially if the supplier is going to argue that they are functionally identical - that would lead you down a dark path of expert witness opinion on whether a difference is "material" or not).  The thing to major on is that some cells are much older than the others.  That's undeniable and as I mentioned, even with maintenance charges, they will still have used up some of their calendar (or "shelf") life, sitting on the shelf.
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Eleanor
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« Reply #107 on: May 06, 2011, 02:08:43 PM »

Talk about going down a dark path, you couldn't make up the contents of the attached document  horror

It's a bizarre story of the "Separator Wars" between the two major manufacturers which ended in a merger between them (Daramic and Microporous). Price fixing, holding the largest battery manufacturer in the world (search the document for "EnerSys") to ransom by saying they won't deliver any separators unless they sign a long term contract leading to the potential loss of $1 Billion of business. Throw in a fire and a strike and all sorts of skullduggery to get orders. Who said separators are boring?!  whistlie

http://www.ftc.gov/os/adjpro/d9327/090717ccposttrialfof.pdf

I've worked out from the document that the newer cells probably have CellForce separators http://www.mplp.com/cellforce.htm - not sure what the others are yet. The Flaky Five seem to have been manufactured just at the height of all the controversy and during a worldwide separator shortage. I think they are probably made from old bin liners  ralph

I wonder if it would ever to possible to balance the charging of the cells as if the separators in the newer cells are voltage regulating as long as the charger is putting out a high enough voltage won't they always have a "top of charge" voltage of 2.67V (when new) with the other cells just getting what's left over  Huh. It's actually the opposite way round to the way I first thought with the cells at a lower voltage being more in danger of overcharging due to a higher current. However, with only 50A available it doesn't seem to be a huge risk.

Outtasight, I think you're right about keeping it simple. Maybe I've gone off on a diversion about the separators. However, I'd have to leave it to the Expert Witness to decide what the most relevant points to argue are. I suppose most people are aware that it isn't a good idea to leave a car battery in the car or on a shelf without charging it  wackoold
« Last Edit: May 06, 2011, 02:15:23 PM by Eleanor » Logged

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Outtasight
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« Reply #108 on: May 07, 2011, 10:41:35 AM »

 genuflect for finding that court case document.  It's a jolly good read.  Maybe I need to get out more...

 tumble
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« Reply #109 on: May 24, 2011, 01:00:10 PM »

I've had a little trip down the dark path  horror
I tried contacting the separator manufacturer and was doing quite well (they even thought the cells may have been manufactured by different companies  ralph) until they cottoned on why I was asking questions. Radio silence now prevails but they haven't said the separators won't make a difference.
We're going to snip a bit off each type of separator and I'm hoping that in the next couple of weeks we'll have some electron microscope images to look at. The separator man said that they needed testing to show what they are made of and this seems a reasonable way of doing it. Just a lucky coincidence perhaps that the brainy half of the duo works for an electron microscope manufacturer. Could this be the proof that is required for the Bingo moment?  Huh
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Eleanor
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« Reply #110 on: October 24, 2011, 09:52:36 PM »

Some good news today - 12 shiny new 2V cells are winging their way in this direction and should be here in time for Christmas!  reindeer

 extrahappy
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Backache stuff!!


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« Reply #111 on: October 24, 2011, 10:09:33 PM »

It just goes to show, you should never be without an electron microscope, you just never know.................or something.

Fingers crossed.

Desp
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Justme
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« Reply #112 on: October 24, 2011, 10:48:06 PM »

Sweet.


 Grin Grin Grin Grin Grin

What was the killer shot for them?
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Navitron solar thermal system
30 x 58mm panel 259L TS
1200watts solar 120vdc
FX80 Solar controller
Victron 12v 3000w 120a
6kva genny
6 x 2v cells 1550amp/h 5C
24 x 2v cells 700amp/h 5C
Total bank 4350 amp/h 5C
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« Reply #113 on: October 25, 2011, 12:10:57 AM »

Desp, I guess you could prove most things with an electron microscope  tumble

Justme, being a bit of a masochist I ended up writing a 14 page report but the SEM images of the separators were La Pièce de Résistance (no pun intended)  tomatosplat

Thanks to everyone who contributed and helped as I would probably have given up without them. Outtasight was on the right track early on with regard to antimony poisoning. The separators of the 5 older cells were manufactured from polyethylene and the newer ones from a mixture of polyethylene and rubber. By knowing the materials and measuring the thicknesses of the separators it was possible to identify the exact brand of separator used. Rubber or polyethylene/rubber separators cause higher on charge voltages than separators not containing rubber as they are able to inhibit antimony poisoning which causes a reduction in the on charge voltage. Lots learned and will post more if people are interested ..


* Polyethylene.jpg (57.85 KB, 500x460 - viewed 295 times.)

* Polyethylene-Rubber.jpg (60.07 KB, 500x460 - viewed 305 times.)
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Justme
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« Reply #114 on: October 25, 2011, 11:47:45 AM »

Lots learned and will post more if people are interested ..


Did you really need to ask?
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Navitron solar thermal system
30 x 58mm panel 259L TS
1200watts solar 120vdc
FX80 Solar controller
Victron 12v 3000w 120a
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6 x 2v cells 1550amp/h 5C
24 x 2v cells 700amp/h 5C
Total bank 4350 amp/h 5C
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« Reply #115 on: October 26, 2011, 10:55:33 PM »

Will post a bit more over the next day or two - just enjoying a few battery free days  Grin
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Justme
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« Reply #116 on: October 26, 2011, 11:47:32 PM »

Did they let you keep the old ones?
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Navitron solar thermal system
30 x 58mm panel 259L TS
1200watts solar 120vdc
FX80 Solar controller
Victron 12v 3000w 120a
6kva genny
6 x 2v cells 1550amp/h 5C
24 x 2v cells 700amp/h 5C
Total bank 4350 amp/h 5C
Eleanor
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« Reply #117 on: October 27, 2011, 12:05:18 AM »

Hasn't been mentioned  Lips Sealed We've been thinking that we can make a reasonable 12V bank from 6 of the 7 good ones. Then we could make a 6V bank of 3 of the bad ones and so it goes on .. I've been wondering about trying different methods of recovering individual 2V cells and seeing what works best. I've asked for a quote to include an electrolyte circulation system with the new cells as we're on a mission to see how long we can keep them going. They did offer the option of a full refund - maybe they wish we'd taken that now ralph
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« Reply #118 on: October 27, 2011, 06:46:06 AM »

If you have 7 good ones its worth buying 5 more matching ones to make a super battery bank.
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Eleanor
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« Reply #119 on: October 27, 2011, 01:02:10 PM »

Hi Roger, when I say "good" ones I probably mean "better" ones. They will be 2 years old by the time the replacements arrive  Cool
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