Very Low OCV on Full Batts???

[jrnymn7]

I've been seeing lower than usual resting ocv this winter, compared to last. Figured it was either wear and tear, too high a charging voltage, or a parasitic draw.

So I tried throwing a lower voltage charge on the bank, to see if that would make any difference, as I know my four 6's prefer to be topped off at less than 15v (t.c.). On Tuesday Feb 3rd, ocv was 12.71 (use to hold strong at 12.78v, last winter), so I topped them up to within 1a @ 14.6v. But before I could give them a week or two to see the results, ocv suddenly plummeted.

Wed, ocv was 13.08. Fri, ocv was 12.88. Sat, ocv was 12.84.

On Saturday I re-wired the bank, c/w new bus bars, and hooked everything up as it would normally be, including the Trimetric, but just no fridge controls, and the cables from the charger were hooked to the bank, but not to the charger. These two cable ends were wrapped with cloth and taped with electrical tape, so they could not short out in the charger cabinet.

Mon, ocv was only 12.54!

So, I checked across all four 6v's, and voltages were all within 0.01v; all in the ~6.27v range. I confirmed the ocv with the trimetric (was rounded up to 12.6v, which is normal for the Trimetric). I switched it to amps, and the Trimetric said 0.00 amps draw. Ah's used was zero, as well. SOC was 100%. I also checked all connections.

So, I hooked up the pm4b-45, and voltage immediately rose to the bulk setting, and amps quickly tapered from 45a to 4a in 15 minutes. So, clearly the bank was full, or at least very near full. But according to the resting ocv, the bank should have been in the 75% soc range, and down at least 100 Ah's.

So why did I get a 12.54v reading on full batts???

Thanks!

[jrnymn7]

So, Monday was the day I first found the bank down to 12.54v. I charged at the pm4b's boost setting of 15.6v for 15 minutes, and then continued at 14.6vabs until 1.1a... about 2hrs 15min in total.

Today, Wed., after just two days, they were at 12.56v. Tomorrow I will charge again, at a much higher t.c. voltage, then disconnect everything. Hopefully it's just a case of progressive capacity loss due to undercharging.

I also did some light reading and found that heavy sulfation/stratification can give false (higher than actual) voltage readings. So, maybe the 12.8+ readings a few days after charging may have been misleading. That coupled with the slightly low SG readings may be an indication of undercharging, over the last month or so. Of course, that may still not explain the 12.71v - 12.74v readings I was getting a week after charging with the Peak at 15.6 - 16v, previously. But then again, I was only taking the bank to about 6a, at those voltages... so again possible undercharging.

I guess I'll know, soon enough.

[Canadian_Rainbi]

:S:S OldTimers. What can I say?

[grizzzman]

Open-circuit voltage

[Canadian_Rainbi]

Pardon my ignorance, but what is "ocv" ?

[mena661]

I could see the voltages changing a little after the recabling while they "equalize" themselves, that's happened to mine before but not that large of a drop. Anyways, keep us posted.

[jrnymn7]

When I took the readings on Jan. 23rd, the ambient temp was about 35*F. So when corrected, the batts were actually in the 1.265 - 1.280 range. Back in October, after I did the EQ on A1 and A2, and then a regular charge cycle to NEAR full, at 14.8v, the SG's were at;

A1 1.280, 1.275, 1.275.

A2 1.275, 1.275, 1.275.

At true full, they would read a little higher than that.

So it does look as though a few cells were somewhat on the low side, on Jan 23rd... 1.265 vs. 1.275-1.280

I think before I pull everything back apart, I'll do a long 8 hour charge at the proper T.C. voltage, and then pull things apart. Then I will know how each individual batt responds, and for sure if there's one bad batt. If they all hold their own over a couple of weeks, then I'll be looking for another explanation.

It just seems rather odd to me, though, that this all happened immediately after re-wiring everything with the new bus bars. I did add the Trimetric back into the mix this time (turned off), as well as those two cables to where the charger would go, but neither of those things should have any negative effect on resting ocv.

I was really hoping to just be done with the batts for a while, as I need the bed (which is above the cabinet) for storage space. So, this is really throwing a wrench into my plans. Oh well, grin 'n' bear.

[BFL13]

In the cold, when it reads 1.300, that corrects to being equivalent to 1.280 or whatever.

I don't know what a "bad" cell reads like but a "weak" cell will have a lower SG than the others. A guy last summer with two older 6s had one with the three cells all just barely in the green and the other 6 had one cell just barely in the green and the other two were way in the red. That pair of 6s lasted him all summer though (with the help of the 130w panel I sold him 🙂

[jrnymn7]

Thanks, guys.

Yes, I do have temp compensated charts for abs and float (courtesy of P.T.) and an SG correction table.

The higher voltages seemed to be causing the ocv to drop, like back in the fall, so I lowered things back down... of course, if it is one bad cell, that may have been the real problem all along.

Could one bad cell pull down the overall voltage readings without actually depleting the bank?

Regarding cold SG readings, if it reads say 1.290, that may only really be say 1.270, and so not as full as it appears to be? IOW, if my SG's are usually in the 1.280 range, when cold they should read higher than that? Or do the readings show the same on the hydrometer, and I have to add or subtract from the usual readings?

And if one cell is bad, wouldn't it show in the SG readings, or can the stronger cells pull the weak cell's SG up a little?

Thanks!

[pianotuna]

Hi,

At -20 C (-4 F) you need a gassing voltage of 17.82 volts.

-15 , 16.86
-10 , 15.9
-5, 15.57
0 (32 f) 15.24

[mena661]

jrnymn7 wrote:

Could the unequal series vs. parallel cable lengths be part of the problem?

No, that won't do it. Sounds like you have a shorted cell and, yes, it can show up all of a sudden. I had a 12V sitting in my garage just pop up one day with a shorted cell. Separate the batts and check voltages over the next few days to see if one drops. My guess is you have one pulling the rest down.

[BFL13]

SG is higher in the cold. With the fluid more dense due to lower volume, the bobber floats higher.

There is a correction table for that somewhere how much to add or subtract from the observed reading. The correction may vary a little by battery spec per brand but here is Trojan's (that 14.6 in this one may have recently been increased to 14.8 but the SG correction would be the same)

http://www.trojanbattery.com/Tech-Support/FAQ/Temperature.aspx

Note the temp vs voltage too--it says crank up the charger's voltage by x amount for a cold temp.

PS, I guess you don't want to know about daffodils just now! 🙂

[jrnymn7]

BFL13 wrote:
Hard to say. I would want to know the SG in all cells to see if the SG is up where it belongs when the batts are "full" SG will be a little higher in cold temps too.

I took sg readings about two and a half weeks ago... they were in the 1.280-1.295 range... nothing out of the ordinary. After EQ'ing last fall, they went back to pretty much where they had been; although the two weak cells are doing a little better than they were.

I admit I'm still confused on how to interpret the readings when cold, though. For example, does 1.275 mean more or less, in reality? Perhaps the batts have not been as full as I thought, lately?

EDIT:

Friday January 23rd, ocv @ 12.83v:

A1 (6.41v) B1 (6.42v)
1.295 1.290
1.280 1.295
1.280 1.290


A2 (6.42v) B2 (6.41v)
1.285 1.290
1.290 1.290
1.280 1.295

A1 and A2 are the ones I EQ'd last fall.

[jrnymn7]

I threw this out in a different thread, sometime back, but am wondering if it could have some bearing on what's going on. It has to do with balanced wiring. Early this winter I rotated the batts one spot clockwise:

before,

B1 B2

A1 A2

... series and parallel cables were of identical length... + from A2, - from B1.

now,

A1 B1

A2 B2

... series cables (A1-A2 and B1-B2) are both 14" long, but the parallel cables (A1-B1 and A2-B2) are only 8" long... + from B2, - from A1.

Could the unequal series vs. parallel cable lengths be part of the problem?

[BFL13]

Hard to say. I would want to know the SG in all cells to see if the SG is up where it belongs when the batts are "full" SG will be a little higher in cold temps too.

[jrnymn7]

BFL,

Last tuesday, the bank was resting at 12.71v. I charged with the Peak for about half an hour at it's 4a setting, until voltage rose to 15.2v. I switched it to it's 10a setting and continued until voltage rose to 15.4v. I then switched over to the pm4b and charged until 2.3a @ 14.86v, at the 15 minute mark, then tweaked Vabs to 14.6v and 1.5a, and took the batts down to 1a; which took quite some time.

I don't see how batts that were resting at 12.71v, and then charged for several hours, even at less than optimal voltges, can slowly drop to 12.84v over 4 days, and then suddenly drop from 12.84v to 12.54v in just two days. My intention was to do sort of a high voltage (albeit short term) float, which is why I used 14.6v. If anything, all this extra charging should have helped sustain a good ocv, no?