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!

[BFL13]

To get 14.7v at 25C results, you need 15.3v at near freezing ambient.
13.6v at 25C is 14.2v at freezing ambient.

The acceptance rate is down on cold batts so they need higher voltage to accept the same amps as usual. That may lead to a "false full" charging at the usual 25C voltages I suppose.

[jrnymn7]

Niner,

I just EQ'd them a few months back, and have not had them in full service since. At that time, while top charging and EQ'ing, I noticed they didn't respond well to the higher voltages; ocv wise. A follow up charge at about 14.8 volts always resulted in better resting ocv.

Until just recently, I have been cold weather charging them with the Peak 40 amper, but its voltage was getting out of control. One rather cold day I saw it go as high as 16.2v, so I switched it to a lower amp setting (it has 4, 10, 20, and 40a) in order to drop the voltage a bit. The resulting ocv's have been lower than usual, so I figured maybe it was the high voltages?

I switched to the modded pm4b, last Tuesday, for better control of the charging voltage. I did a top charge to 2.3a @ 14.86v and then dropped it to 14.6v and continued that 'til ~ 1a. After 4 days, surface charge still had them at 12.84v.

Everything seemed normal until after I rewired everything. But if the batts were indeed down in soc, the pm4b-45 would not have behaved the way it did today. It usually stays at 45a 'til about 83% on half the bank (a single 215Ah series string). So at 75% soc on the 430Ah series/paralleled bank, it should have stayed at 45a for quite a while. It did not. Both it's voltage rose and amps tapered immediately, so the bank was not depleted... nor should it have been. There are no measurable draws, and the cold weather should help sustain a high ocv.

While charging today, voltages across all 4 batts were well within reason... varying, around 0.01- 0.02v, changing occasionally. (I took readings several times throughout the charge).

[NinerBikes]

If you are charging at such cold temperatures, your charge voltage needs to be temperature corrected also, for the batteries. The colder it gets, the higher the charge voltage you need to get your batteries topped off.

Have you ever equalize charged any of them? Perhaps it's time.

[jrnymn7]

SMK,

The ambient temps have been consistently below freezing, but we've had even colder days earlier this winter (like -22*C) with no such drop in ocv. I was top charging in the 15.4-15.6v range so I dropped it down to see if that would bring the resting ocv back up. Never got to see if that was the deal. This sudden drop in ocv is completely new. As stated, last winter the bank would hold strong at 12.78v for 3 or 4 weeks. Once, after 5 weeks, they were down to about 12.7, IIRC.

Roy,

12.6v is what I usually see after two days of light (summertime) use. I was hard pressed to find a chart that reflected my batts, but I did find one that goes to 12.8v. I've confirmed it with the Trimetric's Ah count, as well. My batts have always had higher than average ocv and sg readings (1.275-1.290 range).

[time2roll]

What is the battery temperature? You may need a temperature compensated chart.

[RoyB]

I have been using this chart to get into the ball park of the State OF CHARGE using a DC multimeter when camping off the power grid.


This is showing 12.5VDC is close to 90% charge state.

This may be apples and oranges for your test but in my case this is what I use to get me in the ball park. I will run my battery bank down to 12.0VDC as indicated on my BATTERY MONITOR METER PANEL which is close to being at 50% charge state and that is when I start re-charging using my 2KW Generator connected to the trailer shore power cable using an RV30A to 15A long adapter which fires up my PD9260C 60AMP converter/charger unit for my 255AH battery bank.

My batteries seem to like this procedure as I get pretty much full performance of the batteries again after approximately three hours of generator run time. My GP24 Interstate Batteries were installed in 2008.

After I do some 12-14 50% to 90% state of charge cycles I will then run my PD9260C Converter/Charger for around 12 hours to get a full 100% charge state on my 255AH Battery bank.

I have no need to be absolute on my readings - ball park is close enough for my purposes...

Roy Ken

[Johno02]

Sounds like a cell going bad.