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!

[mena661]

jrnymn7 wrote:
I'm wondering if it's something that occurs in all temperatures, or just in very cold temps?

The resting voltage on my batts (12.82) has not changed but the coldest I've ever checked them was 47F/8C (this was battery temp not ambient). I top charge once a month.

EDIT: Let me correct the top charge. That should say, I perform a de-stratification charge once a month (tall batteries require this...you guys don't have to do that).

[jrnymn7]

I'm wondering if it's something that occurs in all temperatures, or just in very cold temps?

[mena661]

Are you talking about lower resting voltages in cold temps or just in general?

[jrnymn7]

BFL, It has long been my plan to eventually move out west. I've been wanting to do some wwoof'ing on the Island for quite some time now. Green grass and daffodils would sure be a nice change at this point!

Re, stratification... yes, I've been wondering about that myself. In fact, I always do a few electrolyte re and re's, and give the batts a bit of a gentle knocking around/tilting to mix things up before taking readings. But I can certainly see there being some settling. Perhaps a longer dip tube would be helpful in determining the amount of stratification, and for mixing things up?

Landyacht asked an interesting question in his 'frigid weather charging' thread:

"So what is happening to the battery floating at 13.2v at say 10 degrees F. Is it still self discharging? Is it being discharged by the charger?"

If top-off and/or float charging at too low a voltage actually reduces the resulting resting ocv, then that may answer his question, and explain his buddy's battery's low readings. So I guess the real question is, should lower resting voltages be of any real concern, provided one is aware they are simply the result of lower voltage charging, and not necessarily an indication of lower soc? (once again indicating that SG readings are a more dependable method of determining soc)

[jrnymn7]

Niner, I did top charge them before putting them into hibernation, but no EQ; that was done back in Oct, on two of the batts. Since disconnecting all wiring last week, at first I was kinda surprised to see that the "weaker" (lower SG) batteries are holding higher voltages than the "stronger" (higher SG) batteries. But then again, it shouldn't be any surprise, seeing as that's just how my batts behave.

But overall, the resting ocv's appear to have been on the low side; even after allowing for temps; indicating my batts' self-discharge has most likely increased. And if a noticeable increase in self-discharge is clearly evident at cold temps, I can certainly imagine how the warmer weather is going to effect things. Is self-discharge at an alarming rate? No. But it is something I want to keep an eye on. It appears I may be losing my 'high resting ocv' bragging rights. 🙂 So I'm now trying to figure out if this is indeed the case, or just a result of too low a voltage when top charging cold batts. It's tricky, given so many variables.

[BFL13]

I forget why, but high SG batts like golf car ones don't like floating very much. UPS batts are lower SG and don't mind the long-term float. I think this might relate to this situation.

When very cold, the electrolyte volume is less and density higher. This might explain observed SGs and voltages but don't know enough to say for sure.

What might happen, with the more dense electrolyte even though that makes the bobber float higher when taking readings, so SG reads more, is stratification.

What if the very cold electrolyte being "heavier" has more stratification than usual so your hydrometer only sees the "thinner" top part and gives a lower reading than it should. Would that explain anything observed? Then you charged (mixed) them so your SGs are closer to the truth? Some where in between?

I don't have a clue, just tossing random stuff in here.

Have to laugh at all these cold weather concerns. (Tee hee) Had to cut the lawn today. The daffs are out, etc. 🙂

[NinerBikes]

jrnymn7 wrote:
Niner,

I knew there was capacity loss due to cold, but I was not aware there was an associated drop in resting voltage. I mean, it does kinda make sense, but I had never seen it mentioned before. I'm just trying to establish what is normal or to be expected, and thus know when I should be worried.

The bank will remain in hibernation for the next couple months, but I've been concerned with whether or not last years use(abuse?) had any ill effects? All seems well and good though.

If you top charged them, then a few days later, fully equalize charged them before putting them away for the fall and winter season, then revisit them come spring if they've been completely disconnected the whole time. At that time, re top charge them, measure CG again, and if they are not equal, re equalize charge again shortly before going out to use them on a trip.

[jrnymn7]

Niner,

I knew there was capacity loss due to cold, but I was not aware there was an associated drop in resting voltage. I mean, it does kinda make sense, but I had never seen it mentioned before. I'm just trying to establish what is normal or to be expected, and thus know when I should be worried.

The bank will remain in hibernation for the next couple months, but I've been concerned with whether or not last years use(abuse?) had any ill effects? All seems well and good though.

[NinerBikes]

You are worrying too much... just go and use your batteries, cycle them a few times. Disregard the voltage and pay more attention to temperature correcteyd Specific Gravity, and you'll do fine.

Charge up the batteries, let them bubble a bit to stir up the electrolyte so that it's homogenous and uniform, and measure the specific gravity per cell, and call it good.

Your topped off voltage will vary due to ambient temperature affecting the chemical reaction capabilities of the battery acid. Atoms vibrate or move faster the higher the temperature they are at. And move slower the colder the temperature they are at, hence voltage will drop slightly in colder temps, when battery is at rest.

[jrnymn7]

Well, I've gone back over my notes, in light of the new info provided by Bend, and here are my conclusions;

My batts were resting at 12.71 - 12.73v, after being charged at sufficient, i.e; temp compensated, voltages. I thought those resting voltages were too low, but in reality, those resting voltages were actually a little higher than they should have been, given the very cold temps. So, those voltages were actually an indication that the surface charge had yet to fully self-discharge; obviously due to slowed self-discharge rates. So after a couple more weeks at such cold temps, they still would have been fully charged, even once the voltage dropped to below 12.7v. So everything was fine, the batts were indeed full, and charging was not necessary. Nevertheless, I did charge them, at that time, and the readings, while charging, indicated they were indeed full. Hence my confusion at the time.

But here's the kicker. When I charged them, I lowered Vabs to 14.6v, in hopes to see better resting voltages, but then the voltages plummeted to 12.54 over the next 6 days. How and Why? Well, for one thing, temps stayed consistent for the next 3 days, but then dropped consistently the following 3 days. When I compared self discharge, resting voltages, and daily temps, they all seem reasonable for the first 3 days. But then (EDIT; it appeared) there was a sudden increase in discharge rate as temps dropped, which does not make sense, and by the 6th day, resting voltages had dropped drastically, to well below what they should have been at 18F.

So, clearly, dropping Vabs results in much lower resting ocv. Fine. But what really confused me was how charging FULL batts at 14.6v for 2 hours could result in lower resting ocv??? Now think about it... that is like fully charging warm batts at the appropriate Vabs (say 14.8v) and having an ocv of say 12.75v a few days later, after surface charge has dissipated, but then throwing a 13.2v float charge on the full batts for a couple of hours, and then having a lower ocv a few days later.

So the question arises, is that normal? IOW, does float charging result in lower resting ocv, than if one was to terminate charging at the end of the abs stage, and not allow the charger to float the batts? OR, is this perhaps a cold temp phenomenon?

[mena661]

My batts self discharge about 1% per week so don't be alarmed if your SG is lower. For now, you're just concerned with voltage.

[jrnymn7]

Bend,

Thank you so much!

I was just on that website this morning, but didn't see that part. In fact, I remember looking at that freeze point chart. If only I had scrolled down a little.

This has been very helpful. Thanks again.

[Bend]

I cannot find anything on line regarding the effect of cold temps on resting voltages though.

See the charts under the freeze chart:

http://jgdarden.com/batteryfaq/carfaq4.htm#freeze_points

[jrnymn7]

As I mentioned, I warmed up the bank before charging on Thursday. The result was resting ocv rose from 12.57v at 12F, to 12.63v at 44F. So apparently cold temps do have an effect on resting voltages; as well as discharge rates, charging voltages, and SG readings. I cannot find anything on line regarding the effect of cold temps on resting voltages though.

So, before the charge, ocv was 12.63 (and rising?). T.C. SG readings were in the 1.265 - 1.280 range... so a little low. I let the pm4b do its 15 min thing at 14.76v, which ended with the bank accepting 8a. I tweaked it to 15Vabs and 9a, and charged for an additional 6 1/2 hours, which ended with the bank accepting 5.6a. By this point, cabinet temp was 50F... No idea what temp the electrolyte was.

Resulting SG readings varied a lot... some rose while some remained the same, but overall were slightly higher... 1.265-1.285 range.

I disconnected all wiring and left batts to sit individually.

Two days later (Saturday), T.C. SG readings had risen ever so slightly, but appeared to be a bit better balanced. One thing I noticed, as I have in the past, was the batts with the higher resting voltages had lower SG readings. String A (the two "weaker" batts, based on SG readings) were at 6.52v (A1) and 6.53v (A2) while B1 was at 6.50v and B2 was at 6.49v.

Today (Sunday), voltages were A1 6.49, A2 6.51, B1 6.48, and B2 6.46 volts... each down about .02-.03v... at 17F. So now the waiting game, to see the long term results of Thursdays charge. My initial thoughts are the low ocv's I was seeing were from undercharging, but time will tell.

[jrnymn7]

Thought I should mention, even well after lowering charging voltage from 15+v to 14.6v, there was gentle bubbling/gassing going on... probably a result of increased electrolyte temperature, regardless of low surrounding ambient temps. So isn't that the best indicator of sufficient charging voltage?

Can I use a basic glass thermometer for checking electrolyte temp? If so, are there any available in the required temp range?

( ... I was looking at S.S. ones, but I highly doubt it's a good idea to stick a metal probe down into the cell.)

I ask because surrounding temps seem to be too misleading. For example, Thursday (2 days ago) I pre-warmed the cabinet and batts, and maintained a 45-50F surrounding temp, from below and around the bank. I charged at 15v for about 6 1/2 hours, and was able to maintain gassing throughout (will add exact details later).