When does electrolyte stratification occur?

vtmaps · November 2012

I have been reading about the evils of battery stratification. Most sources describe it as a process that occurs during charging. Here is an explicit reference from Victron:

The sulphuric acid (H₂SO₄) produced as the battery is being charged has a higher density than water and does tend to settle downwards so that the acid concentration at the bottom of the battery becomes higher than at the top. Once the gassing voltage is reached, charging is continued with plenty of current (and therefore a high voltage). The resulting gas generation ‘stirs’ the electrolyte and ensures that it becomes well mixed again.

My question is: does stratification occur at other times such as during discharge, while floating, or while in storage?

It seems to me that during discharge, as water is produced, the water should float up to the top and result in stratification, but I have not found any explicit references to stratification occurring during discharge.

It also seems to me that if the charge (discharge) rate is slow enough, the sulfuric acid (water) that is produced should diffuse into the electrolyte without causing stratification, but once again I haven't found any references to it.

It is of note that several traction battery manufacturers sell air bubbling systems to use during charging, but they do not use them during discharge. This could be because stratification doesn't occur during discharge, or because it is impractical, or because the motion of the battery (golf cart, forklift, etc) during discharge stirs the electrolyte.

--vtMaps

inetdog · November 2012

Re: When does electrolyte stratification occur?

vtmaps wrote: »

I have been reading about the evils of battery stratification. Most sources describe it as a process that occurs during charging. Here is an explicit reference from Victron:

My question is: does stratification occur at other times such as during discharge, while floating, or while in storage?

It seems to me that during discharge, as water is produced, the water should float up to the top and result in stratification, but I have not found any explicit references to stratification occurring during discharge.

It also seems to me that if the charge (discharge) rate is slow enough, the sulfuric acid (water) that is produced should diffuse into the electrolyte without causing stratification, but once again I haven't found any references to it.

It is of note that several traction battery manufacturers sell air bubbling systems to use during charging, but they do not use them during discharge. This could be because stratification doesn't occur during discharge, or because it is impractical, or because the motion of the battery (golf cart, forklift, etc) during discharge stirs the electrolyte.

--vtMaps

So many good questions, where shall I start? :-)

1. Traction batteries tend to get shaken or sloshed while in use, which will minimize any effects of stratification during discharge, so I think that you are at least partially correct about that. However, I would not refer to the process during discharge as water production but rather conversion of sulphuric acid to water by capturing the sulphate on the plate an leaving OH in its place. Which leads to:

2. The electrical conductivity of the electrolyte will be higher with higher SG. This leads to a feedback effect which is negative feedback during discharge (the areas where the SG is lower will carry less current and so will lose SG at a lower rate than the areas where the SG is higher.) On the other hand, during charging the higher SG region will be carrying more current and so the SG will increase faster where it is already higher.

3. So the stratification does not result from the high SG electrolyte sinking. But the higher density does keep the different vertical layers from mixing once the gradient in SG has gotten started.
A cell with uniformly mixed electrolyte will not develop stratification just sitting there without charging. At least not in a gravitational field as low as ours. :-)

4. The secondary effect of the stratification is that more activity on the plates takes place at the bottom, leading to a variation in thickness and physical structure of the active material from top to bottom which can have a long term effect on battery life even if you later mix the electrolyte. Ideally, as the traction battery manufacturers know, you should be mixing the electrolyte during the whole charging process. This is especially important when you discharge the battery deeply before recharging as they do, since that side effect is cumulative over multiple charges.
But for stationary batteries, used at lower discharge levels as for RE applications, it is not so critical as long as you mix the electrolyte from time to time to keep the stratification from increasing.

BB. · November 2012

Re: When does electrolyte stratification occur?

This article is behind a pay wall:

Effects of Electrolyte Stratification on Performances of Flood Lead-Acid Batteries

Abstract
The electrolyte stratification in the charge/discharge of lead-acid cells was monitored by a sensor of $mml-math-1.gif$ specific gravity and an in situ electrochemical scan technique was used to measure the current distribution directly on the plates in the rest state. A simulated cell was set up to study the electrochemical behavior of lead-acid batteries with electrolyte stratification. The active mass (AM) in the lower part is discharged in preference to that in the upper part. During recharging, however, the AM in the upper part is more easily charged. The more concentrated the $mml-math-2.gif$ solution, the finer the formed AM becomes. Two mechanisms of sulfation have been proposed. For the batteries often at rest or in the discharge state at very low current, the sulfation occurs in the lower part of the plates, but for the batteries in continual cycles at a high current, it often appears in the upper part.

And there are other studies:

Dependence of Lead Acid Battery Performance on Electrolyte ...

The above appeared to discuss some of the same types of testing that John P did at his work and published here (mixing of electrolyte).

Anyway--It appears that the electrolyte will [not? -BB] stratify on its own accord... My guess is that the bubbles from charging and even the heat from charging/discharging helps mix the electrolyte (and the discharge heat helps very cold battery performance--mixing cools the plates and reduces cold weather performance)...

In the end, flooded cell lead acid batteries want discharge from the bottom and recharge at the top--Certainly not ideal. And I would suggest that very light loads and charging currents are not helpful for long battery life.

-Bill

PS: Member "John P" tests on pumping air to mix/agitate electrolyte in storage batteries. The setup and test results are in this thread:

Air pumps for better battery charging

PPS: I am not sure what I was writing at the time--Either I missed the "not" or I have changed my mind and think charging/cycling causes stratification (when low current/not gassing). -Bill B. 7/10/13

Cariboocoot · November 2012

Re: When does electrolyte stratification occur?

Well you're right, and so are they.

The "sulphuric acid produced during charging" is made by the reverse current forcing the sulphor off the palates and back into the electrolyte. At that point, since it is heavier, it is lower in the battery. Then when the bubbling starts it gets mixed back in.

By the same token, batteries left sitting with little or no activity will stratify as the chemical action of self-discharge takes its toll gluing the sulphor on to the plates and thus removing it from the electrolyte. Again you have a situation where the fluid density is in a state of flux and slowly the heavier fluid will sink to the bottom.

This is almost never a problem with standard batteries. Literally the taller the battery the greater the chance of separation affecting operation.

You're spot-on about the traction batteries too. They bubble them (sometimes) during charging to ensure a more even mix and therefor charge; plate sections with more sulphor on them aren't going to have the same physical activity across them as areas without. Stirring it up just makes everything come out more even.

It's low activity at any stage that exacerbates the problem. Batteries in motion/use will have enough activity to keep things stirred. I don't know if you've ever seen the instructions on "magic eye" batteries (built-in hydrometer) but they tell you to rock the battery after charging to get the magic eye to read properly.

mtdoc · November 2012

Re: When does electrolyte stratification occur?

BB. wrote: »

And there are other studies:

Dependence of Lead Acid Battery Performance on Electrolyte ...

Thanks for the link.

For those of us with AGMs this is interesting:

K.Higashimoto et al [17] have investigated the effects of
stratification in a stationary sealed lead acid battery during
the life pattern of recharging after a deep discharge. The test
batteries are of 200 Ah capacity using plates with heights of
140, 220, 440 and 660 mm and employ a non-woven fabric
composed mainly of minute glass fibers for electrolyte
retention as well as separator. It is demonstrated that during
cycling the stratification makes a large difference in the
electrolyte specific gravity and also a great amount of lead
sulphate is produced in the lower parts of the plates resulting
in a decreased battery capacity. It is observed that with the
increase in height of the plates there is an increase in the extent
of stratification. It is concluded that the tendency for
stratification in a sealed type battery is very much similar to
that of a flooded cell.

I can't equalize my AGMS but I can place them on their side or upside down. Maybe I should be turning my AGMS every few months. Sounds like a lot of work... (visions of an AGM rotisserie type rack coming to mind.....:roll:)

inetdog · November 2012

Re: When does electrolyte stratification occur?

BB. wrote: »

Anyway--It appears that the electrolyte will stratify on its own accord... My guess is that the bubbles from charging and even the heat from charging/discharging helps mix the electrolyte (and the discharge heat helps very cold battery performance--mixing cools the plates and reduces cold weather performance)...

...
-Bill

PS: Member "John P" tests on pumping air to mix/agitate electrolyte in storage batteries. The setup and test results are in this thread:

Air pumps for better battery charging

Nothing in the article seemed to indicate that stratification would develop in an idle cell (except maybe as the result of self-discharge??), but would develop spontaneously during discharge/charge cycles without mixing. I guess I got that part right. Looks like a lot of the other things I said may be wrong though. :-)

inetdog · November 2012

Re: When does electrolyte stratification occur?

Cariboocoot wrote: »

I don't know if you've ever seen the instructions on "magic eye" batteries (built-in hydrometer) but they tell you to rock the battery after charging to get the magic eye to read properly.

That could be the effect of stratification or just the effect of bubbles sticking to the float of the magic eye. Same way you may have to tap your hydrometer after filling it to get bubbles off the float.

Or maybe it is more closely related to having to shake the magic 8-ball to get a different answer to come up. :-)

inetdog · November 2012

Re: When does electrolyte stratification occur?

mtdoc wrote: »

Maybe I should be turning my AGMS every few months. Sounds like a lot of work... (visions of an AGM rotisserie type rack coming to mind.....:roll:)

Turned by a little dog inside an exercise wheel....

FWIW, some AGMs warn against charging them while upside down, so you would probably want to just go from one side to the other and never use them with the terminals on top.

vtmaps · November 2012

Re: When does electrolyte stratification occur?

I still haven't seen an explicit statement that stratification occurs during discharge. In the link that BB. provided, is this:

the flow of high-density acid to the bottom of the cells of traction batteries during discharge – charge cycles

This means that stratification occurs in a complete discharge-charge cycle, but does it mean that stratification occurs during discharge and also during charge?

If stratification does occur during discharge (as well as charge), that would mean that stratification is exacerbated by the intra-day mini charge-discharge cycles that are common in RE systems. It also means that if batteries are not fully charged each day, they should at least be charged to the point of heavy gassing every day.

--vtMaps

inetdog · November 2012

Re: When does electrolyte stratification occur?

vtmaps wrote: »

It also means that if batteries are not fully charged each day, they should at least be charged to the point of heavy gassing every day.

--vtMaps

An interesting problem for two reasons:

1. Is it even possible to get the cells above their gassing voltage during Bulk or Absorb just by increasing the charger voltage but still limiting the current?
2. A fully charged battery which has been Absorbed for a longer time at a lower voltage will not gas at all, I think. So you could end up with a fully charged battery every day but still have stratification.

vtmaps · November 2012

Re: When does electrolyte stratification occur?

inetdog wrote: »

A fully charged battery which has been Absorbed for a longer time at a lower voltage will not gas at all, I think. So you could end up with a fully charged battery every day but still have stratification.

If you DEFINE a battery as fully charged when its SG (measured by hydrometer) is at manufacturers specification, you will never reach full charge without gassing (in a stationary battery without an electrolyte circulator).

I suppose there are other ways to define 'fully charged'.

--vtMaps

niel · November 2012

Re: When does electrolyte stratification occur?

maybe exorcise your batteries once in awhile?
seriously, shaking could help them.

stephendv · November 2012

Re: When does electrolyte stratification occur?

When googling about charging forklift cells I noticed that there's also a discrepancy between the old and new style batts. The old ones used to use a lot of antimony in the plates which meant that they gassed more at the absorb voltage, so there was less of a problem with stratification. A lot of the newer batts proudly advertise that they are "low antimony", so they have less self-discharge but also gas much less at absorb.

Preventing stratification seems to be good for more than just prolonging batt life, the air-bubbler systems all claim that the charging efficiency is increased from 80% to 90%. Of course there are the added power needed to run the air pump, but I can't imagine this is too significant (I think john p confirmed this).

EQ is supposed to break up stratification, but if this only happens once a month seems like there's an awful lot of time for stratification to do its damage. Would be really nice if charge controllers had a second EQ setting that could be run more regularly, but for a shorter period of time, or an IUIa charge curve that effectively does an EQ at the end of absorb, but this would be more difficult to implement.

Came across this PHd thesis where the author suggests doing the high voltage destratification step before absorb: http://tel.archives-ouvertes.fr/docs/00/44/36/15/PDF/these_Phuong_Nguyen_bis.pdf their method is a bit more complex because they calculate the current to use for this mini-EQ step based on a charge acceptance measurement step. What I took away from it was that doing a mini EQ before or mid-absorb could be useful.

BB. · November 2012

Re: When does electrolyte stratification occur?

A 1981 report on storage batteries (directory). From Chapter 3:

During a recharge, the oxidation of PbSO4to PbO2 at the positive plates and the reduction of PbSO4 to Pb at the negative plates forms sulfuric acid of higher concentration in the pores of the positive plates.

This higher density acid settles to the bottom of the cell giving higher specific gravity acid near the bottom of the plates and lower specific gravity acid near the top of the plates. This stratification accumulates during non-gassing periods of charge. During gassing periods of charge partial stirring is accomplished by gas bubbles formed at and rising along the surfaces of the plates and in the separator system.

During discharge, acid in the pores of the positives and near their surface is diluted; however, concentration gradients set up by longer charge periods are seldom compensated entirely during shorter discharge periods.

Diffusion processes to eliminate these concentration gradients are very slow, so stratification during repetitious cycling can become progressively greater. Two methods for stratification control are by deliberate gassing of the plates during overcharge at the finishing rate or by stirring of cell electrolyte by air-lift pumps. The degree of success in eliminating stratification is a function of cell design the design of the air-lift pump accessory system, and cell operating procedures.

Overcharge at the finishing rate may be increased to obtain the required electrolyte circulation after deep discharges. Very large, tall cells may require air-lift pumps which use an outside air pressure source to operate one or more air-lift pumps to circulate
dense acid from the bottom to the top of the cell.

Correct electrolyte circulation can increase the utilization of positive and negative active material, increase capacity and energy output and increase life. In most solar photovoltaic systems which use shallower cells, adequate circulation is achieved by judicious control of the percent of overcharge. A battery or knowledgeable systems engineer should participate in the system design and in setting up the operating procedures.

-Bill

Photowhit · November 2012

Re: When does electrolyte stratification occur?

The midnite classic will allow you to set the time and interval between equalizing cycles, address 4162 and 4163. I think one of the presets is for a 2 week interval as well, I couldn't find the presets just now.

stephendv · November 2012

Re: When does electrolyte stratification occur?

Photowhit wrote: »

The midnite classic will allow you to set the time and interval between equalizing cycles, address 4162 and 4163. I think one of the presets is for a 2 week interval as well, I couldn't find the presets just now.

Yeah this is common to probably all the MPPT charge controllers. But what it (they) don't allow you to do is set 2 different times and intervals, e.g. 1 very short EQ every week where the only purpose is to break stratification, which is not long enough to EQ the cells. And another once a month, or once every 2 months which is a longer proper EQ to actually equalise the cells and ensure a 100% charge.

vtmaps · November 2012

Re: When does electrolyte stratification occur?

From that 1981 report on storage batteries (directory), Chapter 3:

During discharge, acid in the pores of the positives and near their surface is diluted; however, concentration gradients set up by longer charge periods are seldom compensated entirely during shorter discharge periods.

Huh? I don't understand what this means... are they saying that discharging a battery can (if done slowly enough) compensate for stratification? In my original post I speculated that discharging battery would cause stratification, but I still haven't seen an explicit statement that this is so.

--vtMaps

BB. · November 2012

Re: When does electrolyte stratification occur?

I am guessing that they are saying that high discharge rates create heat and/or that since discharging tends to occur at the lower portion of the plates (where the denser electrolyte somehow appears (and therefore would seem to reduce stratification), which can help circulate the electrolyte/reduce stratification--But that short periods of discharge do not generate enough heat/de-stratification to matter.

-Bill

stephendv · January 2013

Re: When does electrolyte stratification occur?

Here's an interesting product from a forklift batt manufacturer which seem to use short pulsed EQ charges to mix the electrolyte:

Attachment not found.

niel · January 2013

Re: When does electrolyte stratification occur?

stephendv wrote: »

Here's an interesting product from a forklift batt manufacturer which seem to use short pulsed EQ charges to mix the electrolyte:

Attachment not found.

interesting as it sounds like a desulfator on steroids. of course they aren't claiming to rejuvenate batteries with the technology, but rather mix the electrolyte with higher current pulses. this will increase gassing for those out there thinking of doing this without a proper venting system. sounds promising. anybody who tries this please report back on your observations and opinions.

vtmaps · January 2013

Re: When does electrolyte stratification occur?

stephendv wrote: »

Here's an interesting product from a forklift batt manufacturer which seem to use short pulsed EQ charges to mix the electrolyte:

It says "current induces gas pulses during the final charging phase".

Why is this better than the normal gassing that occurs late in absorb? One of the advantages of an air bubbler is that it prevents stratification during all stages of charging.

--vtMaps

stephendv · January 2013

Re: When does electrolyte stratification occur?

vtmaps wrote: »

It says "current induces gas pulses during the final charging phase".

Why is this better than the normal gassing that occurs late in absorb? One of the advantages of an air bubbler is that it prevents stratification during all stages of charging.

Yup, dunno

There was a PhD thesis on an idea to do a similar pulsed EQ charge at the start of absorb so that the electrolyte was destratified for the duration of absorb, apparently allowed for faster overral charging: http://tel.archives-ouvertes.fr/docs/00/44/36/15/PDF/these_Phuong_Nguyen_bis.pdf

I've configured Mango + Midnite classic to do this type of charge every 10 days now as an experiment. EQ for 1 second, then sleep for 3 minutes, repeat 10 times. If this works, it could be less hard on the batts because there isn't as much time spent at EQ voltages.

NorthGuy · January 2013

Re: When does electrolyte stratification occur?

BB. wrote: »

The active mass (AM) in the lower part is discharged in preference to that in the upper part. During recharging, however, the AM in the upper part is more easily charged.

Wouldn't that mean that during discharge, the acid is consumed at the bottom, but diring re-charge the acid is produced at the upper part.

If so, the acid should move to the top, which would not make stratification worse, but rather would cure it.

vtmaps · January 2013

Re: When does electrolyte stratification occur?

NorthGuy wrote: »

Wouldn't that mean that during discharge, the acid is consumed at the bottom, but diring re-charge the acid is produced at the upper part.

If so, the acid should move to the top, which would not make stratification worse, but rather would cure it.

Doesn't matter where it is produced... the acid is heavier so it sinks.

Also, when charging, it is easier (electrochemically) to push the sulfate into the dilute acid (at top of cell) than into concentrated acid at the bottom. However there isn't much sulfate at the top of the cell. That's the problem with stratification... during discharge if the acid is at the bottom of the cell, the sulfate will deposit on the lower portions of the plates.

--vtMaps

Blackcherry04 · January 2013

Re: When does electrolyte stratification occur?

vtmaps wrote: »

Doesn't matter where it is produced... the acid is heavier so it sinks.

Also, when charging, it is easier (electrochemically) to push the sulfate into the dilute acid (at top of cell) than into concentrated acid at the bottom. However there isn't much sulfate at the top of the cell. That's the problem with stratification... during discharge if the acid is at the bottom of the cell, the sulfate will deposit on the lower portions of the plates.

--vtMaps

Since VT mentioned that a lower SG will in fact recombine Sulfate easier, there is a published procedure where you lower the SG to 1.10 to 1.050 by withdrawing Electrolyte and diluting with Distilled water and then doing a EQ and stop when there is no rise, you lower the SG again. After doing the procedure as many times as it takes until you no longer get a rise in SG, you have removed as much sulfate as can be removed, you reverse the process and raise the SG back to it's normal strength.

It's probably something that you wouldn't want to do on a cheap battery, sounds like a lot of work, but a $1,000 battery, I would.

vtmaps · September 2013

Re: When does electrolyte stratification occur?

From the OP in this thread:

vtmaps wrote: »

It seems to me that during discharge, as water is produced, the water should float up to the top and result in stratification, but I have not found any explicit references to stratification occurring during discharge.

I have finally found a reference that does state explicitly what I have suspected:

from: http://www.battcon.com/PapersFinal2013/11-Larry%20Meisner%20-%20The%20Effects%20of%20Mechanical%20Electrolyte%20Mixing%20VLA%20Cells%20in%20Renewable%20Energy.pdf

After discharging, acid stratification is observed and level of stratification will depend on the discharge current and the internal resistance of the cell.

--vtMaps

vtmaps · September 2013

Re: When does electrolyte stratification occur?

inetdog wrote: »

A fully charged battery which has been Absorbed for a longer time at a lower voltage will not gas at all, I think. So you could end up with a fully charged battery every day but still have stratification.

Yes! good thinking. The battery can be charged up and reach an equilibrium, an 'end amps', but that equilibrium is in a stratified cell. --vtMaps

Blackcherry04 · September 2013

Re: When does electrolyte stratification occur?

vtmaps wrote: »

Yes! good thinking. The battery can be charged up and reach an equilibrium, an 'end amps', but that equilibrium is in a stratified cell. --vtMaps

Did you buy their Electrolyte mixer yet ??

vtmaps · September 2013

Re: When does electrolyte stratification occur?

Blackcherry04 wrote: »

Did you buy their Electrolyte mixer yet ??

No, I mix them up by overcharging. --vtMaps

Blackcherry04 · September 2013

Re: When does electrolyte stratification occur?

vtmaps wrote: »

No, I mix them up by overcharging. --vtMaps

There you go, you found the solution to the problem that's been going on since the Battery was invented.

vtmaps · September 2013

Re: When does electrolyte stratification occur?

Blackcherry04 wrote: »

There you go, you found the solution to the problem that's been going on since the Battery was invented.

It is a solution, but not THE solution. Overcharging is hard on batteries. Stratification is hard on batteries. Sounds like electrolyte circulation pumps can extend the life of the battery by eliminating both stratification and the need to overcharge.

--vtMaps

When does electrolyte stratification occur?

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