When does electrolyte stratification occur?

Blackcherry04 · September 2013

Re: When does electrolyte stratification occur?

vtmaps wrote: »

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

A pump of some kind may be a solution, the folks that wrote the paper you referenced thought so. I can't see where injecting oxygenated air into the process would be helpful in preventing grid corrosion. I am sure there is a cost / benefit analysis that could be done. A passive separator with a air lift built in seems like something that would work, but there are at least 3 patents on that already and no one is using them.

NorthGuy · September 2013

Re: When does electrolyte stratification occur?

vtmaps wrote: »

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

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

This winter I spent tremenious amount of time measuring SG. During discharge, it falls lineraly with decreasing SoC. If there was a stratification during discharge, SG would be lower than this.

Recently, someone posted that partial (opportunity) charging is bad for forklift batteries. I think this is because such charging creates stratification compared to the usual "discharge only" mode, which keeps homogemous electrolyte. This stratification creates difference between bottom and top parts of the batteries, which ... etc. etc.

vtmaps · September 2013

Re: When does electrolyte stratification occur?

NorthGuy wrote: »

This winter I spent tremenious amount of time measuring SG. During discharge, it falls lineraly with decreasing SoC. If there was a stratification during discharge, SG would be lower than this.

I'm not saying you're wrong about the SG falling linearly, but I don't understand the logic that says stratification can't be developing while this happens. At what rate were you discharging?... development of stratification is a function of discharge rate and internal resistance of the battery. I speculated in the OP that if the discharge rate were low enough, stratification wouldn't occur.

--vtMaps

NorthGuy · September 2013

Re: When does electrolyte stratification occur?

vtmaps wrote: »

I'm not saying you're wrong about the SG falling linearly, but I don't understand the logic that says stratification can't be developing while this happens. At what rate were you discharging?... development of stratification is a function of discharge rate and internal resistance of the battery. I speculated in the OP that if the discharge rate were low enough, stratification wouldn't occur.

I discharged with low rate, typical to RE.

It might be possible to get stratification at higher rates, but I don't know anything about this.

I don't see the mechanism - acid is evenly removed from the solution throughout the battery during discharge - why should it stratify?

Cariboocoot · September 2013

Re: When does electrolyte stratification occur?

I know I've written this before but ...

In order for stratification to occur you must have discharge, which separates the sulphur from the electrolyte solution and adheres it to the plates, and recharge which removes it from the plates and places in back in solution. After discharge the SG will be low, but consistent. After recharge it will be high but inconsistent until enough physical activity (bubbling) in the battery remixes it. Otherwise when you first charge the heavier sulphur-laden solution sinks to the bottom. This is why Absorb time is so important, especially on the tall case batteries where there is literally more space for the separation to occur in.

westbranch · September 2013

Re: When does electrolyte stratification occur?

I just disturbed some cobwebs in the old brain vault...

From my beekeeping information:

Some years I use Formic Acid to treat for Varroa mite.

IFF the Formic acid is above 65% concentration, and applied to an absorbent (dispersal) material (paper towel or cloth, etc). researchers have found that it will NOT disperse into the air, ie lower its concentration , and it 'flows' to the bottom as a 'ball' and will pool there or flow out the hive entrance.

However if the concentration is in the 50 - 55% range it WILL disperse evenly throughout the hive, which is what is wanted...

Makes me think that there could be something similar happening in the battery electrolyte...??

just thinking.

inetdog · September 2013

Re: When does electrolyte stratification occur?

Cariboocoot wrote: »

I know I've written this before but ...

In order for stratification to occur you must have discharge, which separates the sulphur from the electrolyte solution and adheres it to the plates, and recharge which removes it from the plates and places in back in solution. After discharge the SG will be low, but consistent. After recharge it will be high but inconsistent until enough physical activity (bubbling) in the battery remixes it. Otherwise when you first charge the heavier sulphur-laden solution sinks to the bottom. This is why Absorb time is so important, especially on the tall case batteries where there is literally more space for the separation to occur in.

I have seen a credible reference stating that stratification can occur during discharge as well, just not as strongly as during charging.
There may be some dependence on whether the battery is already somewhat stratified at the time discharge takes place.

Cariboocoot · September 2013

Re: When does electrolyte stratification occur?

inetdog wrote: »

I have seen a credible reference stating that stratification can occur during discharge as well, just not as strongly as during charging.
There may be some dependence on whether the battery is already somewhat stratified at the time discharge takes place.

Possibly related to age as well? After all the plates do not remain consistent across their height throughout lifespan, so the reactions wouldn't either. Something along the lines of a small effect amplified.

inetdog · September 2013

Re: When does electrolyte stratification occur?

Cariboocoot wrote: »

Possibly related to age as well? After all the plates do not remain consistent across their height throughout lifespan, so the reactions wouldn't either. Something along the lines of a small effect amplified.

My feelings also. At high charge rates the spongy active material will undergo chemical action faster than the more solid stuff. Sulfation can affect this too.

Vic · September 2013

Re: When does electrolyte stratification occur?

inetdog wrote: »

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.

Am definitely not an expert on these matters, but believe that the speced absorb voltage for common FLA batteries is a bit above the minimum gassing V.

Believe, also, that one needs a significant current to remain at the absorb voltage. Limiting the current should work against even reaching Vabs.

Think that one must have an Absorption stage to convert the last portion of the sulphate on the plates and get it into solution, which requires a lot of energy due to the already relatively high concentration of acid in the electrolyte. And, of course, the full Absorption stage also re-mixes the acid and removes stratification.

Opinions. Vic

Cariboocoot · September 2013

Re: When does electrolyte stratification occur?

There is electrolytic action on the solution regardless of current: it is the Voltage (or Voltage differential) that offers the potential for chemical separation.

Without this action the battery won't actually be fully charged because there is little remix of the electrolyte.

Note that AGM's do not have large solution amounts; just enough to transfer the electrons between plates. As a result they can take larger currents than flooded cells, but can not take the higher Voltages (which would cause too much chemical separation of hydrogen and oxygen leaving them with no solution to keep the plates and separators wet).

Blackcherry04 · September 2013

Re: When does electrolyte stratification occur?

There is one way to prove and disprove all these theories, get your drill out and insert a pipette tube to the bottom and take some samples and put them on your refractometer and tell us what you find. You could test it about 20 different ways, then we'll all know.

stephendv · September 2013

Re: When does electrolyte stratification occur?

For what it's worth the stratification I've seen in my tall cell forklifts (about 3ft high) is not that much using "normal" absorb voltage of 2.4V. With shallow cycling of about 10% and an hour long absorb I measure 1.28 SG at the top of the cells. Fully charged should be 1.29. I never see 1.29 using absorb of 2.4V no matter how long I leave it in absorb. But a quick EQ at 2.6V for even 20 minutes will bump the SG up to 1.29.
So if there is stratification, it's only a 0.01 difference in SG, which I guess is not that significant. Of course that's only for shallow discharges, it doesn't say anything about stratification during partial charging at less than 90% SoC.

vtmaps · September 2013

Re: When does electrolyte stratification occur?

Blackcherry04 wrote: »

There is one way to prove and disprove all these theories, get your drill out and insert a pipette tube to the bottom and take some samples and put them on your refractometer and tell us what you find. You could test it about 20 different ways, then we'll all know.

I don't have the facilities that "John P" has, so I can't do the experiment. There's no need to... others have done that and published their work.

In the published studies, not every possible charge/discharge cycle has been tested on every possible battery. Therefore, its quite possible that the lessons learned from these studies may not be relevant to you and your system. Lower current charging/discharging seems to minimize stratification, shorter batteries are less susceptible, and higher internal resistance makes them more susceptible. The study I referenced was done on stationary batteries, and batteries on a boat or vehicle probably get some mixing from being in motion.

--vtMaps

Blackcherry04 · September 2013

Re: When does electrolyte stratification occur?

vtmaps wrote: »

I don't have the facilities that "John P" has, so I can't do the experiment. There's no need to... others have done that and published their work.

In the published studies, not every possible charge/discharge cycle has been tested on every possible battery. Therefore, its quite possible that the lessons learned from these studies may not be relevant to you and your system. Lower current charging/discharging seems to minimize stratification, shorter batteries are less susceptible, and higher internal resistance makes them more susceptible. The study I referenced was done on stationary batteries, and batteries on a boat or vehicle probably get some mixing from being in motion.

--vtMaps

Yeah, a study from someone selling the fix at a price that is more than cost of most batteries. Again, putting Oxygenated air into a battery might not be a good thing over the long haul.

When does electrolyte stratification occur?

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