Measuring Stratification in 2V Cells

[Eleanor]

I want to determine if there is any electrolyte stratification in our 2V cells. I'm thinking of trying two different methods :

1. Add bits of tube of varying lengths to the hydrometer so I can measure SG at different depths

2. Make a simple aeration system using an aquarium pump (just for one cell to start with) and measuring SG before and after aeration and until it gives a constant reading

Has anyone tried either of these and if so are there any tips or things to watch out for?

I know it's sulphuric acid  and electricity 

Thanks

[Justme]

Might be hard to suck up from the depth with a standard bulb type tester.

Air turbulence is a standard way of mixing it up. Or you could charge them hard (which they need every now & then), the gassing does the same thing.

Or if they are loose & small enough give em a jiggle but watch for splashes.

[Eleanor]

Justme

Thanks, I had wondered if the hydrometer has enough pulling power. I can only try. I suppose I could extract the acid into a container using something else eg big syringe if the hydrometer won't do it. I want to determine if there is stratification present so I'm trying to avoid launching into a big charge until I know. I'll probably buy or make an aeration system but I need to set something up quickly which can be used on one cell at a time.

[Philip R]

Lead Acid cells can stratify if they are continually float charged after discharge and the cell voltage not exceding 2.3 volts per cell (onset of cell gassing). If this undercharged state persists, the cells can end up non uniformly charged.

As Justme says, a boost charge, where some gassing takes place will set up some induced convection of the electrolyte and get the acid stirred up.

Dont go poking hydrometer tubes down to the bottom of the mud space, you will end up knocking active material off the plates and inducing a short circuit within the cell, thus trashing it/them. If your cells have clear containers, you can ascertain a lot about the state of charge from the colour of the plates. The positive plate will go a deep red brown colour, as it discharges it will develop a greyish hue, due to the formation of soft lead sulphate, reversible by charging promptly. If left in a discharged state for too long, these deposits consolidate and choke off the cell surface.

PhilipR

[Eleanor]

Philip

Thanks. Potentially bashing things when putting a hydrometer tube down there is something I was concerned about, I don't know how easy this will be. We don't have clear sides on the cells so have to work in the dark. I want to measure the amount of stratification because it has been suggested that there could be a significant amount but I'm not convinced (to explain the whole situation would make it too complicated). I want to determine if there is stratification or not, and if there is how much. How it got there or didn't get there is the subject of several more threads I expect. Maybe just aerating the cells and measuring the SG before and after is the answer.

I haven't seen a manufacturer supplied aeration system. How far do the tubes project into the cells and do they sell them as something a customer can retro-fit 

[stephendv]

The distributor I bought my bank from said that I could fit the air system after the fact.  Cost something like 250ish Euro for 24 cell system.  He also said that there's no need for it as long as there'll be a 2.4V charge every now and then.  However, I keep reading about the benefits of these systems especially with regards to battery life extension, charging efficiency and reduced need for EQ - a bit sorry that I didn't order the bank with the system now.
The Battery Technology Handbook (googleable pdf floating about) says that they can reduce the charging factor from 1.2 to 1.1 (i.e. you only need to put in 110% of what you took out, instead of the more typical 120%) - which could be great news for winter months.  Also they would increase life expectancy of the bats in a number of ways:
- Reduced need for EQ so not so hard on the positive plate
- Keeps temperature down during charging.  Along with deep DoD high temps are what kill batts prematurely.

If you're going to be the guinea pig, would love to see the results you get 

[Justme]

Wont your boats movement keep the cells from stratifying? Just like in a cars battery.

[Eleanor]

Stephen, thanks. I think it's something we'll invest in as I've always thought that relying on gassing and equalisation to keep the electrolyte mixed didn't seem ideal. I need to speak to our supplier about it. I've noticed recently that 5 of our cells are charging at just below the gassing voltage and the other 7 are charging at 2.54V so I want to check the 5 for stratification. If I do it by gassing the SOC will change so I won't be able to do a before and after comparison. After that the bank will need charging at a higher voltage to bubble the electrolyte in the 5 cells and then equalisation.

I've had an interesting discussion with the supplier and if there is stratification the remedy (to reverse any sulphation which may have occurred as a result) is to discharge the cells to 80% DOD (20% remaining)   and recharge including equalisation. FIVE times  

So, this is why I want to get an idea of the amount of stratification first as it will take days to do this  

Also, he said that traction batteries should be periodically discharged several times to 80% DOD so the plates can rebuild themselves and reduce sulphation and this deep discharge will prolong the life of the bank  

Apologies to anyone who may have thought my replies were a bit grumpy last night  

Justme, I wish I did but I haven't got a boat, just a tin shed! 

[Stuart]

What about a big syringe attached to a piece of tube, just squirt some air in there to get some mixing, and do your measurement to see how much difference it makes. I wouldn't have thought you need a continuous system.

[Eleanor]

Stuart, thanks sounds like a good idea  I don't think I'll put the tube below the tops of the plates as I don't want to risk causing any damage as Philip has pointed out 

[Stuart]

in that case you could suck a bit of electrolyte up and squirt it downwards into the bottom of the cell?

[Eleanor]

I decided to think on it a bit longer so haven't done anything yet. That sounds a better plan. As the cells are under warranty I don't want to do anything that can harm them (apart from recently unwittingly overcharging 7 and undercharging 5  ). I think we've got cells from different manufacturing batches as since noticing the strange charging I've found codes etched on the tops and guess what? 7 have one code and 5 have a completely different code 

Anyway, probably best not to discuss it anymore while we work it out with the supplier who has been very helpful

[Eleanor]

TSOM remembered that we've got one of these and I've found it in the shed and it works! So, hopefully tomorrow after careful adaptation there will be some gentle bubbling of electrolyte for a few hours  

[billi]

Hope all will be fine Eleanor

I  would be interested in more information about that aeration idea , and how it would benefit an off grid idea

When to apply air , for how long , etc ?  And how would a charge controller know  that their is a benefit ( or is voltage rising faster ?)

All the parts are  available on-line (only one pump i found through ), but sure  one can improvise


Found 2 photos 

[knighty]

careful with the pump Eleanor, most of those tyre inflater pumps are only made to run for a few min.... after that they start to overheat !

you could probably take the cover off and add a fan to help it out a bit ?