DIY Flow battery ? possible ?

[justsomeguy]

I was looking at batteries for my "off-grid" shed, and wanting to keep costs down (it is only a toy after all) any reason why i can't "just"
Create a simple flow battery using a few litres of battery acid a few holes (and a spot of plastic welding)  and 2 storage containers.

Thought using gravity would be safer than pumping,

SO:

When charging can use rapid charge to 80% then (disconnect), open drain tap,  and allow charged acid to drain into store 1 (close drain tap),
Open fill tap and lift store 2 (using electric jack)  when battery is full repeat....

And of course when actually wishing to retrieve the power reverse it...

I assume I'm missing something simple why it wouldn't work.. and i've thought about this for all of a good 30 minutes....

[johnrae]

Think maybe you are ignoring the requirement to convert lead sulphate into lead oxide

[knighty]

I'd never heard of a flow battery until now.... so just googled it...

didn't someone post a link to a pumping a system a couple of days back ? (I can't seam to find the link now)
it was made to improve charge/discharge rates iirc...

looks like a good idea... but I think you would be much better off with a pump... so you don;t have to 'handle' the acid...

personally... I'd want it all mounted inside a big stainless steel tank... (the batteries, the pump, and the storage tank)

looks like a cheap way of increasing capacity....you could easily have 2 or 3 times as much acid as needed... increasing your battery capacity 2 or 3 fold ? (or more?)

another advantage (from googling) is that you can discharge/charge any of the batteries, so with a 48volt bank, you can charge/discharge half of it at 24v or even quarter at 12v... and the pump will even it all out

you never have to worry about equalisation charging.... or checking the battery levels... (just check your storage tank?)

this is just my thinking tho... from what I've googled... so might well be totally wrong !

[EccentricAnomaly]

Expanding on what johnrae said, isn't the capacity limited by the area of lead as well as by the volume of acid?  I'd have thought that a "normal" battery has about the right amount of acid for the surface area of lead available; after all, if adding more acid would increase the capacity then the manufacturers would do that as acid is cheaper than lead.

[knighty]

thats a good point..... so how do flow batteries work then ?

[myozone]

http://www.youtube.com/watch?v=GbLRKN3srUs

Dave

[pontiff]

I don't think the lead acid chemistry is suitable for flow battery design. I think most flow battery research is focusing on relatively inert electrodes ( platinum, graphite ) with a membrane separating the two electrolytes. The main issue/problem in their development  is producing a stable membrane which will last a reasonable length of time with little maintenance.

The idea is not new but I think there is huge potential in this technology, especially if they can scale it down for home use. VRB technologies in Australia ( though did lots in Canada) were convinced their Vanadium based flow battery was suitable but ran out of funding in 2008.

In my opinion a lot more investment is needed before we will see a suitable product but the idea is great. For example, imagine a flow battery powering a car, instead of recharging for hours, you pumped in/out the  electrolyte at your local electrolyte station and off you go again.  

[pontiff]

p.s. Here's a link to the vanadium flow battery if anybody wants to know the fascinating chemistry behind it. I love chemistry me! 

http://www.science.org.au/nova/newscientist/ns_diagrams/037ns_001image2.jpg

[justsomeguy]

My assumption was that the lead plate size in for example car batteries was more to do with surface area required to extract large amounts of amps at one time, I understand that there would be a max amount of acid in these cases as the plates are only so far apart etc
but maybe 2 - 3 times the acid would be ok.....

oh i would of been using a couple of servos for the taps and 2 electric jacks to move the acid around... the idea of strong acid under pressure is scary to me....at least I would be able to see what pressure is where with a gravity fed system. (2 stores on the jacks and the battery(s) at the 'mid height'

I'm sure there are better battery compounds for it as well...but bet they cost an arm and a leg...so not much use for my shed.

Dave,

[biff]

hi pontiff,
            this sounds an extremly good idea,but where could one find the necessary gear such as the vanadium and would the tanks have to be specially lined,? i would definatly give this a try,
                     biff

[pontiff]

I would think the tank would have to be plastic or plastic lined. I'm trying to build a demo model for my school kids which could use a simple salt bridge instead of a membrane. The vanadium compounds are pretty toxic, you would need a licence to buy them I suspect. The membrane really is the key bit here as it is the weakest part of the design and the most likely thing to degrade. I assume its composition is kept under wraps 

[noelsquibb]

I love chemistry me!

Hi Pontiff,

You could be just the chap Im looking for .....

http://www.navitron.org.uk/forum/index.php/topic,8946.60.html

We seem to be edging towards understanding why my heatstore has battery like qualities ( well this is a thread about batteries )  but Im not exactly sure how I can switch them off.

Any thoughts ?

noel

[Outtasight]

My assumption was that the lead plate size in for example car batteries was more to do with surface area required to extract large amounts of amps at one time, I understand that there would be a max amount of acid in these cases as the plates are only so far apart etc
but maybe 2 - 3 times the acid would be ok.....
Dave,

The Vanadium thing looks interesting but you can't do that with a lead-acid battery.  The quantity of acid in the cells is the right amount for the amount of lead paste in the grids (they aren't solid lead).  If you had more acid than needed to convert all the lead into lead sulphate then you'd have over discharged the cell and it will be destroyed in short order as two things will happen.  First, it will start to set as hard lead sulphate and won't convert back to lead and second, the paste will lose cohesiveness and fall out of the grids (shedding).  Both destroy the cell in a few cycles.

Gel cells and AGM cells are more robust because they are mechanically stronger (the silica gel and the pressed AGM sponge between the plates mean the paste can't fall out of the grids). Some are even "acid starved" designs (like mine). Being acid starved means there is less acid than lead and so they can't be over discharged so easily. The acid runs out before the lead does so they never get completely discharged and so can survive more "100% discharges".  In fact, the reason they survive is because they never actually get 100% discharged.  As the acid runs out, the lead stops reacting prematurely and the cell voltage collapses (triggering the load to stop, hopefully).  This saves the grid from physical disintegration and also means there isn't enough acid left to cause the grid to then hard sulphate so much if left standing in the discharged state.

For a flow cell to work, both types of fuel need to be liquid and the cell is just where the electron and ion exchange happens though an exchanger membrane that isn't changed by the chemical reactions in the fuel.  The lead grid in the normal battery is both the physical mechanism of exchange (electrode, permeable membrane) and half the fuel itself.  The problem has always been how to make the lead fuel into a physical shape that allowed it to work well as an ion exchange structure (permeable, sponge-like, large surface area) without it falling to bits or going non-conductive when it gets "used up" as fuel at the same time.