Single Cell AH limit

[Northernchateau]

100, 200 and 280 ah Lifepo4 batteries seem pretty common.
I am waiting on 8pcs of 310 ah batteries and now I see offerings for 420 ah cells.
My question is, if I don't care about size, is there a current limit of single cell because of chemistry or something else?

 

[toms]

The only limit is cost. Winston make a single cell @ 1000Ah.

 

[ShockinglyAverage]

The 272-280 seem to be the largest that i've seen thoroughly tested. I'd be interested to see any tests of the larger cells as I would definitely be interested in them if someone could prove they are legit as the 280 ones.

 

[HighTechLab]

A lot of the 420ah cells we are seeing are scams. One of the other threads even had an obviously terrible photoshop of a label in the picture.

Also, these socalled "420ah" cells, are the exact same size of a 270-310ah cell. How do you fit 25% more into the exact same space without drastic chemistry changes?

Have you ever seen a capacity test for a 420ah cell? Doubtful.

 

[Steve_S]

As HighTechLab mentions there are scams appearing on this. LFP & LYFP (Yttrium Doped) are specific chemistries, "no magic". As such the weights are all most or less uniform, for example 105AH cells weighs 2kg, and a 280AH is 5.6kg. Dimensions vary, some are thinner but taller & wider and so on, because there is no "set uniform standard size" because each company & their particular chemistry blend & casing/packaging varies.

I should also point out a Subtle but important delail which only now some people are figuring out for themselves.
Bulk Commodity cells (most commonly sold by Ali* vendors) tend to "just" make the grade, IE, a 280AH cell will test to capacities between 275AH-285AH real capacity, whereas the Grade-A Matched & Batched (properly) cells are logging in TESTED at 290AH-300AH which is as it should be to guarantee the 280AH capacity.

I'll also add the Pro battery assemblers such as BattleBorn, SimpliPhi, Relion and so on ALL use properly Matched & Batches cells, which exceed their actual Labelled Capacity Ratings which they then even further derate by 2-5% and set their internal BMS's to support that... This maximizes (ensures) that all assembled packs meet or exceed specs which is a big bonus for warranty, longevity & reliability.

 

[Stan Kopernik]

Although I can't comment on the limits of this battery chemistry, I think that max capacity may be limited for practical reasons. I would suspect that larger capacity cells will have to be compressed pretty hard to keep them from swelling. Even the shorter walls may need to be compressed when they get longer than 4 inches or so. It may become impractical to handle heavy cells, mount them in RV compartments and so on. Any thoughts on this?

 

[OffGridInTheCity]

To me - BMS/balance is a consideration. I have Batrium longmons on 260ah 'cells' and based on this, I can see where they'd go up to 500ah, 750ah range if you were patient. However, with Batrium you can do multiple longmons per cell - its the only BMS that supports this that I know The max for a single Watchmon 4 or newer Core is 250 longmons - https://support.batrium.com/article...l-monitors-can-be-installed-with-one-watchmon.

So, as an extreme example, one could do 250 longmons /16s LifePo4 48v battery... with 15longmons per cell. 15 * 500ah = 7500ah / cell would be a 'max'. This would be on the order of a 360kwh battery - and that be so much fun to build/own!!!

However, I think one would want multiple 'batteries' so you can work on one while the other allows the system to to run. If one went with 3 separate batteries of 16s5p of 500ah cells - then you could do 5 longmons per cell and have 2500ah per cell.

 

[Hasakei]

The only limit is cost. Winston make a single cell @ 1000Ah.

=10000ah http://en.thundersky-winston.com/?cnxdc/301.html

 

[RCinFLA]

Capacity of LFP cells is dependent on weight of graphite in anode and LiFePO4 in cathode at the optimum ratio. Together they account for about 80% of the weight of cell.

There a a couple of things that can squeeze a few percentage increase in AH's from cells.

1) thick electrodes gives more AH's of capacity but poorer peak current performance. Already known to be done on 'blue' cells.

2) pre-lithiating anode. This is relatively new process and not sure it is ready for primetime. At most it saves 5% of the normal lithium loss from cathode during initial charge forming of cell. It comes close to a legit 310 AH cell. With the recent rash of small amount of cell bloating showing up I have been wondering if this is being used now.

3) silicon doping of anode. Definitely not ready for primetime unless you are happy with few hundred cycles life.. It has added anode expansion during charging that fractures causing increased SEI regrowth that eats up cell capacity, defeating its purpose. Tesla LFP will likely have polymer wrapped silicon added to graphite anode to contain the silicon cracking but it is likely about 2 years out.

There are a few other additives and processes that can squeeze out a little more capacity but most have their own negative side effects. Nothing that will create a 420 AH cell with same size and weight of 280 AH cell with good cycle life.

 

[Stan Kopernik]

Capacity of LFP cells is dependent on weight of graphite in anode and LiFePO4 in cathode at the optimum ratio. Together they account for about 80% of the weight of cell.

There a a couple of things that can squeeze a few percentage increase in AH's from cells.

1) thick electrodes gives more AH's of capacity but poorer peak current performance. Already known to be done on 'blue' cells.

2) pre-lithiating anode. This is relatively new process and not sure it is ready for primetime. At most it saves 5% of the normal lithium loss from cathode during initial charge forming of cell. It comes close to a legit 310 AH cell. With the recent rash of small amount of cell bloating showing up I have been wondering if this is being used now.

3) silicon doping of anode. Definitely not ready for primetime unless you are happy with few hundred cycles life.. It has added anode expansion during charging that fractures causing increased SEI regrowth that eats up cell capacity, defeating its purpose. Tesla LFP will likely have polymer wrapped silicon added to graphite anode to contain the silicon cracking but it is likely about 2 years out.

There are a few other additives and processes that can squeeze out a little more capacity but most have their own negative side effects. Nothing that will create a 420 AH cell with same size and weight of 280 AH cell with good cycle life.

Interesting R&D but we may be reaching the point of diminishing returns. Cost of cell will be rising faster than the benefits gained from the newer technologies.

I am still wondering about the physical size limits of cells. I remember when breaking 100Ah was newsworthy. Aluminum case cells have been getting larger and larger. 280Ah cell is over 3 inches thick. How far can we go before the internal expansion forces become too much. Stronger cases will be needed which adds weight.

 

[Substrate]

In the case of prismatic cells, which merely house paralleled pouches, theoretically the only limitation is overall size to hold them all.

The real problem is even if you could parallel one-thousand - 50ah pouches together inside one case to make a prismatic cell, the odds are against you that out of those thousand, they would all be the same capacity and internal resistance themselves.

It can be done with high quality manufacturers. At a cost of course to pay for all that QC and precision.

 

[RCinFLA]

Size is limited only by internal self heating which is dependent on maximum discharge/charging rate. 280 AH 'blue' cells really should not be pushed beyond 0.5 CA discharge rate.

At 0.5 CA they have about 8-10 watts internal heating. At 1.0 CA they have about 30-35 watts of internal heating.