New, I think, variation on the "mixing Lishen & Eve cells" question

[David S]

Question: is it acceptable to create a battery made up of (2) 3p supercells of Eve 280ah LiFePO4 cells and (6) 3p supercells of 272ah Lishen LiFePO4 cells. See attached configuration image.

Details: I currently have an 8s configuration of Eve 280ah cells managed by an Electrodacus SBMS0. I'd like to expand this to a 3p8s configuration. I understand that, due to intentional choices in the way the SBMS0 works, it cannot communicate with another SBMS0 in any meaningful way. I also understand that it is expected that the battery managed by an SBMS0 present as a 3s-8s config. Finally, I intend to implement this in the method defined for the Electrodacus, with the SBMS0 in control of all charging/loads.

Currently, it is not possible to easily obtain 280ah Eve LiFePO4 cells due to recent changes in the marketplace. Lishen 272ah LiFePO4 cells are now common and relatively inexpensive. Their dimensions are similar enough that intermingling them should be approachable from a physical perspective. Two of the current Eve 280ah LiFePO4 cells are lower performing (but in spec). In the proposed configuration, those 2 cells would be scrapped, creating (2) 3p Eve supercells per the illustration.

Thanks!

 

[snoobler]

I would intersperse the Eve cells into Lishen groups, i.e., have 6 "blocks" consisting of 2X Lishen cells and 1 Eve cell.

This make the average capacity of each cell group closer and more uniform. Rather than having 2 cell groups with nearly 10% more capacity, you have 6 cell groups with 3% more capacity.

This assumes that there are no issues with terminal heights between the cells.

 

[David S]

I would intersperse the Eve cells into Lishen groups, i.e., have 6 "blocks" consisting of 2X Lishen cells and 1 Eve cell.

This make the average capacity of each cell group closer and more uniform. Rather than having 2 cell groups with nearly 10% more capacity, you have 6 cell groups with 3% more capacity.

This assumes that there are no issues with terminal heights between the cells.

@snoobler thanks for responding. If that's a workable scenario, it seems that scrapping the 2 "extra cells" might not be necessary? In that case, you'd create 8x supercells, each with 2x Lishen and 1x Eve. Interesting... People often seem to have a very, very negative reaction to mixing cells. Could be a fun experiment, assuming the cell heights are manageable.

 

[snoobler]

There's a military spec for Lithium batteries for use in military communication equipment. It requires cell capacity variation to be within 5%. What does that mean? Nothing directly applicable, but I expect field military equipment battery reliability is pretty critical, so if it's good enough for that application, it's good enough for me.

The IR difference between those cells is absurdly low, typically 0.12 - 0.15mΩ. That's so low, it's almost absurd. Even if the lower IR cells raise to meet the higher IR as many claim... still damn low... and 3P means 0.05mΩ worst case.

My only concern is any physical constraint that make connections/clamping impractical.

Alternatively 3X batteries: 1X 8S Eve and 2X 8S Lishen each with their own BMS and fixed in parallel as 3 separate batteries means higher current capability from the BMSs and redundancy.

If you have a cell fail, it will take out any paralleled cells with it if you don't catch it right away. If you only have 8S cells, the bad cell is obvious, and you can take the bad battery out of the group and continue operation on two.

 

[David S]

There's a military spec for Lithium batteries for use in military communication equipment. It requires cell capacity variation to be within 5%. What does that mean? Nothing directly applicable, but I expect field military equipment battery reliability is pretty critical, so if it's good enough for that application, it's good enough for me.

The IR difference between those cells is absurdly low, typically 0.12 - 0.15mΩ. That's so low, it's almost absurd. Even if the lower IR cells raise to meet the higher IR as many claim... still damn low... and 3P means 0.05mΩ worst case.

My only concern is any physical constraint that make connections/clamping impractical.

Alternatively 3X batteries: 1X 8S Eve and 2X 8S Lishen each with their own BMS and fixed in parallel as 3 separate batteries means higher current capability from the BMSs and redundancy.

If you have a cell fail, it will take out any paralleled cells with it if you don't catch it right away. If you only have 8S cells, the bad cell is obvious, and you can take the bad battery out of the group and continue operation on two.

Totally agree and thanks for the clarification. The SBMS0 removes the current capability issue with the BMS as it's not in the circuit. That said, I completely understand the risk of a bad cell taking down a "supercell". I'll do some more research on the size differences. Worst case scenario, creating a 2x3 Eve cell cluster means only two custom bus bars (bent to adjust height).

 

[DLTooley]

Mixing batteries of different ages and types in a bank is a consistent issue across chemistries.
One of the issues with buying grade B cells is variation between cells.
Those variations and the variations in age of the cells may be more of an impact than the difference in brand as actual performance of Eve and Lishen cells is quite similar.
That 5% number sounds right.
In general s/p configurations will be superior to p/s configurations in that regard, all the more so with greater variation.
Lifepo4 ages much slower than lead acid also.
On the market side, I just ordered Eve cells a week ago. They have not yet shipped though.
I spoke with Luyuang and Amy’s replacement at Xuba prior to purchasing new Eve cells at a higher price rather than the new manufacturer Rept’s product on in the end was a shipping detail. I trust both of these sellers on managing quality.
Neither has Lishen sells for sale, at least as of two weeks ago.
I suspect that what is going on is that Lishen is a relatively new manufacturer that has now an established quality record and can get a higher price for its grade A cells. There real grade B cell market has not matured enough to the satisfaction of these known good resellers, theoretically.

 

[cinergi]

Personally I would parallel same cells together like you have. With the Lishen's testing at a higher capacity than the EVE's, you won't be dragging down your pack capacity (from 280 to 272) by introducing them.
Note also the physical differences are significant enough that getting a rigid busbar between an EVE and Lishen isn't going to properly happen (the terminals are at different heights) unless you raise the EVE cells.

 

[Hendo7]

what's wrong with having two separate battery banks (one lishen and one eve )with both having its own bms connected to the same buss bars ?

 

[David S]

what's wrong with having two separate battery banks (one lishen and one eve )with both having its own bms connected to the same buss bars ?

I’m using an electrodacus sbms0 bms and am having it control all loads and charging. One of the limitation/choices about sbms0 is that it can’t communicate with other sbms0’s and setting up discrete charging & load paths is complex.