LFP with L-Ion Shim-cells

[venquessa]

"Shim-cells". Small cells used in parallel to match a cells capacity with peers.

I have a lower capacity cell. It's the first down and the first up and over voltage.

It actually seems quite difficult and quite expensive to buy small cylindrical LFP cells.

I do have a "clatter" of LiIon 18650s.

Why can't this work?

The range I operate my LFP in, per cell, 3.65V to about 3.00V, is voltage compatible. I might only get about 50% capacity on the LiIon cell at 3.65V, but I think I only need 2-5Ah of shim.

Current wise, a 3Ah cell in parallel with a 90Ah, will be "fine!".

Is there anything I am missing before I put an 18650 2.9-4.2V in parallel with an LFP, just to add 1 or 2 Ah of capacity?

Also... any suggestions on a UK source for "tiny" LFP cells?

 

[venquessa]

This is temporary.

Forward plan is to pair these 8 cells with another 8. First I will grade the capacity of these 8 in order. Then I will swap the new 8 cells and take a baseline capacity order for them. Simply pairing the lowest with the highest, should, I hope, prevent the need for shim cells longer term.

Or I could send twice as much on cells. I'm not convinced though.

 

[Ampster]

Is there anything I am missing before I put an 18650 2.9-4.2V in parallel with an LFP, just to add 1 or 2 Ah of capacity?

Yes you are missing the significant voltage difference between those chemistries. Depending on how many cells in series you may want to use fewer of the 18650 in series so the pack voltages are closer. Also it would be useful to have the ability to measure current from each pack so you can get a sense for the current between packs because of the different shapes of each of those discharge curves. Also before paralleling those packs try to get the voltages somewhat the same or you will have a big spark as the current will flow from the higher voltage to the lower voltage pack.
If you are just talking about one of each chemistry then the non LFP cell will always be at the bottom of its charge discharge curve when the LFP is at a resting voltage of 3.3 volts so you are not getting much additional capacity and taking the non LFP into the lower knee of its discharge curve. At at a full charge of 3.6 volts the resting voltage of the non LFP cell will keep the LFP cell at a higher voltage for a long time which will be detrimental to its already weakened state.

 

[venquessa]

At at a full charge of 3.6 volts the resting voltage of the non LFP cell will keep the LFP cell at a higher voltage for a long time which will be detrimental to its already weakened state.

It's a good point.

I suppose this would be the better option:

ENIX Energies, 18650 LiFePO4, 3.2V, 18650, Lithium Phosphate Rechargeable Battery, 1.8Ah | RS

uk.rs-online.com

 

[Supervstech]

"Shim-cells". Small cells used in parallel to match a cells capacity with peers.

I have a lower capacity cell. It's the first down and the first up and over voltage.

It actually seems quite difficult and quite expensive to buy small cylindrical LFP cells.

I do have a "clatter" of LiIon 18650s.

Why can't this work?

The range I operate my LFP in, per cell, 3.65V to about 3.00V, is voltage compatible. I might only get about 50% capacity on the LiIon cell at 3.65V, but I think I only need 2-5Ah of shim.

Current wise, a 3Ah cell in parallel with a 90Ah, will be "fine!".

Is there anything I am missing before I put an 18650 2.9-4.2V in parallel with an LFP, just to add 1 or 2 Ah of capacity?

Also... any suggestions on a UK source for "tiny" LFP cells?

A NMC cell, never reaching over 3.65V would never have much capacity… 3.7V is the nominal voltage…
And pushing LFP cells to 3.65 all the time is bad for them… so… not a good mix.