Mixing Li-ion and LifePO4 26650 cells in series

[teal_mngri]

My question is, can i combine LifeP04 with Li-Ion cells in series (both are 26650 format)? and my secondary question is, how can I tell if a 26650 cell is a LifePO4 or a Li-Ion cell (with a multimeter).
I have a 48v ebike battery pack (13s3p) that isn't charging. I opened it up and checked all the cell groups and one group was at zero volts . My first surprise was that the cells were much bigger than the 18650s that I was expecting. It seems that they are 26650 size. I ordered some new 26650 cells, thinking i would just remove the bad cells and add in good ones. However it seems the new cells I ordered might be a different chemistry to the ones in the battery pack. My reasoning is based on the voltage readings and charging characteristics. The new cells I ordered seem to like being at about 3.7v and I can get them up to 4.2 very quickly (this seems very consistent with the curve for LifePO4). The existing cells in my battery pack are all at about 4.2v. I assumed they are Li-Ion based on the voltage, but I am not 100% sure. There is no marking on the cells. they are all a light teal (turquoise) colour and the have a very noticeable "safety caps" (not sure what they are called, but my understanding is that it's some kind of safety mechanism to cut off the voltage if the cell experiences pressure?. As a side note, I did try to revive the zero volt cells by using the end of a flat head screwdriver and trying to depress the diaphragm (Yes, I know this is VERY dangerous and I was not intending to re-use these cells, I was just curious to see if that's why they went to zero volts) but that didn't work (I followed a youtube 'tutorial' so I am sure I was doing it right, but anyway). So, to summarize, I am trying to replace the 3 bad cells in the ebike pack. They seem like 26650 size (based on my measurements of the cell dimensions), and the existing cells seem like lithium ion based on they all being at 4.2v, and the new cells I want to add into the pack (as a group of 3 parallel LiPO cells, connected to the rest of the pack in series) are the same dimensions (26650) but seem life LifePO4 (based on their voltage and charging characteristics). Also, the pack does have a BMS, I probably should have mentioned that.

My question is, can I add these LiFePO4 cells into the pack and what bad things could happen?

Many many thanks,
Dan

 

[john61ct]

No never do that.

Even between different 3.6-3.7V chemistries the mapping between SoC% and volts can vary greatly.

LFP is so far out do not even consider it.

Really even brand new same model cells all Grade A is not optimal if from different suppliers

should all be the same production run.

And ideally sorted so your batch are well matched in capacity.

But that's rare for suppliers or even the factory to do these days, especially little cells under 20Ah

 

[teal_mngri]

Ok, thank you for your quick reply. I have some follow up questions:

1) how will the different mappings of SOC% to Volts affect the battery are we talking a risk of fire and explosion, or a risk of ruining cells or just poor battery performance? What can actually go wrong by mixing LFP and LIION in series?
2) If, instead of putting LFP cells in place of the dead Li-Ion 26650 cells, I put 18650 Li-Ion cells, would that be ok? I could make sure that the 18650 Li-Ion group has at least as much mAH capacity as the 26650 groups so that the 18650 group isn't the weakest link in the chain.
3) what is the easiest way to fix a battery with one bad cell group? Could I change it to a 12s3p (instead of a 13s3p) and would I need to change the BMS? Is there any way to "trick" the BMS into ignoring the missing cell group? Also would I need a different charger because the max voltage is different now (12*4.2=50v vs 13.4.2=54.6v) so the old charger would be overcharging it (or would the BMS stop this from happening?

many thanks,
Daniel

 

[john61ct]

Just build a pack from quality new cells, all the same known-good model and ideally production timestamp, from a well trusted seller.

Messing around with a mixed batch of scrap cells is IMO just wasting your time.

What is the purpose of the pack?

What sort of voltage and amp rates, continuous vs burst?