How to determine chemistry?

[Rednecktek]

Sooo I've made a few LFP batteries, but now I'm looking at a project with cells stripped from old solar generators. I see a regular 5-wire BMS leads coming out which tells me 4s BMS and I've ordered some of those, but the whole pack is wrapped up in the blue heat shrink and I'd really, REALLY like to not have to cut the pack open.

What I think I always see is that LFP batteries are 4s, 8s, 16s, and batteries using Li-Ion 18650;s is 7s, 9s, 23 & 1/3s and so on with odd numbers of cells. Is that really the case or are there 4s 18650 rigs out there? My understanding is that LFP is 3.55v fully charged but 18650's are 3.7? 3.9? 4.1?

I guess I need to know so when it comes time to program the battery I can get the settings right. So, is there some sort of dead giveaway or something to make sure what I think is a 4s LFP pack really is a 4s LFP pack without cutting the battery apart?

Thanks y'all!

 

[Hedges]

You just have to Wick da Wed Wire, and see what the voltage is.

I think knee of curve would be a dead giveaway. There is an upper and lower knee, probably unique to the chemistry but I haven't confirmed.

 

[Rednecktek]

You just have to Wick da Wed Wire, and see what the voltage is.

I think knee of curve would be a dead giveaway. There is an upper and lower knee, probably unique to the chemistry but I haven't confirmed.

Ok, now how do I determine if it's hitting the knee or just undercharged because of settings without a graphing calculator and a Phd in Solarology?

 

[Hedges]

Need to log data.
Starting with a full charge, load with a current source (ideally) or a resistor.
Log voltage over time. With resistor load you can calculate current.
Use Excel or other program to calculate Amp-hours (SoC).
Graph voltage vs. SoC. There should be two inflection points, the upper and lower knee.

You will then have an Internet PhD in Solarology.

 

[Rednecktek]

Starting with a full charge, load with a current source (ideally) or a resistor.

And there's the crux of my problem. Without cutting open the pack do I charge as if they're 3.2v LFP cells or 3.7v 18650's?

 

[Hedges]

Specs?
Does BMS function, and do you trust it to protect battery (from over-charge and over-discharge)?
Can you read settings from BMS?

 

[Rednecktek]

BMS is still on the way, but it's a 4s LFP Smart BMS so I don't know if I need to change the settings for Li-Ion or not. Will the BMS connect up and "Just Know" or will it default to something else? I've never done any chemistry besides LFP and I don't want to undercharge 1860 cells or pop LFP cells by having the wrong voltages.

The JBD 40a 4s Smart BMS is what I have coming.

 

[Hedges]

New BMS will have no idea what the cells are.
If your cannibalized solar generators still have their BMS, it's settings would match the cells and it should protect during charge and discharge.

I don't know if there is a way to electrically measure a cell or pack and determine what you have without possibly damaging it. I would think the two knees could be found, but to do so safely you'd need to basically implement a BMS that detects the inflection point of the curve then stops charge/discharge. Me being an electrical guy, this might be my approach.

In a similar vein, I could test breakers and determine their ratings. At first glance it would seem fuse testing must be destructive, but because they go into thermal runaway, I think they could be pulse tested and their resistance change noted without damage.

If you had spare cells of same batch, I suppose they could be destructively tested, electrically and/or chemically.

 

[45North]

Google the ID # on the side of each cell to get it's manufacturer and description etc..

 

[Rednecktek]

So without having the guts of the old unit, it's sounding like I have no option but to cut open the packs.

 

[Hedges]

I think discharging with a resistor would reveal the lower knee. And charging would reveal the the upper knee. But if you don't detect that and stop in time, would discharge or charge too far, then the cells are damaged. (If that happens, chemistry may be identified by the color of the flame.) Automated monitoring would be required, looking for when rate of voltage change increases.

 

[Yurtdweller]

Do you not know the make and model from which you removed the cells?

 

[Ellcon123]

This shouldn't be to hard to determine as the curve is quite different between 3.2V & 3.7V cells. The Li-ion curve is really stretched out vertically compared with LFP. If you can't datalog then take regular voltage measurements from 3V up to a max of 3.65V. LFP will have gone into a knee by 3.65V.

 

[RCinFLA]

Blue lines are the cell terminal voltages for LFP, NMC, NCA, LTO cells.

Battery terminal voltage is cathode potential minus the anode potential.

 

[RCinFLA]

I would love to see a LFP cathode with LTO anode. It would have low battery terminal voltage of 3.5v LFP cathode potential minus the 1.5v LTO anode potential, netting 2.0v battery, but it would have a nearly flat discharge voltage curve and the cell longevity would last almost forever.

LTO does not suffer the graphite expansion/contraction degradation on SoC/cycling.

LFP cathode is the most rugged, having the iron to provide vertical lattice support when most of the lithium leaves the cathode at full SoC.

But alas, like making a medicine that actually cures a disease, making a battery last forever makes terrible business sense for recurring revenue.

 

[Doggydogworld]

I would love to see a LFP cathode with LTO anode. It would have low battery terminal voltage of 3.5v LFP cathode potential minus the 1.5v LTO anode potential, netting 2.0v battery, but it would have a nearly flat discharge voltage curve and the cell longevity would last almost forever.

I've heard LTO is really expensive. Is that just because it's only made in low volume or is the chemistry just really hard to manufacture?

It seems existing LTO cells will usually hit calendar aging issues long before the 10-20k cycle life limit. That's already 30-60 years at 1x per day.

Any ideas how LFP/LTO max power would compare to existing cells? Any thoughts about Toshiba's new NTO cells?

 

[RCinFLA]

Yes, high LTO price is primarily due to low demand, which is driven by their low terminal voltage and therefore low power density. Material is also more expensive than graphite which is very low cost.

One other comment, a lot of people pick up on full charging damaging lithium-ion batteries with cell phones and EV's recommending not fully charging batteries. This applies to nickel based lithium ion cathode chemistries (NMC, NCA) and not LFP cathodes. LFP takes full charging well. You may have heard Elon Musk talking about LFP for EV's making up much of their lower energy density by allowing full vehicle charging.

On NCA, NMC lithium-ion cathodes, it is the cathode that determines longevity of battery, to several hundred full cycles. By comparison, LFP cell's graphite anode determines longevity of battery which is several thousand full cycles.

High power is primarily cell material conductivity. LFP was recognized as a long-lasting cathode but initially given up on due to its poor electrical conductivity. Later on, the addition of very fine carbon black powder made LFP a viable cathode. Graphene is much better than carbon black powder but very expensive. I think 304 AH cells have a small bit of graphene mixed in.

If you are interested in finding out more about graphene, this video is interesting. Graphene is an amazing material. A lot of smart people are trying to develop a cost-effective manufacturing process to mass produce graphene. If it can be accomplished, it would be as revolutionary as the invention of the transistor.

I should mention the video targets strength of graphene, but electrical conductivity is also much better for graphene, almost a superconductor. Another magical property of graphene is heat conductivity.

 

[Rednecktek]

Fortunately I got the make & model of the unit they came out of and it says LFP so all is well!