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What is going on with this cell?
#1
I'm geeking out (EE here) while doing some precision testing on some used Headways and I have noticed one cell is losing about 2-4% capacity every cycle, and it's nominal voltage during discharge was lower. So I captured it's discharge curve at 2C and compared it with another cell also at 2C. Both cells originally tested the same capacity, but you can see how this bad one, in orange, is now lower capacity, and you see how it's curve is 0.1V lower than the other. What phenomenon can cause such results in a LiFEPO4? Electrolyte oxidation from over charge? Design defect? I'm assuming this cell isn't going to last many more cycles, and it has a high internal resistance (explaining the 0.1V lower voltage) but I do not have a way to directly measure that.

[Image: EeDy_OmXYAU0YIQ?format=png&name=large]
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#2
Definitely something going on, obviously not for use in your battery pack...
Some sort of contamination or maybe air leak (bad seals?)
Does it have any obvious physical damage?
Running off solar, DIY & electronics fan :-)
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#3
I encountered something I called destabilized lattice in some liitokala Li-ion 26650s.
Basically, they had full capacity, good sustained current, but every discharge they would lose a small % of the capacity. I got from 5200 to 4200mAh after just a few full discharges.

Probably such cells are cells that failed QC testing and got dumped and re-labeled.

In LiFEPO4 it's even more strange because they suppose to last significantly more cycles than Li-ion.

Can you test a large number of cells to see if this is an isolated case or something that is widely spread among your cells ?
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#4
(07-29-2020, 11:26 AM)Overmind Wrote: Can you test a large number of cells to see if this is an isolated case or something that is widely spread among your cells ?

I bought 48 from BH and I have 40 I'm testing. 4 show self discharge, wide range of capacities on the rest. But this is the only one with high resistance and diminishing returns per cycle.
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#5
I started with 18650 lithium-ion (many small cells) and am very far down that road but have 'grass is always greener' for those doing larger cells (fewer to process) for similar power.  However, this info illustrates that you can't just buy the exact number as some may not test out - same as 18650 but with larger consequences.   I'm sure its a bit discouraging to find suspect cells.  Just curious - did you buy a couple extra with this possibility in mind?    I wonder if there is a general consensus strategy when buying larger LifePo4 cells in 48v context such as 'buy 1 extra per 16'? 
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#6
I didn't have any plans for most of the batteries I buy mostly, I'm just buying them because I'm a geek engineer and I don't have experience with all these battery chemistry so I buy them to play with them to gain that experience. One reason Ibought these to experiment with supplementing the weak cells in my BYD packs with these to try an balance my BYD packs a bit better. But that is proving to have diminishing results anyway.

I recently became a boat owner so now some of these cells plus some of the LTO BMW batteries I bought are going into the boat. LTO will be the starter battery and these headways will be a hotel battery.

I mean fair rule of thumb for most projects is buy 10% extra. Where I'm lost is what the expectations should be when you buy from places like Battery Hookup. Tom is a great guy, and if you do ask about issues he takes care of it. I wish he would be a little less passionate in his item descriptions and focus more on what testing he does and will do, what capacities he guarantees, where these batteries came from and how they were used(if he knows it), etc. He has most recently gotten better at this on the newest batteries. When I bought these headways, all that was said is they were "Engineering samples testing near full capacity" so I have no idea what that means for expected age / life / quality / % bad cells. And if I ended up with 10% bad batteries from my order, should I even talk to them about it (I know he will refund / replace if i do) or is that the nature of buying used batteries? I want Tom to stay in business after all.

Anyway, sorry the tangent.
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#7
(07-29-2020, 07:01 PM)madsci1016 Wrote: I didn't have any plans for most of the batteries I buy mostly, I'm just buying them because I'm a geek engineer and I don't have experience with all these battery chemistry so I buy them to play with them to gain that experience. One reason Ibought these to experiment with supplementing the weak cells in my BYD packs with these to try an balance my BYD packs a bit better. But that is proving to have diminishing results anyway.

I recently became a boat owner so now some of these cells plus some of the LTO BMW batteries I bought are going into the boat. LTO will be the starter battery and these headways will be a hotel battery.

I mean fair rule of thumb for most projects is buy 10% extra. Where I'm lost is what the expectations should be when you buy from places like Battery Hookup. Tom is a great guy, and if you do ask about issues he takes care of it. I wish he would be a little less passionate in his item descriptions and focus more on what testing he does and will do, what capacities he guarantees, where these batteries came from and how they were used(if he knows it), etc. He has most recently gotten better at this on the newest batteries. When I bought these headways, all that was said is they were "Engineering samples testing near full capacity" so I have no idea what that means for expected age / life / quality / % bad cells. And if I ended up with 10% bad batteries from my order, should I even talk to them about it (I know he will refund / replace if i do) or is that the nature of buying used batteries? I want Tom to stay in business after all.

Anyway, sorry the tangent.
I'm with you.  I've purchased well over 6,000 cells from BatteryHookup - and agree that they have been responsive.  For example, I bought a batch of 2,000 modem cells but 10% of them were dented inside the case!! - nothing I did or BatteryHookup did or visible till the case was opened.   BatteryHookup made good on them.
However - I agree that you have to take off the rosy glasses and really think about what the descriptions 'don't say'  Big Grin

Another recent example is that I made 4 seperate purchases from BatteryClearing hosue for RING packs.  They come 100packs or 150packs in a box depending on when I ordered them.  Overall I had 11 seperate boxes with 11 seperate FedEx trackings.   One of them got lost in Kentucky by FedEx but FedEx says Battery Clearning House's problem to make a claim. I can't. Battery clearing house won't respond by email or phone so I'm stuck missing 10% of my overall orders.   On the other hand, the cells are pristine / new / only 2 bad out of 500 so far.... far exceeding my expectations from other DIY packs.

1.5 years ago Power2Spare had a super sale on raw packs - with 8 NCR18650As per pack.   They're description was pretty rosy and the good cells were indeed 90% or better.  However, 30% of the cells were no good / 0v / heaters - huge loss % wise and much worse than I expected. 

So it seems like you cannot reach 100% but relative to flat out false advertisting...   I would reorder from any of these 3 places and just try to be more critical when I read the 'glowing descriptions' Smile
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#8
I think I've gotten to the point where I'm happy with the tested capacities, but really bummed that I'm up to 8 cells so far (8 left to test) showing self discharge. IE, they lose .3V in about a day after charging up to 3.65V. So I may ask for a refund on those.

Any advice on how I could use these cells otherwise? Or if I'm out of line to expect a refund for cells that demonstrate self discharge?
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#9
^^^ Is the IR on those much higher than others?
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#10
>One of them got lost in Kentucky by FedEx but FedEx says Battery Clearning House's problem to make a claim. I can't. Battery clearing house won't respond by email or phone so I'm stuck missing 10% of my overall orders. 

Update - Battery Clearing House got me my missing cells.   @floydR provided a good email to Battery Clearing House and they responded and made good on the missing box of cells.    Just wanted to make a note since I complained about it above.   Thank you @floydR and Battery Clearing House. 
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