OK, so just to be sure we're on the same page: the pack was 20 Ah pouch cells in 14, 15, or 16S, 1P. Max pack current was 20 Amps, or 1C. Correct?whatever wrote:only 20amps on ebike in 48v pack,
OK, so the pack was discharged to around 32 Volts (assuming 16S). I can believe that. I have a hard time believing that this particular cell didn't see some time at a very low voltage, or maybe was even driven negative at one point. Several of your photos suggest that the copper anode had begun to dissolve into the electrolyte. Flaking of the powder layer on the anodes often accompanies this, as do soft pouches. The cell has to come to rest below 300 mV to do this. Driving the cell negative is even worse. What was cell performance like after it "went soft"? Do you have any photos of the cell before it was dissected?whatever wrote: lowest voltage was 2.4 v
that was voltage when pack was opened up
Cells can be soft for several reasons, not all of which represent a defect. The most common cause in my experience is overdischarge (which internal evidence also strongly suggests). Sometimes a cell will get a bit more electrolyte than normal, and this can make the cell less rigid, but it's not generally a problem.whatever wrote: I think the difference between hard cells and soft cells will be when the graphite layer on
the copper starts to go off, was surprised just how good the iron phosphate layers held up.
They were pretty much unblemished ( iron phosphate layer), the graphite had starting peeling off the copper in places.
The pouch is made of aluminum with a thin plastic layer on both the inside and outside of the pouch. Think about what you're saying here about Mylar--an insulator--having an "electrical connection" to anything. It makes no sense. The aluminum pouch is electrically isolated from the electrode stack. The defect you describe with liver spotting and electrolyte under the outer poly layer is usually the result of breached isolation between the anode and the aluminum pouch.whatever wrote: When you get some electrolyte starts to be visible from the outside of the pack ( grey bubbles along seams), it looks like that
is due to breakdown of the mylar outer skin, not due to contact with copper or aluminium plates. The inner surface of the mylar pouch appears to have electrical contact with the plastic membrane, first layer under the plastic separator membrane is copper.
There can be some capacitive "surface charging" of the pouch by the electrode stack, but it represents a tiny amount of power. Trying to salvage cells with tabs that have been cut down below the pouch edge is a recipe for disaster. Tweak the cell just right and you've got a dead short across the pouch itself--no fuse can save you now.whatever wrote:The aluminum
Innerside of mylar pouch has no iron phosphate coating, but possibly a very thin plastic coating ( not sure on that)so should play no role in battery capacity, but may hold some charge or at least a conductive path, so cells with cut down tabs and mylar removed to make tabs longer might short from mylar pouch to copper tab ( a bit of silicone seams to solve that issue)
The separator does not "hold" the electrolyte, BTW. The electrode stack with separator go into the pouches before the electrolyte is added. The separator is a plastic that has high enough dielectric strength to prevent the electrodes from shorting together, but is still porous enough to allow Li ions to pass through from anode to cathode and back. The lithium ions are suspended in the electrolyte.