So out of the 640cells that have been installed in the 5kWh bank 1, 4 packs were 36cell packs using the slightly lower cell group as at the time I didn't have the numbers of 2000mah cells.
I have done around 50 half cycles over the last 2 months on the powerwall with this pack installed. Charging to 4.1 and discharging to around 3.7v. Over the last 30 days i have been running the powerwall every night and its been going well up till this pack died.
Over the last few weeks and as it turns out to be very timely i have solder up my next set of parks and working through the pile. ( Lots of soldering).
Out of the 640 old cells losing around 10 cells isn't too bad, Given we are working with old cells it was bound to happen sooner than later.
Remember these cells have lasted around 50 cycles so it would have been very hard to test and find those problems before the cells make it into the pack in the first place.
I think now after I start to solder up the next lot that letting the cells rest for a month is very important too. I have found 2 cells that tested at 1800-1900 that have now after 2 months or so reading 0.6 and 1.6 ish . A big drop. So as I build these packs I am voltage testing each of the rested cells and maybe i will find more that now will never make it into the pack and will go straight to the bin
I am sure that another pack problem will happen at some point but by adding more 5kWh banks together it will mean i can take banks offline for maintenance which will be great.
Remember we are working with old cells and its just what happens, a pain but still very worth it. The key is to have a spare pack ready to go as there isn't anything else to do. More testing or double testing wouldn't of helped. Things will fail.