Sorting for optimal safety and power in a powerwall is your question.
Depending on the actual Amp draw on the pack that can make a huge difference.
Wolf
YEAH a HUGE difference!
Experience is a big teacher and found out an idea I had is definitely not viable, at least not with cheap holders.
I made up a "battery bank" of 20 cells, (ones I won't be using in a powerwall coz they're below acceptable specs), and used the little black plastic chinese holders you see people on youtube using to make banks for charging. Decided to use this little bank and run my air pump from it as I had a trailer with a flat tyre and as I did I felt the batteries with the back of my fingers and it worked nicely I can say and no problems with heat in the cells. After about 3 minutes I noticed a little smoke coming from the top of one of the holders and promptly pulled the positive end of that cell from the holder. The cell itself wasn't hot but the holder had started slight meltdown at the spring end, seems this battery had more amps passing than it's neighbors, then another in a different "group" started doing the same.... I stopped the pump and checked everything, springs now 70% shorter on holders, 3 in fact. I did consider buying holders and making up packs this way instead of soldering or welding but now I'm glad this lesson happened this way. Lesson learned, and not with a major fire burning down a shed or something
Jehu Garcia's (on yootoob) cell holders must be immensely more capable of handling higher current, but he does use 7 batteries in each bank.
I ran 5 cells parallel joined to a thick wire at the back -then series- to the base of the next 5, etc etc for 4 "groups" of cells creating a bank that when low would be low a min 14.2V and high 16.8V at the ends.
I think the pump drew far too many amps from the 20 cells as configured and using the holders this way is a huge no-no, even thick wires don't help as the weak spot becomes the spring, heating faster as it is thinner metal and higher resistance than the connecting wire. I think these cheap holders are ok for charging use as not much current is passing to do that but as a holder for high current outwards - seriously DANGEROUS and shouldn't be considered period!
With safety the biggest concern unless I can get Jehu's holders cheap enough, spotwelding on nickel strip is now all I would consider. I think a localized high current situation could even cause solder to melt and disconnect (or worse, pool and short) the hot cell.
So I guess I've answered my own question about mixing cells, whereas amps is the biggest consideration, ie; not throwing in any say 1500mAh cells with 2400mAh together as failure will be imminent in that local area. I'll now consider I should match cells as close as possible in "rating" together as per what my tester said.
Anyone with knowledge of an Aussie supplier of holders as least as good as Jehu's, or a reasonably priced nickel strip would be appreciated.