Technology

Whoa, there is a LOT to unpack here so I will do my best. Please understand as I'm an engineer by training I will always advocate towards caution rather than YOLO so I'm going to spell out a lot of doom and gloom so read at your own peril. I'm not trying to poop on your hobby/business.

Inquiries; What kind of cells, 18650s? What brand cells do you buy and do you know their use history? Do you mix model numbers? I assume these are bare cells with no protection circuitry and you add the 5A fuse? What kind of fuse, a PPTC? And is that a typo, 64 in PARALLEL? 4S64P? That is bonkers, what the heck are you doing with them that requires that kind of amperage?

OK OK, so, do you use a BMS? How are you handling these packs other than the fuse? You NEED a BMS to at least balance the 4 series strings and monitor the packs temperature and shut it down if any part of the pack gets too hot. Lithium cells have a very narrow range where charging is allowed and a slightly wider range where discharge is allowed but at some point you need to turn it off. In terms of massively paralleling the batteries this is dangerous territory unless you are acutely aware of the cells, their limits, age and tight monitoring to notice when a handful of the cells are aging (getting higher ESR and forcing the others to handle the increased current which might exceed their safety limits). The end goal is to balance the current evenly between the cells in parallel and its not trivial, if you force all the current through a handful of cells you will ignite a lithium fire.

Regarding charging to 4V and discharging to 3V, most modern 18650 cells from good vendors charge up to 4.2V with a float charge so you're leaving charge behind but 3V seems way too low. I guess it depends on how long you want the pack to last. There is max and min voltages the cell vendor will recommend but then if you ask they can give you plots of the cycle life at different depths of discharge and typically in my world we don't go much lower than 3.2V to ensure the packs last well past 500 cycles. The deeper you discharge the more stress you put on the cathode.

Finally, loose wires are certainly a cause for fires and since you're hand making these its likely your biggest risk but in addition to that dendrite growth in the cells. If you abuse the battery either with temperature, over current/current imbalance you slowly introduce dendrites which keeps increasing the chance the cell will catastrophically fail with continued cycling. Its statistics and with the number of batteries you're playing with any little bit of abuse will grossly magnify the chance of a big failure to happen. Cells can and do spontaneously fail from internal damage but its a lot more likely to happen during charging.

In summary, you are playing with some serious power levels here, I would read up some more before you get much further. Especially before you step up to 26V! I hope this helps!

Cell quality is important. You want to be using known good quality cells like those from Panasonic, Samsung, LG, etc..

How you manage temperature, charge and discharge is also really important, dendrite growth can cause cell failure in time. Charge temperature is extremely important. So you want to make sure you're using a smart programmable BMS where you can set up all the protections properly. Ideally one with as many temperature probes as you can find, 4 is good, 8 is better and some will have that many.

Otherwise making sure nothing can short out internally is important too, but it sounds like you're putting some thought into that. Most critical IMO from what I've seen on pre-made battery packs is making sure your series banks are well insulated from each other and have no chance of vibration causing the cells to wear through their heatshrink and touch each other.