First time building a battery pack. Any glaring flaws in my design?

Just that usually NCM etc chemistries are usually 14S to be more compatible with nominally 48 V inverters. Inverters have traditionally been designed for 20S lead acid, for which 48.0 V is nearly dead flat. 3.7 V for most non-LFP non-LTO lithium chemistries is a middle of the range value, like 12.5 V for a nominally 12 V lead acid battery. So lead acid is roughly 12.5 x 4 = 50.0 V at middling SOC and not being charged. The problem is more often at the lower end, with the so-called 64 V models, which operate at up to 64.0 V on charge, and their output tends to start clipping just above 48.0 V. You would like to run your cells down to 3.6 V or lower, which is only 3.6 x 13 = 46.8 V. So 14S is generally a better fit than 13S, depending on the specific inverter model.

Is it possible to add another pair of cells? Or perhaps this is not for inverter /  UPS use?

14 hours ago, groob2 said:

First time building a battery pack. Any glaring flaws in my design?

I don't know how you physically placed your cells, but you have 14 positives and 12 negatives. That for one is a glaring flaw. Perhaps you have one cell that only has 2 positives? 🤪