NexPower V3 hybrid battery unveil - Sodium-ion battery

It all depends on your expectations. From a mathematical point of view, this is definitely a non-linear graph. But if you look at other types of batteries, then this is quite a linear characteristic.

 

Certainly, in a graph like that, a person could say "this is almost linear", without sounding bonkers, if talking about the portion between Vexp and Vnom. That'd be, eyeballing it, the portion between about 1.28 and 1.22 V in the graph you attached to #99 (on the red trace).

The portion between Vexp and Vfull is a lot harder to call "almost linear" with a straight face, and to try to take the whole region Vfull to Vnom and call it "almost linear" would pretty much demand giving the word 'almost' hazardous-duty pay.

If I remember right, this little detour first arose from Fred_H's question in #94, which I think might be restated as a comparison of two ways to estimate the difference in usable capacity between two battery chemistries if the BMS keeps the working region between the same two voltage breakpoints.

It looks like Fred_H, following mudder, went with an early conversion from V to SoC for each chemistry, and then compared the resulting ΔSoCs.

I think that Fred_H, in #94, is asking why/how it improves the estimate any to use ΔV and a stated assumption that "the characteristic is linear", when the stated voltage ranges extend well up past Vexp and therefore the relationship may not fit the stated assumption very well.