I cannot speak to the technical but I have a similar problem with mine.  I have a 4 battery TP set that is about 3 years old and I am starting to get weak cells.  What I have noticed is the balancing leads are developing breaks from the constant bending.  I don't know if battery life is measured by time or cycles.  I think mine are just getting old.

Ken

30 to 50 cycles on average.

I divided the end voltage by the start voltage and got 90%. That's 19V divided by 21V. I'm not Reddy Kilowatt.           

Some general information on battery life and why capacity slowly gets less.

https://3dinsider.com/lipo-lifespan/

If you start with a new 2800 ma battery and use it for many cycles, the actual capacity may only be about 2500 mah.  You then can only use 80% of 2500 instead of 80% of 2800 ma.  As nosted, storing or keeping at near full charge and over discharging may rapidly decrease the capacity or may directly damage the cells.

Some rules to live by:

No battery is going to last forever.  They all have a limited number of charge/discharge cycles.

Cheap batteries, just like cheap engines, aren't going to last as well as good quality ones.

Cheap batteries also tend to suffer from spec inflation -- basically they'll claim a higher capacity just by moving the goal posts as far as what's altogether charged or altogether discharged.

LiPo cells, specifically, don't like to be over charged, or discharged too deep, and they don't like to spend too much time at either end of the charge curve.  Which is why folks tend to obsess over the whole "storage charge" thing.

So, whatever you get, the more you pamper it the longer it'll last, and you may want to pamper the cheap cells a bit more.  The closer you get to the ThunderPower end of the spectrum, the longer it will last.

50 cycles for Turnigy cells is kind of on the low side for battery life, but it's not absolutely horrible.