Hey, when you charge a 2S battery of Tattu professional UAV battery with your balancing charger, does it charge one cell to 4.2V and the other cell to 4.15V? Is that a normal-sized voltage difference? The player always meet the problem that when charging their battery, by the time the battery is done being flown (discharged). The stock charger that came with the Blade 130X have the same problem which my Thunder charger does (Thunder T610 is the model). What’s the point of having a balancing charger if it doesn’t balance it all the way? Here I will give some suggestions to you.
If after using, one cell is regularly 2.8 and the other 3.6, then the battery is damaged and needs to be replaced. Cells should never be discharged below 3.0v, preferably not even close to 3.0v
It could be that one battery started with a bad cell but by connecting them all together on a parallel board you are pulling down the cells on the other batteries too, and have now damaged them also. If you are using a little plug in cell checker then their accuracy is unreliable.
Also, you should be sure that you’re charging your packs in balance mode. Hope my suggestion can help you.