What's wrong with this lipo?
 

It keeps breaking at this solder joint. I've soldered it before just for it to break again.

Note, I use this battery mostly in series with an identical lipo for 6s power.

Weird. You could try soldering in a piece of copper rod/ thick wire/ soldering braid to reinforce it.

If it were a much thinner solder trace then I's say it was over-current ( is a built-in feature on some lipo packs, designed to act as a fuse of sorts.. ), but a trace that fat... weird....

That fat trace is to lower the resistance a bit between the two connection point. I went through the same issue a few times and fixed it by adding braid wires.

Thank you guys. I never would have thought of soldering on some wires. I'll give that a shot.