I've repaired balance leads that have broken off inside the case. Pre-tin the balance leads, you don't need the iron very hot maybe 300-350c. Hotter just seems to melt the insulation. Put flux on the tabs and solder away. The lipos I've worked with seem to use high temperature solder on the tabs, so didn't have to worry about tacking a balance lead onto the tab or main wires and having the mains come desoldered.