In motorsport wiring they use crimped connections instead, due to it's mechanical strength compared to solder. It withstands the vibrations and movement better. You can get really small crimps so you should be able to find some that fit. If it's soldered to the PCB I haven't got much advice sorry. One option might be to mechanically support the wires near the solder, maybe hot glue the battery or wires in place so they can't pull on the solder.

Solder will wick up the wire and create a 'hard spot' where it breaks upon the wire flexing. Try using a blob of hot melt/sugru(sp?) around the wire in this area, to create strain relief.

Another tip is to use plumbers flux and an old soldering bit to 'tin' the battery first, the solder will fairly leap onto it with such flux. :) Don't forget to clean the flux residue off as it's quite corrosive.