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.