1. Use vacuum pump or solder wick to remove the soldered debris to clear the circuit line from the board and from the battery itself. Should look clean.
2. Use solder coil (thinest coil roll) on the board and create fresh lines over the copper lines of the board.
3. Do it one line at a time.
4. While doing such put a metal instrument such as screwdriver against the hot lining so that you prevent the heat damaging the circuit. It's very important.
5. Once the circuit lines have been freshly lined up with the solder coil, seat the battery on the exact spots and use kepton tape to keep the flex cable seated on place properly. You can use another helping hand in this regard if you wish to.
6. Put the solder coil over the battery flex cable and start soldering one line at a time over solder coil. Take time. Solder first and wait a minute to let it cool. Repeat the process to the next being very careful. Aim to finish one line at 1 to 2 solder press. And press very gently. Hard press may glide the iron or release too much head to the circuit and cause melting.
7. I hope you will make it. Good luck.
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