We have to solder a bodge wire to create a permanent connection between the negative side of the old battery's circuit board and the ribbon cable.

Solder a small enamelled copper wire to bridge the two center pads of the IC (red circles first picture)

Use a multimeter to check the connection between the negative side of the circuit board and the ribbon cable

Add some solder to the positive and negative tabs of the board to prepare them for soldering later

You can also add some solder mask to the bodge wire for extra protection