If the batty casing solder has been disconnected you will see that the pair of red and black wires near the battery pack are no longer connected.

You will need to solder back on these wires to the two silver soldering plates on the battery case. The black wire should be soldered on to the plate closer next to the square plastic piece attached.

Make sure to correctly solder each wire as shown, because if you solder the red wire where the black wire should be your device will not function.