Mrs. Nerdly Endeavours had a LED bicycle tail lamp that had stopped working, even with fresh batteries. It looked very simple so I opened it up to check it out.
The LEDs are mounted on a circuit board that had contacts on the back for the on/off button and the spring clips to provide power from the batteries.
I didn’t take a “before” picture, but the contacts for the battery clips were kind of corroded. I glooped some solder on them to see if that would help.
The arrow shows the new solder. It didn’t stick to the entire contact area on the PCB, but it worked well enough to get the light shining again.