This just a coin 3V battery (and small resistor) with 4mm sockets for test leads. Sometime when you solder SMD LEDs some might not have any marking for cathode (usually it's a some tiny green dots around cathode pin, or triangle under or "T" on side that show diode direction), so instead trying to check diode polarity with coin battery and 2 pieces of wire, use thin (sharp) test leads from your meter.

You need a:

• CR2032(2020/2016) battery holder for PCB where you can slide battery from side (the one with spring contact on top - check the photos),
• 2x (red and black or other colors) 4mm sockets for insulated test lead plugs (thread diameter 12mm, 14mm dia at front, about 31mm in length from front to end of solder tab) - got those for 1.5$/10pcs from ali,
• 20-47 ohm small resistor (1/4W or 1/8W).
• 3 screws, either M3x10(12) or for 2.9x9-10.

You'll need to cut the sides of the battery holder (check photo) so you can "push or slide out" the battery from socket through the slot in chassis without problems. For this reason is the slot on top cover for small screwdriver to push out the battery :smile:
Then cut those small plastic protrusions under the battery socket too

Solder resistor to one of battery holder terminal ( + ), and other side of resistor to the 4mm socket. Other 4mm socket connect with wire to other battery socket terminal. (I know this is pointless to write, but I hope no one will ask for schematic :sweat_smile: :facepalm: )

Then use some hot-glue to hold down the battery socket.