Lithium Battery Power Supply, 12V Charge, 8.4V Output

This is a design for my use. It can be modified for your own. My use is for an Amazon remote switch LED light. The light uses six AA batteries, and I wanted longer lasting lithium power. I modified the light to accept a 3.5mm phone jack and the battery box for the 3.5mm plug. The plug and jack I already had and did not have to buy. For safety there is a STL for a cap to go over the 3.5mm plug.

Battery Box: The first piece is the power supply box itself. The box assembled is 40mm x 43mm x 80mm. To bolt the lid on takes (four) 2mm socket head cap screws. Internals are (two) 18650 lithium cells, lithium charger board from Amazon Amazon.com: ACEIRMC 2S 7.4V 8.4V 8A 18650 Charger PCB BMS Protection Board for Li-ion Lithium Battery Cell (5pcs) : Electronics DC connector 5.5mm x 2.1mm from Amazon Amazon.com: HiLetgo 10pcs DC Power Supply Jack Socket Female Panel Mount Connector 3-Pin 5.5 x 2.1mm : Electronics . The hole in the lid for output is 4.5mm. There is a blank STL also to modify.

Assembly: There are no instructions for the charge card. On the two batteries, positive pole of one goes to B+ on the board, the other battery negative pole goes to B-. On the other battery ends, strap them together and send a wire to the opposite end of the board, that provides balanced charging. The charging input and 8.4V output are the same pads on the board. Positive wires to positive and negative wires to negative. Don't make the mistake I did in the picture. Best to keep the solder away from the sides of the board so the board will go into the slot better. It takes a little finesse to get the DC connector in and the board and batteries slide in their slots. Make output lines to your desire but don't put end on until it is through the cap and make sure the wire ends don't touch, it is a live circuit. Set on lid and screw together, the screws will cut their own threads. I think I made the holes about 10 to 12mm deep to prevent stripping.

Meter Box: Since there is no way to tell when the batteries are charged, I made a tester. It is 40mm x 40mm x 18mm. It bolts together with one screw 3mm x 12mm or less countersunk flathead. The seven-segment display is from Amazon  6pcs Mini Digital DC Voltmeter 0.28 Inch 2 Wire 2.5V-30V Digital Voltmeter Voltage Tester LED Display DC Volt Meter (Yellow): Amazon.com: Tools & Home Improvement . Before installing, calibrate display. Hook to a known power source value and turn the screw on the display to match. Inset display whichever direction you want it to read. I hot glued it into place. The STL with the hole was for a 3.5mm jack. There is a blank STL for you to modify for your own. Display fit is tight. May need a little rework to make display fit. I use this box hooked up when charging to see when I reach 8.4V and can check voltage at any time to see discharge.

Holders: I made two holders for the case. Each has two screw mounting holes and a screw hole to secure the case. There is a recess in the holder that covers the case input jack for safety. The screw hole is 4.5mm. I have a STL for a cap that will press on a 5mm socket head cap screw. A 10-24 or 10-32 SAE can also work. The case can be mounted yet be able to be slid out for charging.