Modular 12V 3S Mini UPS

This is a simple box I created for a mini UPS used to power up my Wifi router and CCTV camera during power cuts.

The box is designed to be modular. The bottom part which houses the 18650 cells can be stacked on top of each other to add more parallel cells increasing capacity as you require.

The 18650 holder I used was a modification of flexible battery holder to fit 18650 cells. Requiring only a 4mm nickel strip to be inserted for both anode and cathode with a wire soldered to the strip. 

You'll need the following:
1. 3x 18650 batteries. (Add more in parallel to increase capacity)

2. 1x HW-544 3S 60A BMS. 

(Link: https://www.aliexpress.com/item/1005006063247992.html )

3. 1x XL4015 DC-DC CC/CV Lithium Battery Step-Down Charging Board Module.

(Link: https://www.aliexpress.com/item/1005005644043024.html )

4. 1x Battery Level Indicator Module 1S-8S.

(Link: https://www.aliexpress.com/item/1005005161413704.html )

5. 1x Rocker Switch KCD1 - 104 4 Pin SPST.
(Link: https://www.aliexpress.com/i/32612054550.html )

6. 1x DC Power Jack Socket Female Panel Mount 5.5×2.1mm.

(Link: https://www.aliexpress.com/item/33021521630.html )

7. 32x M2x8 screws.

8. 4mm nickel strip for the battery holder to be inserted at the cathode and anode for each cell. (I cut pieces strip of an old coffee can)

9. 5.5*2.1 mm Male Plug 12V Dc Power Pigtail Cable Jack.

(Link: https://www.aliexpress.com/item/1005005761340771.html )

10.  2x KF-128-2P 7.5MM 2-pin terminal block (Optional to be soldered to BMS Charge and Discharge making assembly/disassembly a lot easier in the future)

(Link: https://www.aliexpress.com/item/1005006300561330.html )

11. 4x Female Blade Crimp Terminal FDD 1-250. (Optional to be used for wires connected to SPST switch for easy assembly/disassembly)

Assembly instructions:

1. Cut around 15mm of nickel strip for each pole per battery.
2. Insert strip pieces across the 2 holes at each pole of the battery holder.
3. Solder wires to the nickel strip to connect batteries in series. Circuit diagram is shown in attached images.
4. Attach bottom part to the top part that houses the BMS and the XL4015 using 4x M2x8 screws.
5. Optional: Stack more bottom parts adding more cells to increase capacity.
6. Optional: Prepare BMS and battery level indicator by soldering the optional terminals.
7. Attach wires as indicated in the circuit diagram (Note that the switch is connected to the input of the XL4015)
8. Insert everything into position. Switch and battery indicator fit snuggly with no screws needed.

Printing instructions:

No supports needed. Printed nicely on my Anycubic Vyper using a 0.4mm nozzle / 0.2mm layer height. I used PETG but feel free to experiment with other materials.

You'd need to print 2 pieces of cover.stl one for each side.

I included the F3d file in case someone wanted to modify it for different parts that I used.