How to Make a 12V Portable Power Supply

Pre-Assembly: Capacity Sorting and Grouping

Before assembling the lithium batteries, capacity sorting and grouping essential steps. This process involves measuring each battery’s voltage, internal resistance, capacity, and discharge curve. For most DIY enthusiasts, simple voltage detection may be the only feasible option. If any battery shows abnormal voltage during this process, it should be replaced immediately. I recommend having the seller perform capacity sorting and grouping before purchase to save a lot of time.

Battery Assembly

When assembling the lithium batteries, use holders for fixation. Here, we use 18650 single-cell holders, which can be arranged as needed. The advantage of using holders is evident: they simplify installation and offer better heat dissipation, shock resistance, and insulation compared to not using holders.

We are assembling a 12V battery in a 3S4P configuration, so we need to connect two 3×4 holders. Note that 12V ternary lithium batteries use a 3S configuration, while 12V lithium iron phosphate batteries use a 4S configuration. Each cell has a capacity of 2.5Ah, so 4 in parallel provide 10Ah. 

After assembling the holders, place all the cells in the holders and arrange them in a 3S4P configuration. Ensure that the polarities of the four parallel cells are the same and the polarities of the three series cells are opposite. 

After placement, use a spot welder to connect the batteries. Use nickel strips and a spot welder to first connect the positive and negative terminals of the four parallel cells, leaving some length on the nickel strips for later connection to the protection board. After connecting in parallel, use nickel strips to connect them in series. Once one side is welded, weld the other side in the opposite direction to prevent short circuits. With this, the battery pack welding is complete.

Installing the Protection Board

Next, install the protection board. This is a 3S-specific protection board for ternary lithium batteries. If using lithium-iron-phosphate batteries, choose the corresponding protection board. The maximum output current of this protection board is 20A. When selecting the protection board current, pay attention. For example, the battery in this video has a total capacity of 10Ah, so 20A means a discharge rate of 2C. 

If the cells have a discharge rate of 1C, the maximum output current for a 10Ah capacity is only 10A, so a 20A protection board would be unsuitable. Choose one under 10A. If the cells have a maximum discharge rate of 3C, the 10Ah capacity can provide 30A, and a 12V battery with 30A current equals 360W. If the output load is 300W, a 20A protection board is unsuitable as it won’t drive the load.

Protection boards come in shared and separate port versions, with or without balancing functions, and active or passive balancing. These factors should be understood before selecting a protection board. During installation, choose an appropriate position and place the battery’s total positive and negative terminals close to the protection board. Fix the protection board with double-sided tape, preferably placing an epoxy board underneath for enhanced insulation. 

Once fixed, connect the collection cables. The seller usually provides a wiring diagram; follow it for connections. In this video, a shared port protection board is used, so the charge and discharge interface is the same wire. For general DIY purposes, there’s no need to buy a separate port protection board as it’s more complicated to connect.

The protection board monitors the voltage of each cell series. If the voltage is too low or too high, the protection board will cut off the circuit to prevent overcharging or over-discharging, protecting the cells and preventing accidents. Some protection boards also have a temperature control system. 

After welding, measure the charge and discharge interface of the protection board. If there’s around 12V, the welding is correct. Use high-temperature-resistant silicone wires for connecting the protection board. The regular wires used in the video are not recommended as they are prone to insulation damage, leading to short circuits.

Final Steps

Finally, cover the battery pack with heat shrink tubing. If possible, wrap the battery with an epoxy board for enhanced insulation and strength. After placing the heat shrink tubing, use a heat gun or hairdryer. The tubing will shrink when heated, tightly wrapping the lithium battery. Seal the gaps with 704 silicone for improved waterproofing. 

Connect appropriate plugs to the battery’s output for easy connection to loads or chargers. Now, the 12V outdoor battery is complete. Test it by connecting a 12V fan; it should run normally. This 12V battery is widely applicable outdoors, such as when connecting to an inverter or other 12V loads.

Safety Reminder

Assembling lithium batteries can be dangerous. If you plan to DIY lithium batteries, ensure you understand the related knowledge to avoid accidents during assembly or use.