UPS for WIFI using 3 cells

 Circuit Diagram:

DIY 12V Mini UPS: 3S Li-ion Pack with 20A BMS and Buck Regulator

Welcome back to the workbench! Today, we are building a high-efficiency, pocket-sized DIY Uninterruptible Power Supply (UPS). Perfect for keeping your routers, microcontrollers, or network switches alive during sudden blackouts, this project utilizes a 3S 18650 Lithium-Ion Battery Pack, a 20A Battery Management System (BMS), and an adjustable DC-DC Buck Converter to output stable, ripple-free voltage on demand.

System Architecture & Power Flow

The system is designed with safety and high current delivery in mind. Power distribution routes directly through the protection stages before hitting the step-down regulator:

[3x 18650 Li-ion Cells (Series)] │ (11.1V - 12.6V Raw) ▼ [3S 20A Protection BMS] ──► [Safety Cutoffs (Overcharge/Short)] │ ├──► [DC Jack Input / Charging Rail] ▼ [Buck Converter Stage] (Step-down to 5V, 9V, or Stable 12V) │ ▼ [Target Load Output / DC Output Jack]

Complete Hardware Wiring Guide

To safely manage lithium chemistry, ensure your connection paths match this exact physical terminal allocation:

3S BMS Battery Pack Balancing Array

Battery Pack Physical Terminal	BMS Target Board Pad	Nominal Voltage Level	Wire Gauge & Build Type
Cell 1 Negative Ground (-)	0V Pad	0.0V Ground Reference	20 AWG Black (Main Return)
Cell 1 (+) / Cell 2 (-) Intersection	4.2V Pad	3.7V - 4.2V Balance Node	22 AWG Balanced Hookup Wire
Cell 2 (+) / Cell 3 (-) Intersection	8.4V Pad	7.4V - 8.4V Balance Node	22 AWG Balanced Hookup Wire
Cell 3 Positive Supply (+)	12.6V Pad	11.1V - 12.6V Total VCC	20 AWG Red (Main Power Rail)

System Load, Buck Converter, & Control Loop Routing

Source Core Component Pin	Target Component Pin	Functional System Operation
BMS Output Positive (P+)	Buck Converter Input (IN+)	Delivers Protected Battery Voltage to Regulator
BMS Output Negative (P-)	Buck Converter Input (IN-)	Completes Main Protected Ground Path
Push-Button Switch (Input Terminal)	Main DC Barrel Jacks Line	Provides Manual System Isolation / Master ON-OFF
Buck Converter Output (OUT+)	Status Indicator LED (Through Resistor)	Provides Visual Feedback on Active Power Output
Buck Converter Output (OUT-)	Status Indicator LED Ground Terminal	Tied to Common Output Regulation Rail Ground

⚠️ CRITICAL BATTERY SAFETY WARNING: Always verify that all three 18650 cells are at the exact same voltage level (within 0.05V) before connecting them to the BMS balancing pads. If you mix cells with wildly different charges, massive currents will flow between them instantly, risking permanent cell damage or fire.

System Deployment & Bench Commissioning

1. Tuning the Buck Converter Output

Before connecting your expensive router or microcontroller to the output jack, use a digital multimeter to measure the voltage across the Buck Converter's OUT+ and OUT- pads. Rotate the brass adjustment trimmer screw on top of the converter until your multimeter reads exactly your target voltage (e.g., 5.0V or 9.0V).

2. Understanding the Flyback Diode Protection

The anti-parallel protection diode included on the output line safeguards your sensitive electronics from reverse voltage spikes. If you are powering an inductive load (like a 12V fan or small DC pump), this diode clamps the voltage spike safely away from your buck regulator.

3. Charging Your DIY UPS

To safely juice up your pack, supply a constant-current/constant-voltage (CC/CV) power source to the P+ and P- pads via your input barrel jack. The charging voltage must match the maximum cell configuration voltage, which is exactly 12.6V. The integrated BMS will automatically stop the charge once the individual cell banks hit their upper limit.

Your robust, clean-running mini UPS is now fully operational! Put it inside a clean, 3D-printed enclosure, hook up your backup gear, and never lose your internet connection to a power drop again.