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12 V battery from three 18650s: 3S BMS, USB-C laptop-charger charging, capacity meter, 12 V and USB outputs

Bench build · Module builds · Intermediate · about 45 minutes

A rechargeable 12 V pack with a fuel gauge, charged from any 65 W USB-C charger. Three 18650 cells in series make 10.8–12.6 V. A 3S BMS board protects them (over-charge, over-discharge, short circuit). To charge, a USB-C PD trigger board asks a laptop charger for 15 V and a CC/CV buck module (XL4015) turns that into 12.6 V at 1 A, exactly what a 3S pack needs. A panel meter set to 3S Li-ion (L3) shows how full it is, a 12 V socket runs a router or LED strip, and a USB step-down gives 5 V for a phone. Set the buck to 12.6 V and 1 A with nothing connected before you wire the pack.

12 V battery from three 18650s: 3S BMS, USB-C laptop-charger charging, capacity meter, 12 V and USB outputs: the finished build running on the simulator's bench

Build it in the labGuided build, test sheet and fault finding, free in your browser.

zencircuitlab.com/l/12v-battery-three-18650s

Parts list

Test procedure

Each test has an expected value and a pass range, like a factory test sheet:

  1. Charger plugged in: charge current
  2. On battery: 12 V output
  3. On battery: USB output
  4. Full pack stops at 12.6 V

How it works

What it does: A rechargeable 12 V pack with a fuel gauge, charged from any 65 W USB-C charger.

Build it (guided or ready-made), then run its test procedure below. Every test has an expected value and a pass range, just like a factory test sheet.

Background

This is the inside of most “12 V portable battery” products: three cells, a BMS, and a charger board.

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