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Single 18650 charger with a 3-LED charge indicator (green 80–100 %, yellow 40–79 %, red below 40 %)

Bench build · Batteries and solar · Intermediate · about 45 minutes

Charges an 18650 from USB and shows its charge on three LEDs: green 80–100 %, yellow 40–79 %, red below 40 %. Charge one 18650 from USB and see how full it is at a glance. U1, a TP4056 module, charges the cell at 1 A to 4.2 V (CC/CV) and protects it (DW01: over-discharge, over-current, short circuit); its own red/blue LEDs still mean “charging / full”. The indicator runs from the module’s protected OUT, so the protection also guards the cell against the indicator’s small drain. U2, a TL431 wired as a 2.495 V reference (REF tied to K, R1 1.2 k gives it about 1 mA at the thresholds), is the yardstick. R2/R3/R4 (32.4 k / 4.75 k / 63.4 k, 1 % parts) divide the cell voltage into two taps: tap A reaches 2.495 V when the cell is at 3.96 V (80 % on a resting Li-ion cell), tap B when it is at 3.68 V (40 %). U3, an LM339, compares the taps with the reference. Its outputs are open-collector: each can only pull down, which is exactly what an LED needs. C1 sinks the green LED above 80 %. C2 sinks the red LED below 40 %. The yellow LED needs “above 40 % AND below 80 %”: C4 gives it a return path above 40 %, and C3 shorts its anode to ground above 80 %, so it only lights in between. Exactly one LED is lit. Read it at rest: while charging, the cell sits about 0.05–0.1 V above its resting voltage, so the indicator runs a little ahead; after charging it settles back within a few minutes. S1 switches the indicator off (it draws about 4–7 mA, which would empty a 3000 mAh cell in about three weeks; the LEDs run at 1–2 mA, plenty for modern LEDs indoors); a push button in its place gives “press to check”.

Single 18650 charger with a 3-LED charge indicator (green 80–100 %, yellow 40–79 %, red below 40 %): the finished build running on the simulator's bench

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

Parts list

Test procedure

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

  1. 90 %, resting: green only
  2. 82 %: still green
  3. 78 %: yellow
  4. 60 %: yellow only
  5. 42 %: still yellow
  6. 38 %: red
  7. 10 %: red only
  8. Reference
  9. Indicator draw at 100 % (the most)
  10. USB in: the TP4056 charges at 1 A

How it works

What it does: Charges an 18650 from USB and shows its charge on three LEDs: green 80–100 %, yellow 40–79 %, red below 40 %.

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.

Things to try

  1. Click the cell and drag its charge from 100 % down to 0 %: green → yellow → red.
  2. Plug in USB (set the supply to 5 V) at 75 % and watch the indicator move up as it charges.
  3. Switch S1 off: the indicator goes dark and stops drawing current.

Background

The same window-comparator idea is inside every 3- and 4-LED battery-level board; the cheap ones use a dedicated chip, but it is four comparators and a reference underneath.

More builds: batteries and solar