18650 battery level indicator
Three LEDs show full, half and low. Three LEDs show how full a Li-ion cell is. A TL431 makes a steady 2.495 V reference; a resistor chain gives three fractions of the cell voltage; an LM339 compares each with the reference. Green and yellow lit: full. Yellow alone: half. Red: charge me.
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:
- Full cell (4.10 V)
- Half (3.75 V)
- Low (3.25 V)
How it works
What it does: Three LEDs show full, half and low.
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
The battery gauge on torches, power banks and e-bike packs.