PAGE 099 OF 150 · DISPLAYS & INDICATORS
Battery Level Indicator
A few comparators can show charge state.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Reading battery voltage
A battery's voltage falls as it empties, so comparing it with set thresholds gives a simple fuel gauge.
Typical voltages
- Li-ion cell: about 4.2 V full to about 3.3 V nearly empty.
- Alkaline AA: about 1.5 V new, down to about 0.9 V flat.
Circuit
Use a resistor divider to scale the voltage to a comparator or microcontroller input, then light green, amber and red LEDs for high, medium and low.
Measure under load where possible. Lithium cells hold a flat voltage for most of their life, so the reading is only a rough guide.
REMEMBER
A voltage reading is an estimate of charge, not an exact measure.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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