PAGE 122 OF 150 · POWER SUPPLIES & BATTERIES
Battery Life Calculation
Hours equal capacity divided by load current.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
The formula
Run time in hours is about battery capacity in mAh divided by load current in mA. Apply a safety factor of 70 to 80 percent for real-world losses.
Example
Two AA alkaline cells at 2,000 mAh powering an LED at 20 mA give 100 hours in theory and about 70 hours with the safety factor.
- Parallel LEDs add their currents, so life falls.
- Series LEDs share the same current, so life does not change.
- Lower current gives disproportionately better life.
High current drains alkaline cells faster than their rating suggests.
REMEMBER
Capacity divided by current gives hours, then reduce it.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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