Battery Life Calculator
Estimate battery runtime for IoT and portable devices given average current draw, duty cycle, self-discharge rate, and depth-of-discharge cutoff. Suitable for LiPo, alkaline, NiMH, and coin-cell batteries.
Formula
I_{eff} = I_{avg} \cdot \frac{D}{100},\quad t = \frac{C \cdot (1 - SoC_{min}/100)}{I_{eff} + R_{sd}}
Reference: Nordic Semiconductor PWR Profiler methodology; Texas Instruments SLUA364
How It Works
Battery life calculation is a critical aspect of power system design, bridging theoretical electrochemistry with practical electrical engineering. The fundamental principle involves understanding the total available charge capacity and how various factors degrade battery performance. Self-discharge represents an often-overlooked phenomenon where batteries lose charge even when not in active use, caused by internal chemical reactions and electron migration. The duty cycle concept reflects real-world intermittent device operation, where peak current consumption differs significantly from average current draw. Engineers must account for battery chemistry specifics, as lithium-ion, lead-acid, and alkaline batteries exhibit distinct discharge characteristics. Temperature, load profile, and internal resistance further complicate battery runtime predictions, making precise modeling essential for designing power-efficient electronic systems.
Worked Example
Consider a 2000mAh lithium-ion battery powering a wireless sensor with 50mA peak current at 10% duty cycle. Monthly self-discharge rate is 3%. Calculate expected runtime. Peak current: 50mA. Duty cycle: 10%. Average current calculation: 50mA × 0.1 = 5mA. Self-discharge per month: 3%. Monthly self-discharge per hour: 3% ÷ 720 hours = 0.0042mA. Runtime calculation: (2000mAh × 0.9) ÷ (5mA + 0.0042mA) = 323 hours or approximately 13.5 days of continuous operation.
Practical Tips
- ✓Always measure actual self-discharge rate for specific battery lot
- ✓Use manufacturer's datasheet for precise battery characteristics
- ✓Consider temperature effects on battery performance
Common Mistakes
- ✗Neglecting self-discharge rate in long-term battery life estimates
- ✗Assuming linear discharge curve for non-linear battery chemistries
- ✗Using peak current instead of average current for runtime calculations
Frequently Asked Questions
How does temperature affect battery life?
Lower temperatures reduce battery capacity and increase internal resistance. Higher temperatures accelerate self-discharge and can degrade battery chemistry faster.
What is battery duty cycle?
Duty cycle represents the percentage of time a device is actively drawing power versus being in a low-power or idle state during a given operational period.
Why do batteries have a cutoff percentage?
Cutoff percentage prevents deep discharge that could permanently damage the battery's chemical structure and reduces available usable capacity to protect battery longevity.
How accurate are battery life calculators?
Calculators provide reasonable estimates but cannot perfectly predict real-world conditions. Actual battery life varies with temperature, load variations, and battery age.
Do all battery types have similar self-discharge rates?
No, self-discharge rates differ significantly between battery chemistries. Lithium-ion batteries typically have lower self-discharge rates compared to lead-acid or alkaline batteries.
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