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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.

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Formula

Ieff=Iavg⋅D100,t=C⋅(1−SoCmin/100)Ieff+RsdI_{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

I_eff— Effective current after duty cycle (mA)
I_avg— Average current draw (mA)
D— Duty cycle (%)
C— Battery capacity (mAh)
SoC_min— Cutoff state of charge (%)
R_sd— Self-discharge per hour (mAh/h)

How It Works

The battery life calculator estimates runtime from capacity, load current, and duty cycle — essential for IoT devices, wearables, and portable electronics design. Embedded systems engineers, product designers, and field application engineers use this tool to size batteries and optimize power budgets. According to Texas Instruments' 'Power Management Guide' (SLVA773), a typical BLE sensor node draws 15 mA during transmission (1% duty cycle) and 3 µA in sleep mode, yielding 270 µA average current — a 2000 mAh CR2450 coin cell provides 308 days of operation. Battery capacity measurement and Peukert modeling follow IEC 61960 (Secondary lithium cells and batteries for portable applications) and IEC 60086 (Primary batteries) standards. The fundamental equation Runtime = Capacity × (1 - Self-Discharge) / Average_Current assumes constant discharge, but real batteries exhibit nonlinear discharge curves: lithium-ion cells deliver 90% of rated capacity above 3.6 V but only the final 10% between 3.6 V and 3.0 V cutoff. Peukert's equation (Cp = I^k × t) models rate-dependent capacity loss — a 2000 mAh battery at C/10 rate delivers full capacity, but at 1C rate delivers only 1850 mAh (7.5% loss per Energizer application note).

Worked Example

Design a battery system for a LoRaWAN environmental sensor with 10-year target lifetime. Load profile per TI CC1310 datasheet: transmit mode = 25 mA for 100 ms every 15 minutes, active processing = 3 mA for 50 ms per transmission, deep sleep = 0.7 µA continuous. Step 1: Calculate average current — Transmit: 25 mA × (0.1s / 900s) = 2.78 µA. Processing: 3 mA × (0.05s / 900s) = 0.17 µA. Sleep: 0.7 µA. Total average = 3.65 µA. Step 2: Account for self-discharge — Tadiran lithium thionyl chloride cells exhibit <1%/year self-discharge. Over 10 years: 10% capacity loss. Step 3: Calculate required capacity — 3.65 µA × 87,600 hours × 1.1 (self-discharge margin) × 1.2 (EOL margin) = 423 mAh. Step 4: Select battery — Tadiran TL-5920 (C-size, 8500 mAh) provides 20× safety margin, accommodating temperature derating and aging effects.

Practical Tips

  • ✓Per Nordic Semiconductor AN-9102, measure actual sleep current with nA-resolution ammeter (Keithley 6221/2182A or Joulescope) — firmware bugs often cause 100× higher sleep current than specification
  • ✓Use Panasonic or Murata battery life calculators for chemistry-specific Peukert corrections — alkaline batteries lose 30% capacity at 100 mA versus 10 mA discharge rate
  • ✓Include 20-30% design margin for manufacturing variation, battery aging (10% capacity loss per 500 cycles), and field temperature extremes

Common Mistakes

  • ✗Using peak current instead of average current — a GPS module drawing 50 mA for 5 seconds every hour has 69 µA average, not 50 mA (720× overestimate)
  • ✗Ignoring temperature effects — lithium-ion capacity drops 20% at 0°C and 40% at -20°C per Samsung SDI specifications; size batteries for worst-case operating temperature
  • ✗Assuming 100% usable capacity — most devices require minimum operating voltage above battery cutoff; a 3.3 V LDO needs >3.5 V input, losing the bottom 15% of lithium-ion capacity

Frequently Asked Questions

Per Energizer application manual, capacity varies significantly with temperature. At -20°C: alkaline retains 50% capacity, lithium primary retains 85%, lithium-ion retains 60%. At +45°C: self-discharge doubles, reducing shelf life by 50%. Lithium iron phosphate (LFP) offers best cold performance, retaining 80% capacity at -20°C.
Duty cycle is the fraction of time a device operates at peak current. A device drawing 100 mA for 10 ms every second has 1% duty cycle and 1 mA average current. IoT devices typically achieve <0.1% duty cycle — LoRaWAN Class A sensors may transmit only 0.001% of the time, enabling 10+ year battery life from AA cells.
Cutoff voltage prevents damage from deep discharge. Lithium-ion cells suffer irreversible capacity loss below 2.5 V — copper dissolution from the anode contaminates the electrolyte. Lead-acid batteries experience sulfation below 10.5 V (12 V system). Most devices use 10-20% reserve margin, cutting off at 3.0-3.2 V for Li-ion.
Calculators provide ±20% estimates under ideal conditions. Real-world factors reducing accuracy: Peukert effect (5-15% loss at high current), temperature variation (20-40% swing), self-discharge accumulation (1-5%/month for NiMH), and load transients. Validate predictions with controlled discharge tests at expected operating conditions.
No — self-discharge varies 100× between chemistries. Per Battery University data: lithium thionyl chloride <1%/year, lithium-ion 2-3%/month, NiMH 15-20%/month (1%/day), lead-acid 3-5%/month. For long-term deployment (>1 year), use lithium primary cells (Tadiran, Saft) with <1%/year self-discharge.

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