Battery Life Calculator
Estimate runtime from capacity and load current
Required Parameters
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Quick Answer
Estimated Runtime (Hours) = (Battery Capacity × Usable Efficiency) / Average Current Draw.
Battery Life Calculator
Estimate how long a battery will last based on its capacity (mAh) and the device current consumption (mA).
Basic Formula
Battery Life (hours) = Battery Capacity (mAh) / Load Current (mA)
With Efficiency Factor
Battery Life = (Capacity x Efficiency) / Load Current
Typical efficiency: 80-90% for regulated circuits, 95%+ for direct connections.
Design Tips
- Use sleep modes to reduce average current
- Consider self-discharge (1-5% per month for Li-Ion)
- Temperature affects capacity significantly
- Derate by 20% for real-world estimates
Related Tools
- Ohm's Law Calculator
- Energy Conversion Calculator
Design Notes
Battery capacity (mAh) is measured under ideal, low-current discharge conditions at room temperature. If your circuit draws high burst currents (like turning on a cellular modem or motor), the internal resistance of the battery causes a voltage drop (Peukert's Law effect for lead-acid, less severe but present in Li-Ion). Always derate heavily: a safe benchmark is using 80%-85% usable capacity for Li-Ion/LiPo, and as low as 60% for coin cells running high-pulse loads.
Common Mistakes
- 1
Assuming 100% capacity is usable: Most devices cut off when voltage drops below 3.0V-3.3V, leaving 10-20% chemical energy stranded.
- 2
Ignoring standby/quiescent current: A 100µA sleep current might seem small, but over months it completely drains a coin cell.
- 3
Not factoring in extreme cold, which can cut effective LiPo capacity by up to 40%.
Engineering Handbox
1. Calculate Usable Capacity: 2000 mAh × 0.85 = 1700 mAh 2. Divide by Average Load: 1700 mAh / 50 mA = 34 Hours
Knowledge Base
Does battery life really equal mAh / mA?
Only approximately. The formula Hours = mAh / mA assumes 100% efficiency and constant discharge rate. Real-world factors reduce this by 15-30%: self-discharge, voltage cutoff (leaving 10-20% stranded), Peukert effect (high currents reduce effective capacity), and temperature (cold cuts Li-ion capacity by up to 40%). Use 80-85% efficiency for Li-ion and 60-70% for alkaline.
What current should I use for battery life calculation?
Use the AVERAGE current draw, not the peak. Measure with a current logger over a full duty cycle. For example, an IoT sensor that draws 50mA for 100ms every 60 seconds and 10uA in sleep effectively averages about 93uA — giving months of runtime on a CR2032 instead of the hours suggested by peak current.
What is the difference between mAh and Wh?
mAh (milliamp-hours) measures charge capacity at a specific voltage. Wh (watt-hours) measures energy regardless of voltage: Wh = mAh × V / 1000. A 3000mAh battery at 3.7V stores 11.1Wh. Always use Wh when comparing batteries of different voltages — a 12V 1000mAh battery stores more energy (12Wh) than a 3.7V 2000mAh battery (7.4Wh).
How does temperature affect battery life?
Cold temperatures significantly reduce battery capacity: Li-ion loses 10-20% at 0°C and up to 40% at -20°C. Alkaline loses 50%+ in freezing conditions. Hot temperatures (above 45°C) temporarily increase capacity but dramatically reduce cycle life. Optimal operating temperature for most chemistries is 20-25°C.
What is Peukert's Law?
Peukert's Law states that higher discharge rates reduce the effective battery capacity disproportionately. A battery rated at 2000mAh at 200mA discharge might only deliver 1600mAh at 1A discharge. The effect is strongest for lead-acid batteries (Peukert exponent 1.2-1.4), moderate for alkaline (1.05-1.15), and minimal for Li-ion (1.02-1.05).
How long will a CR2032 coin cell last?
A CR2032 has approximately 220-240mAh capacity at low drain rates. At 1uA continuous draw (e.g., RTC backup), it lasts about 220,000 hours or ~25 years (theoretical max is limited by 10-year shelf life). At 10mA continuous, capacity drops significantly due to high internal resistance. Typical use: 2-5 years in watches, key fobs, and low-power sensors.
Can I put batteries in parallel for more capacity?
Yes, paralleling identical batteries adds their capacities (2x 2000mAh = 4000mAh) while maintaining the same voltage. Critical rules: only parallel identical chemistry, brand, and age batteries. Never parallel batteries with different charge states — the voltage difference causes large equalizing currents that can cause heating or fire. Add individual fuses per cell for safety.
What voltage cutoff should I use?
Li-ion/LiPo: 3.0-3.2V per cell (going below 2.5V causes permanent damage). Alkaline AA: 0.9-1.0V per cell (usable to 0.8V for some devices). Lead-acid: 10.5V for 12V battery (1.75V/cell). NiMH: 1.0V per cell. Set your device's low-voltage cutoff at these levels to prevent over-discharge damage.
How do I estimate battery life for a wireless IoT device?
Sum the energy per cycle: E_cycle = (I_active × t_active) + (I_sleep × t_sleep). Then: Battery Life = Capacity × Efficiency / E_cycle × t_period. For example: 10mA for 500ms active, 5uA for 59.5s sleep, every 60s = 88.1uA average. A 2400mAh AA at 85% efficiency: 2040mAh / 0.088mA = 23,181 hours = 2.6 years.
Is self-discharge significant for battery life?
For long-duration applications, yes. Li-ion self-discharges at 2-3% per month. NiMH loses 20-30% per month (low-self-discharge NiMH like Eneloop: 15% per year). Alkaline loses about 5% per year. CR2032 lithium primary cells lose less than 1% per year. For multi-year deployments, always account for cumulative self-discharge loss.