Battery Life Calculator: Complete Guide to Runtime, Capacity & C-Rate
Whether you are designing an IoT sensor, a portable medical device, a consumer wearable, or an EV battery pack, accurately predicting battery life is one of the most critical steps in the hardware design process. Overestimating battery life leads to device failures in the field; underestimating it adds unnecessary cost and weight. This guide explains the underlying formulas, practical considerations, and real-world examples behind accurate battery life calculation.
The Battery Life Formula
The fundamental formula for estimating battery runtime is straightforward:
Battery Life (hours) = Battery Capacity (mAh) / Load Current (mA)
However, this theoretical value must be derated by an efficiency factor (typically 70–85%) to account for real-world losses. The practical formula becomes:
Battery Life = (Capacity × Efficiency) / Current
For example, a 3,000 mAh lithium-ion battery powering a device drawing 150 mA at 85% efficiency will last approximately 3,000 × 0.85 / 150 = 17 hours — not the naive 20 hours from the ideal calculation.
How to Calculate Required Battery Capacity
When you know the target runtime and load current, you can calculate the minimum battery capacity needed:
Capacity (mAh) = Current (mA) × Runtime (h) / Efficiency
Always add a 20% safety margin to the calculated capacity to account for battery aging, temperature derating, and unexpected load spikes. For example, if you need 48 hours at 50 mA with 80% efficiency: Capacity = 50 × 48 / 0.80 = 3,000 mAh. With 20% margin, spec a 3,600 mAh battery.
Understanding C-Rate
The C-rate is the rate of discharge relative to the battery's maximum capacity. A 1C rate discharges a battery in exactly 1 hour; a 0.5C (or C/2) rate in 2 hours. High C-rates (above 1C) reduce the actual usable capacity due to increased internal heat generation and voltage sag. Lithium-ion cells are typically rated for continuous discharge at 0.5C to 2C; high-rate LiPo cells can sustain 20C or more. Computing C-rate helps you verify that your battery is not being stressed beyond its specification.
Real-World Factors That Reduce Battery Life
- Temperature: Cold temperatures dramatically reduce lithium-ion capacity; at -20°C, a cell may deliver only 50–60% of its rated capacity.
- Peukert's Effect: Higher discharge rates cause a non-linear reduction in delivered capacity, particularly relevant for lead-acid batteries.
- Aging: Li-ion batteries lose 2–4% capacity per year; after 500 full cycles, expect 80% or less of original capacity.
- Self-Discharge: Lithium-ion self-discharges at 1–5% per month; nickel-metal hydride at 15–20% per month.
- Depth of Discharge: Discharging to 0% repeatedly degrades Li-ion faster; limiting depth to 80% significantly extends cycle life.
Common Battery Life Examples
- Smart home sensor (10 mA average, 2×AA = 2500 mAh, 80% eff.): ~200 hours ≈ 8.3 days
- Smartphone (300 mA average, 4000 mAh, 85% eff.): ~11.3 hours between charges
- Wireless earbuds (25 mA, 55 mAh per earbud, 80% eff.): ~1.76 hours per charge
- GPS tracker (50 mA, 5000 mAh LiPo, 80% eff.): ~80 hours in the field