Engineering · Electronics · Electronics
Battery Life Calculator
Calculates estimated battery life (runtime) from battery capacity and device current draw, with optional efficiency derating.
Calculator
Formula
t = battery runtime in hours; C = battery capacity in milliamp-hours (mAh); \eta = efficiency or derating factor (0 to 1, dimensionless); I = average current draw of the device in milliamps (mA). When \eta = 1, the formula gives ideal runtime. In practice, \eta accounts for battery chemistry inefficiencies, temperature effects, and Peukert losses, typically ranging from 0.70 to 0.95.
Source: Derived from fundamental Coulomb capacity definitions; consistent with IEC 61960 standard for secondary lithium cells and batteries.
How it works
Battery capacity is rated in milliamp-hours (mAh) or amp-hours (Ah), representing the total charge the battery can deliver before depletion. A 3000 mAh battery can theoretically supply 3000 mA for one hour, or 300 mA for ten hours. By dividing the total available charge by the rate at which it is consumed (the current draw), you obtain the theoretical runtime. This relationship is a direct application of the definition of electric charge: Q = I × t, rearranged to t = Q / I.
In practice, batteries do not deliver 100% of their rated capacity under all conditions. Factors such as operating temperature, discharge rate (Peukert effect), battery age, and circuit inefficiencies reduce usable capacity. The efficiency factor \u03b7 (eta) captures these real-world losses in a single scalar multiplier. A conservative value of 0.80 is common for lithium-ion cells under moderate load, while alkaline cells under heavy drain may use 0.60–0.70. The complete formula is: t = (C × η) / I, where t is runtime in hours, C is capacity in mAh, η is the efficiency factor (0 to 1), and I is average current draw in mA.
This calculator is widely applied across consumer electronics (smartphones, wearables, wireless earbuds), industrial IoT sensors, medical devices, remote controls, drones, electric vehicles, and emergency lighting systems. Power system engineers use it during the design phase to select appropriate battery packs, while field technicians use it to estimate maintenance intervals and replacement schedules. Combining this calculation with sleep/wake duty cycle analysis or power profiling tools gives a comprehensive picture of total energy consumption.
Worked example
Suppose you are designing a wireless IoT temperature sensor powered by a 2000 mAh lithium-ion cell. The sensor's microcontroller and radio module draw an average of 12 mA during normal operation (accounting for periodic sleep modes and transmission bursts). You estimate a derating efficiency of 0.85 to account for low-temperature operation in an outdoor enclosure and the battery's partial discharge curve.
Step 1 — Calculate effective usable capacity:
Effective capacity = 2000 mAh × 0.85 = 1700 mAh
Step 2 — Calculate runtime in hours:
t = 1700 mAh ÷ 12 mA = 141.67 hours
Step 3 — Convert to days for practical planning:
Runtime in days = 141.67 ÷ 24 = 5.90 days
This result tells the engineer that the sensor will operate for approximately 5.9 days before requiring a battery replacement or recharge. If a 30-day service interval is required, the engineer could increase the battery capacity to roughly 12,000 mAh (e.g., using multiple cells in parallel), further reduce the average current draw through aggressive power management, or both.
Limitations & notes
This calculator assumes a constant average current draw. Real devices often have highly variable loads — a smartphone may draw 50 mA in standby but 600 mA during GPS navigation. For variable loads, compute a time-weighted average current across all operating modes before using this tool. The Peukert effect means that at very high discharge rates, effective capacity drops significantly below the rated value; this calculator does not model Peukert's equation directly, so the efficiency factor must be set conservatively for high-drain applications. Battery capacity also degrades with cycle count and age — a lithium-ion cell may retain only 80% of its original capacity after 500 full charge-discharge cycles. Temperature is another critical variable: capacity can fall by 20–30% at 0°C and even more at sub-zero temperatures. The calculator does not account for self-discharge, which is relevant for long-term storage applications. Always validate calculator estimates against empirical measurements using a battery analyzer or power profiler for safety-critical and medical device applications.
Frequently asked questions
What is a typical efficiency factor to use for a lithium-ion battery?
For most lithium-ion batteries operating at moderate loads and room temperature, an efficiency factor between 0.80 and 0.90 is a reasonable starting point. Use 0.85 as a general-purpose default; lower it to 0.70–0.75 for cold environments, high discharge rates, or aged cells. Always validate with empirical testing for critical applications.
How do I find the average current draw of my device?
The most accurate method is to use a power profiler or bench power supply with current measurement to log current consumption across all operating modes (active, idle, sleep, transmission). Multiply each mode's current by the fraction of time spent in that mode, then sum the results to get a true time-weighted average current draw.
Can I use this calculator for alkaline or NiMH batteries?
Yes, but adjust the efficiency factor accordingly. Alkaline batteries are more sensitive to the Peukert effect and suffer significant capacity loss at high discharge rates — use efficiency values of 0.60–0.75 for heavy loads. NiMH cells perform better under sustained load and typically use 0.70–0.85. Always check the manufacturer's discharge curves for the most accurate data.
How does temperature affect battery life?
Temperature has a substantial impact on battery capacity and runtime. Lithium-ion cells can lose 15–25% of their rated capacity at 0°C and up to 40% or more at -20°C. High temperatures accelerate chemical degradation and reduce long-term cycle life. To account for cold-weather operation, reduce the efficiency factor in this calculator to reflect the lower usable capacity.
What is the difference between battery life and battery lifespan?
Battery life (or runtime) refers to how long a battery powers a device on a single charge — this is what this calculator estimates. Battery lifespan refers to how many charge-discharge cycles a battery can complete before its capacity degrades to an unacceptable level (commonly defined as 80% of original capacity). Lithium-ion batteries typically have a lifespan of 300–500 full cycles for consumer-grade cells and 500–1000+ cycles for higher-quality cells.
Last updated: 2025-01-15 · Formula verified against primary sources.