Battery Life Calculator – Estimate Your Device Runtime Accurately

The battery life calculator converts battery capacity, device load, and real-world losses into a runtime estimate expressed in hours and days. It supports capacity in Wh or mAh, constant current or constant power loads, pack configuration (series/parallel), duty cycle for sleep modes, regulator efficiency, temperature derating, and an optional Peukert exponent. With it, you can move from guesswork to numbers and plan batteries that actually meet your product’s runtime targets.

How a Battery Life Calculator Works

At its core, runtime equals usable energy divided by average power draw. When capacity is in watt-hours (Wh), the formula is straightforward:

Runtime (hours) ≈ Effective Wh / Average Input Power (W)

If your capacity is only in mAh, you convert to Wh with nominal pack voltage. The battery runtime calculator handles these unit conversions and then applies the adjustments most designs need: efficiency of regulators, duty-cycle averaging, temperature capacity loss, and (optionally) Peukert effects at higher C-rates.

Capacity: Wh vs mAh

Wh already encodes both voltage and charge; it’s the most portable metric for mixed chemistries and pack topologies. mAh on its own is ambiguous until you specify voltage. If you enter mAh, this battery capacity calculator multiplies mAh by nominal cell voltage and scales by series (S) and parallel (P) cells to estimate pack Wh.

Pack Configuration (Series/Parallel)

• Series (S): increases voltage; Wh scales with S.
• Parallel (P): increases Ah; Wh scales with P.

For example, four 3.7 V cells in 2S2P have about twice the voltage of a single cell and twice the Ah — overall ≈ 4× the Wh of one cell (ignoring losses). The battery life calculator incorporates this when converting mAh to Wh.

Load Modeling: Constant Current vs Constant Power

Some devices are more like constant-current loads (e.g., LED drivers), others behave closer to constant power (e.g., processors, DC-DC regulated systems). Our tool lets you select the mode that best matches your design:

• Constant current: average current (A) × pack voltage (V) ≈ average power (W).
• Constant power: enter W directly; current is inferred from power and voltage.

In both cases, conversion efficiency (buck/boost/DC-DC) is applied to translate output-side power to battery-side input power.

Duty Cycle and Sleep Modes

Modern devices sleep aggressively to save energy. The battery life calculator blends an active state (high current/power) and a sleep state (µA or mW) using a duty cycle percentage. For instance, with 20% active and 80% sleep, your average load is dramatically lower than “always on.” Measure the real duty cycle with your firmware and instrument it on bench runs for accuracy.

Regulator / Conversion Efficiency

No regulator is 100% efficient. A 90% efficient buck/boost means battery input power is output power / 0.9. The calculator includes this efficiency so that runtime reflects real losses. For linear regulators (LDO), efficiency ~ Vout/Vin under load; consider modelling as constant power with a reduced effective efficiency if you’re dropping voltage significantly. See TI power design guides and application notes (e.g., TI Power topics) for typical efficiencies.

Temperature Derating

Capacity falls at low temperatures due to increased internal resistance and slowed electrochemistry. To approximate this, set a percentage loss per degree below 25 °C (for example, 0.4–0.8 %/°C for many Li-ion cells). The battery discharge calculator applies a linear derate to effective Ah. For mission-critical design, consult the cell’s datasheet curves. Battery University offers excellent primers on temperature effects.

Peukert Exponent (Optional)

At higher discharge rates, batteries yield less capacity than their nominal rating. Peukert’s law models this effect. Lead-acid cells exhibit strong Peukert behavior (k ~ 1.1–1.3), while Li-ion tends to be closer to 1.02–1.05. The calculator uses your average C-rate to reduce effective Ah by ≈ Ah_nom × C^(1-k). If you’re working at modest C-rates on Li-ion, you can set k ≈ 1.03 or simply 1.00 to ignore the effect.

Step-by-Step: Using the Battery Life Calculator

1. Enter capacity (prefer Wh of the entire pack). If you only know mAh, add cell voltage and pack S/P.
2. Choose load model:
   • Current: set active mA and sleep µA; duty cycle blends them.
   • Power: set active W and sleep mW; duty cycle blends them.
3. Set regulator efficiency (%). For buck/boost, use bench-measured values if possible.
4. Set ambient temperature and per-degree derating if cold operation matters.
5. Optionally set the Peukert exponent (chemistry-dependent).
6. Press Calculate → the battery life calculator shows runtime, effective Wh/Ah and average load.

Worked Examples

Example 1 — IoT Sensor (Duty-Cycled)

Pack: 1S Li-ion, 50 Wh. Load: 200 mA active, 50 µA sleep, duty 20%, efficiency 90%, 25 °C, k=1.03, derate 0.5 %/°C. Average output current ≈ 0.2×0.2 + 0.00005×0.8 ≈ 0.04004 A. Avg output power ≈ 3.7 V × 0.04004 A ≈ 0.148 W. Input power @90% ≈ 0.164 W. Runtime ≈ 50 Wh / 0.164 W ≈ 305 h (≈12.7 days), before Peukert/temperature refinements — the tool will apply those precisely for your parameters.

Example 2 — Embedded Device (Constant Power)

Pack: 2S Li-ion (7.4 V nominal), capacity 4 Ah → ≈ 29.6 Wh. Load: 2.5 W active, 10 mW sleep, duty 50%, efficiency 92%, k=1.03, 20 °C with 0.5 %/°C derate (≈2.5 % loss). Average output power ≈ 1.255 W; input ≈ 1.364 W. Effective Wh ≈ 28.85 Wh → runtime ≈ 21.2 h.

Accuracy, Cutoff Voltages, and Real-World Gaps

This battery life calculator uses nominal voltages and average power. Real devices shut down at certain cutoff voltages; some regulators are more tolerant than others. Profiles with bursts (radios, motors) can sag voltage and trip early cutoffs. Always validate on hardware across temperatures.

Chemistry Notes (Quick Guide)

• Li-ion / Li-poly: high energy density, mild Peukert, sensitive to cold; avoid deep discharge.
• NiMH: more Peukert effect, self-discharge higher, tolerant to cold vs Li-ion.
• Lead-acid: strong Peukert effect; capacity drops sharply at high C-rates; heavy but cheap/widely available.
• Primary cells (Alkaline/Li-primary): good shelf life; check pulsed-load capability and internal resistance.

Design Tips for Longer Runtime

• Lower the active duty cycle; push more work to sleep modes.
• Use efficient DC-DC regulators and right-size conversion (buck vs boost).
• Tune radio TX power and retry logic; batch transmissions.
• Measure real sleep currents (µA); datasheet “typical” can mislead.
• Derate for cold; pre-warm or insulate packs in frigid environments.

Useful References

• Battery University — chemistry, aging, temperature effects.
• DigiKey Power Fundamentals — regulators, measurements.
• Texas Instruments – Power Management — design notes, efficiency data.
• Energizer – Datasheets — discharge curves for primary cells.

Related Tools on Our Site

• Ohm’s Law Calculator
• LED Resistor Calculator
• Watts to Amps Calculator
• Watts to Volts Calculator

Disclaimer: This battery life calculator provides estimates for educational purposes. Actual runtime varies with hardware, temperature, discharge profile, cutoff voltage, and cell aging. Always validate on the bench.