Article — Battery Life Calculator
Battery Life Calculator: From mAh and Wh to Real Runtime
A battery life calculator divides available energy by average power draw to estimate runtime. The simplest form is runtime (h) = capacity (mAh) / draw (mA), but realistic estimates also need voltage, usable-capacity fraction, and converter efficiency.
Manufacturer specs use mAh (charge) or Wh (energy). Devices specify mA (current) or W (power). The calculator above accepts any of those four input combinations and shows runtime in hours, minutes, and days simultaneously.
What is a battery life calculator?
A battery life calculator predicts how long a battery will power a device at a given draw. The basic physics is energy conservation: total energy stored, divided by average power consumed, equals runtime. Capacity sets the numerator; draw sets the denominator.
Real devices never reach the ideal value. Reserved capacity, conversion efficiency, and Peukert losses each shave off a fraction. The calculator above exposes usable-capacity and efficiency inputs so the estimate can match real-world expectations rather than spec-sheet ideals.
The IEEE 1725 standard for cellular phone batteries requires manufacturers to publish nominal capacity, but lets them choose the test current. Reported mAh values use a low-current "discharge for 5 hours" test that overstates available capacity for high-current use by 10 to 20%.
Battery life formulas
The clean form of the runtime equation is t = E / P, with energy in watt-hours and power in watts. Converting from mAh to Wh requires the cell's nominal voltage: Wh = mAh × V / 1000. So a 3000 mAh, 3.7 V cell holds 11.1 Wh.
If the device draws current rather than power, convert with P = I × V. A 500 mA draw from a 3.7 V cell is 1.85 W. Runtime is then 11.1 Wh / 1.85 W = 6 hours. The calculator handles unit normalization automatically.
t (h) = mAh / mA t (h) = Wh / WWh = mAh × V / 1000 W = V × AE_avail = E × usable% × eff% days = hours / 24mAh vs. Wh: which to use
mAh is charge; Wh is energy. You can compare two batteries with mAh only if they share a voltage. A 10,000 mAh power bank at 3.7 V (37 Wh) holds far less energy than a 10,000 mAh laptop pack at 11.1 V (111 Wh).
Airline carry-on rules use watt-hours specifically to avoid this confusion. The FAA caps lithium batteries at 100 Wh in carry-on without approval and 100-160 Wh with prior approval. A 20,000 mAh power bank labeled "20K" passes if it is at 3.7 V (74 Wh) but fails if mislabeled at a higher voltage.
Battery life by device type
Different devices live at different points on the capacity-draw plane. Smartphones combine moderate capacity (3000-5000 mAh) with moderate average draw (80-300 mA), giving 12-48 hour real-world runtime. Smartwatches use small batteries (250-500 mAh) but very low draws (10-30 mA), so they last 18-72 hours despite the small cell.
IoT sensors are at the extreme low-draw end. A BLE temperature sensor pulling 0.5 mA average from a 500 mAh coin cell lasts about 1000 hours (42 days). Smoke detectors run on 9 V cells for 5 to 10 years because their quiescent current is microamps. At the other end, mobile gaming and 5G hotspot use can pull over 1500 mA, cutting a 5000 mAh phone to 3 hours.
What affects battery runtime
Several factors push real battery life below the simple capacity/draw ratio:
- Reserved capacity — phones expose 80-90% of nominal mAh to extend cycle life. The calculator's "usable%" input accounts for this.
- Conversion efficiency — DC-DC converters that step 3.7 V cell voltage to 5 V USB or 1.8 V CPU rails lose 5-20%.
- Peukert effect — effective capacity falls as discharge current rises. Lithium loses 5-15%, lead-acid 20-40% at high draws.
- Temperature — lithium capacity drops about 15% at 0 °C and another 10-20% below −10 °C.
- Aging — lithium-ion loses about 20% capacity after 500 full-charge cycles.
- Self-discharge — alkaline cells lose 2% per year at room temperature; NiMH loses 1-2% per day.
Spec-sheet mAh is measured at low constant current and room temperature, with a fresh cell. Real-world runtime under realistic conditions is typically 15-30% lower. Set the calculator's usable% to 85 and efficiency to 90 for honest estimates.
Realistic battery life estimates
For a planning estimate, work backwards from your endurance target. A laptop expected to run an 8-hour workday at 15 W average power needs at least 120 Wh of energy, accounting for 25% losses across reserved capacity and converter efficiency. Tablets and phones aim for 10-15 Wh of net energy for a full day at moderate use.
For embedded design, the dominant factor is sleep current. A device awake 1% of the time at 50 mA and asleep 99% at 50 µA has an average of 549.5 µA. A 1000 mAh primary cell at this draw lasts about 2000 hours (83 days). The trick to multi-year battery life is brutal duty cycling, not bigger cells.
Convert everything to watt-hours and watts before estimating. Energy and power are voltage-independent, so a single calculation works across power-bank voltages, laptop pack voltages, and cell voltages without conversion errors.
Common battery calculator mistakes
The most common error is mismatched units: dividing mAh by W or Wh by mA gives nonsense. The calculator above forces a consistent energy/power form internally, but you still need to enter capacity and draw in their own correct units. Forgetting to enter nominal voltage when mixing units is the second-most-common error.
Treating manufacturer mAh as available capacity over-estimates by 10-20%. Ignoring converter efficiency over-estimates by another 5-15%. Using peak draw rather than average draw under-estimates runtime by 2-5x. Use average current measured over a typical hour of use, not the maximum instantaneous draw.
Battery life quick reference
A 3000 mAh smartphone at 100 mA average lasts roughly 27 hours after accounting for 90% usable capacity and 95% efficiency. A 500 mAh smartwatch at 20 mA average lasts 21 hours. A 20,000 mAh power bank delivers about 12,500 mAh of phone-charging current after the 5 V conversion loss — enough for three full phone charges. A 100 Wh laptop battery at 15 W average lasts about 6 hours. A 9 V smoke detector cell at 10 µA lasts roughly 5 years before alarm-circuit voltage drops too low.
The pattern across all these: capacity matters, but average draw matters at least as much. Cutting average draw in half doubles runtime. Doubling capacity also doubles runtime, but at higher cost, weight, and cycle-life trade-offs.