Battery Life Calculator

Enter battery capacity and current draw to estimate runtime, energy, and charge cycles.

mAh
mA
%
V
mA
mA
%
h
h
Battery Runtime
21.0 hours
0.88 days  |  Effective capacity: 2,100 mAh
Energy
11.1 Wh
Effective Capacity
2,100 mAh
Avg Current (duty)
100 mA
Charge Cycles / Year
—

Common Battery Reference

Runtime estimates use your current draw and efficiency settings above.

Battery Capacity Voltage Runtime at your load

How to Calculate Battery Life

Battery life is one of the most important specifications for any portable electronic device — from microcontroller projects and IoT sensors to remote controls, flashlights, smartphones, and industrial instruments. This calculator gives you a realistic runtime estimate by factoring in not just raw capacity, but also the real-world losses that reduce it.

The Core Formula

The fundamental formula is straightforward:

Runtime (hours) = Battery Capacity (mAh) × Efficiency Factor ÷ Average Current Draw (mA)

For example, a 3,000 mAh lithium battery powering a 100 mA load with 70% efficiency lasts: 3,000 × 0.70 ÷ 100 = 21 hours. Without the efficiency correction you would overestimate by 43%.

Why the Efficiency Factor Matters

The nameplate capacity on a battery is measured under ideal laboratory conditions. Real-world deployments lose capacity through several mechanisms:

  • Voltage regulation losses — DC-DC converters and LDO regulators are only 80–95% efficient. A 3.7 V lithium cell powering a 3.3 V circuit through an LDO wastes 10–11% as heat immediately.
  • Battery internal resistance — As current flows, voltage drops across the internal resistance, dissipating power as heat rather than doing useful work.
  • Peukert's effect — At higher discharge rates, batteries deliver less total charge. A battery rated at C/20 may only deliver 80% of its capacity at C/5.
  • Depth of discharge limits — Lithium batteries should not be discharged below 2.7–3.0 V. Most protection circuits cut out leaving 5–15% of capacity untouched.
  • Temperature — Capacity drops sharply at low temperatures. A lithium cell loses ~20% capacity at 0°C and ~40% at -20°C.

A conservative 70% efficiency is appropriate for engineering designs. Use 80–85% for back-of-envelope estimates with well-matched circuits.

Duty Cycle Mode

Many embedded systems spend most of their time in a low-power sleep state, waking briefly to take measurements or transmit data. The average current for a system with active and sleep states is:

Avg Current = (Active Current × Active%) + (Sleep Current × (100% − Active%))

An IoT node drawing 500 mA when transmitting and only 5 mA while sleeping, with a 10% active duty cycle, has an average current of just 54.5 mA — far lower than the 500 mA peak. This can extend battery life by an order of magnitude compared to always-on operation.

Energy in Watt-Hours (Wh)

Watt-hours let you compare batteries across different voltages. Energy (Wh) = Capacity (Ah) × Voltage (V). A 3,000 mAh, 3.7 V lithium cell holds 11.1 Wh — the same energy as a 1,233 mAh, 9 V battery. For AC-connected devices, dividing Wh by the wall outlet wattage tells you how much household electricity equivalent you are using.

Charge Cycle Estimation

If you know how many hours per day you use the device, you can project how many charge cycles the battery accumulates per year. Most lithium-ion cells are rated for 300–500 full cycles before capacity degrades to 80%. Partial cycles count proportionally — 10 half-discharges roughly equal 5 full cycles. Keeping lithium batteries between 20% and 80% state of charge (avoiding full discharge and full charge) can extend cycle life to 1,000+ cycles.

Common Battery Types at a Glance

The comparison table above shows runtime at your configured current draw for six common battery types: AA alkaline (2,500 mAh), AAA alkaline (1,200 mAh), 9V alkaline (500 mAh at 9 V), CR2032 coin cell (225 mAh at 3 V), 18650 lithium-ion (3,000 mAh at 3.7 V), and a typical smartphone pack (4,000 mAh at 3.7 V). The 9V battery is notable for its high voltage but relatively low capacity — it is often a poor choice for high-current loads.

Tips for Maximizing Battery Life

  • Minimize sleep current — modern microcontrollers can sleep at 1–10 μA; peripheral standby currents often dominate.
  • Use switching regulators (buck/boost) instead of linear regulators when the voltage difference is large.
  • Reduce active time — shorten transmission bursts, use edge triggering instead of polling.
  • Match battery chemistry to temperature — LiFePO4 handles cold better than standard Li-ion; lithium primary cells outperform alkalines below freezing.
  • Derate capacity by 20–30% for designs that must last across a specified temperature range or product warranty period.

FAQ

Battery life (hours) = Battery capacity (mAh) × efficiency factor ÷ average current draw (mA). For example, a 3,000 mAh battery with 70% efficiency powering a 100 mA load: 3,000 × 0.70 ÷ 100 = 21 hours. The efficiency factor accounts for voltage regulation losses, temperature effects, and that batteries cannot be fully discharged.
The efficiency factor (typically 70–85%) accounts for losses that reduce real-world capacity below the rated mAh. These include DC-DC converter losses (10–20%), battery internal resistance heating, voltage sag as the battery discharges, Peukert's effect at high currents, and the fact that batteries should not be fully discharged to preserve longevity. A conservative 70% is recommended for design work; 85% is reasonable for simple estimates.
Duty cycle describes what fraction of time a device is in its active (high-current) state versus a low-power sleep state. The average current = (active current × active%) + (sleep current × (100% − active%)). This average is then used in the battery life formula, and can extend runtime dramatically compared to always-on operation.
Modern smartphone batteries typically range from 3,500 mAh to 5,000 mAh. With an average current draw of around 150–250 mA during normal use and a 70% efficiency factor, that yields roughly 11–23 hours of continuous runtime. Screen-off standby current is much lower, extending real-world standby time to days.
A charge cycle is one full discharge and recharge of the battery. Most lithium-ion batteries are rated for 300–500 cycles before capacity degrades to 80%. Keeping lithium batteries between 20% and 80% charge significantly extends cycle life to 1,000+ cycles. If you enter your daily usage hours, this calculator estimates how many charge cycles you accumulate per year.