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Battery Life Calculator
Use this calculator to estimate how long a battery can power a device from the battery's capacity and the device's average current consumption. Enter capacity and current in compatible units, then apply a usable-capacity or reserve adjustment when appropriate.
This calculator estimates runtime per charge or per battery. It does not predict cycle life, calendar life, service life, state of health, or the number of times a rechargeable battery can be charged. Actual runtime depends on battery chemistry, discharge rate, temperature, cutoff voltage, age, self-discharge, load profile, converter efficiency, and other system conditions.
How to Use the Battery Runtime Calculator
Find the battery capacity at conditions relevant to the application, preferably from the manufacturer datasheet.
Measure or estimate the device's average current drawn from the battery.
Use matching units: Ah with A, or mAh with mA.
Enter a usable-capacity percentage or reserve percentage if the calculator supports it.
Calculate the estimated runtime.
Check the estimate against the battery's discharge curves, device cutoff voltage, peak-current behavior, temperature range, and converter efficiency.
Basic Battery Life Formula
For an ideal constant-current load:
Runtime in hours = Battery capacity in Ah ÷ Load current in A
The same formula works directly with milliamp-hours and milliamps:
Runtime in hours = Battery capacity in mAh ÷ Load current in mA
If only a fraction of nominal capacity is expected to be usable:
Estimated runtime = Nominal capacity × Usable fraction ÷ Average current
If the interface uses a reserve or “discharge safety” percentage r, then usable fraction = 1 − r, with r expressed as a decimal.
Example: Constant Current Load
A 3,000 mAh battery supplies a device drawing 150 mA continuously:
Ideal runtime = 3,000 mAh ÷ 150 mA = 20 h
If the design assumes that only 80% of nominal capacity is usable:
Estimated runtime = 3,000 × 0.80 ÷ 150 = 16 h
The 80% value is an engineering assumption, not a universal battery rule. Select it from application requirements and battery data.
Battery Capacity and Current Units
| Quantity | Unit | Relationship |
|---|---|---|
| Current | A | 1 A = 1,000 mA = 1,000,000 µA |
| Charge capacity | Ah | 1 Ah = 1,000 mAh |
| Charge capacity | mAh | 1 mAh = 1,000 µAh |
| Energy | Wh | Nominal Wh ≈ Nominal V × Ah |
| Power | W | W = V × A for DC values |
Ampere-hour measures electric charge, while watt-hour measures energy. A 2 Ah battery does not have a complete energy rating until voltage is included. A nominal 3.7 V, 2 Ah battery has a nominal energy of approximately 7.4 Wh, but actual delivered energy depends on its voltage curve and cutoff conditions.
Convert Watt-Hours and Ampere-Hours
At a stated nominal voltage:
Wh ≈ V × Ah
Ah ≈ Wh ÷ V
These are nominal conversions. Battery voltage changes with state of charge, load, temperature, and chemistry. For systems with regulators or inverters, an energy-based calculation is often more useful than dividing Ah by load-side current.
Average Current for Duty-Cycled Devices
Devices such as wireless sensors may alternate among transmit, processing, idle, and sleep states. Calculate time-weighted average current over one complete operating cycle:
Iavg = (I1t1 + I2t2 + ... + Intn) ÷ T
where T is the total cycle time and the state times add to T. An equivalent duty-cycle form is:
Iavg = I1D1 + I2D2 + ... + InDn
Example: Active and Sleep Current
A sensor draws 80 mA for 5 seconds and 0.2 mA for the remaining 55 seconds of every minute:
Iavg = (80 × 5 + 0.2 × 55) ÷ 60 = 6.85 mA
With a nominal 2,400 mAh battery:
Ideal runtime = 2,400 ÷ 6.85 = 350.365 h = 14.5985 days
This ideal estimate excludes self-discharge, capacity derating, startup current, regulator loss, and battery voltage limits. For long-life devices, even small sleep, leakage, sensing, and regulator quiescent currents can materially change the result.
Energy-Based Runtime Calculation
When the device is powered through a DC-DC converter or inverter, estimate runtime from energy and power:
Runtime in hours = Usable battery energy in Wh ÷ Average battery-side power in W
For a converter delivering output power Pload with efficiency η:
Pbattery ≈ Pload/η
Converter quiescent power and other battery-side loads should be added separately.
Example: Battery Through a Converter
A nominal 12 V, 7 Ah battery has approximately 84 Wh of nominal energy. It powers a 10 W load through a converter with 90% efficiency:
Pbattery = 10 W ÷ 0.90 = 11.1111 W
Ideal runtime = 84 Wh ÷ 11.1111 W = 7.56 h
This result assumes all nominal energy is usable and omits converter standby consumption. Apply the battery's discharge data and required reserve to obtain a more realistic estimate.
Factors That Change Actual Battery Runtime
| Factor | Why It Matters |
|---|---|
| Battery chemistry | Different chemistries have different voltage curves, rate capability, self-discharge, and usable operating ranges. |
| Discharge rate | Available capacity can change with current; high current also increases voltage sag and internal heating. |
| Temperature | Low or high temperature can reduce usable capacity and change internal resistance. |
| Cutoff voltage | The device or BMS may stop operation before the battery reaches the endpoint used for its rated capacity. |
| Age and usage history | Storage, calendar aging, and previous cycling can reduce available capacity. |
| Self-discharge | Battery charge is lost internally over time, which matters most in long-duration, low-current applications. |
| Peak current | A short high-current pulse may cause voltage to fall below the device cutoff even when average current is low. |
| Power conversion | Regulator efficiency and quiescent current vary with input voltage and load. |
| Protection and monitoring | BMS, indicator, sensing, and balancing circuits consume energy and impose operating limits. |
Why Battery Size Does Not Define One Capacity
Labels such as AA, AAA, 9 V, and CR2032 describe a form factor or battery designation, not one universal capacity. Products with the same physical size may use different chemistries and may be rated at different currents, temperatures, and cutoff voltages. Do not use a generic capacity table when a reliable runtime estimate is required.
Use the exact manufacturer's datasheet and discharge curve for the selected part number. Match the curve's load, temperature, and endpoint voltage to the application as closely as possible.
Lead-Acid Batteries and Peukert Behavior
Lead-acid battery capacity is often specified at a stated hour rate. Higher discharge current can reduce the effective capacity and runtime. Peukert-type models can improve estimates when the manufacturer supplies an appropriate exponent or battery-monitor configuration, but the simple Ah/current calculator does not automatically model this effect.
Do not apply one generic Peukert exponent to every battery chemistry. Use the battery manufacturer's data or a validated battery model.
Batteries in Series and Parallel
| Connection | Nominal Voltage | Ah Capacity | Nominal Energy |
|---|---|---|---|
| Two identical batteries in series | Voltage doubles | Same as one battery | Approximately doubles |
| Two identical batteries in parallel | Same as one battery | Capacity approximately doubles | Approximately doubles |
For example, two identical nominal 12 V, 100 Ah batteries contain approximately 2,400 Wh in either a 24 V, 100 Ah series arrangement or a 12 V, 200 Ah parallel arrangement.
Only connect batteries in series or parallel when the manufacturer permits it. Follow limits for battery type, count, matching, fusing, cable sizing, charging, balancing, and BMS configuration. Mixing batteries with different chemistry, voltage, capacity, age, or state of charge can be hazardous.
Runtime Estimate Checklist
Use capacity for the exact battery part number and relevant discharge conditions.
Measure battery-side current over a representative complete operating cycle.
Include sleep current, startup events, communication retries, indicators, sensors, and regulator quiescent current.
Check whether peak load causes unacceptable voltage sag.
Use the actual device cutoff voltage and BMS limits.
Apply temperature, age, self-discharge, and capacity reserve assumptions.
For regulated systems, include efficiency across the expected battery-voltage range.
Validate the estimate with bench measurements and representative batteries.
Common Battery Life Calculation Mistakes
Using peak current instead of time-weighted average current, or ignoring peak current entirely.
Mixing Ah with mA or mAh with A without converting units.
Treating nominal Wh as fully usable energy.
Ignoring regulator efficiency and quiescent current.
Using a capacity value for the wrong discharge rate, temperature, or cutoff voltage.
Assuming every battery of the same physical size has the same capacity.
Confusing runtime with rechargeable cycle life.
Predicting multi-year runtime without accounting for self-discharge and calendar aging.
Battery Life FAQ
How long will a 2,000 mAh battery last?
Under an ideal constant 100 mA load, 2,000 mAh ÷ 100 mA = 20 hours. Actual runtime may be shorter or occasionally different because usable capacity depends on battery and system conditions.
Can I divide mAh by mA directly?
Yes. mAh divided by mA gives hours because the milli prefix cancels. Do not divide mAh by A without first converting one of the units.
Should I use average or maximum current?
Use time-weighted average battery current for the basic capacity estimate, but separately verify that the battery can support maximum and pulse currents without excessive voltage sag, heating, or protection trips.
Why did the measured runtime differ from the calculation?
The simple formula assumes a constant load and fully usable rated capacity. Real batteries and devices add discharge-rate effects, voltage cutoff, temperature, aging, self-discharge, pulsed loads, and conversion losses.
Is watt-hour or ampere-hour better for runtime calculations?
Ampere-hour is convenient when battery voltage and battery-side current are compatible and relatively direct. Watt-hour is often better when voltage changes significantly or when converters separate battery voltage from load voltage.
Does a higher-capacity battery always run longer?
Under otherwise identical and permitted conditions, more usable capacity generally increases runtime. Compatibility, voltage, discharge capability, physical limits, charging method, and protection requirements must also be satisfied.
Can this calculator predict battery cycle life?
No. Cycle life depends on chemistry, depth of discharge, charge protocol, temperature, current, voltage limits, and aging. Use manufacturer cycle-life data for that purpose.
