EngineeringFormulaElectrical
Battery Life Calculator
Estimate how long a battery will last, find the capacity needed for a target runtime, or calculate the maximum current draw allowed - using t = C / I.
Battery life is the ratio of stored charge to current consumption: t = C / I. Enter your battery's capacity (in mAh or Ah) and the average current your device draws, and this calculator returns the theoretical runtime in hours. You can also reverse it - enter a target runtime and current draw to find the minimum battery capacity you need, or enter a capacity and runtime to find the maximum safe current draw. All three modes use the same fundamental relationship.
C · The charge stored in the battery, rated by the manufacturer at a specific discharge rate (usually C/10 or C/20). Use the mAh value printed on the battery or its datasheet.
I · The average current your device consumes from the battery. For variable loads (sleep vs. active modes), use the time-weighted average: I_avg = Σ(I_i × t_i) / t_total.
Solution
Enter the required values to calculate battery life.
t = C / I
Formula Sheet
- tBattery Life
- CBattery Capacity
- ICurrent Draw
Variables & Units
| Symbol | Variable | Description | Common Units |
|---|---|---|---|
| t | Battery Life | Theoretical discharge time - how long the battery can supply the specified current before being fully discharged. Actual life is typically 70–85% of this theoretical value. | min, h, days |
| C | Battery Capacity | The charge stored in the battery, rated by the manufacturer at a specific discharge rate (usually C/10 or C/20). Use the mAh value printed on the battery or its datasheet. | mAh, Ah |
| I | Current Draw | The average current your device consumes from the battery. For variable loads (sleep vs. active modes), use the time-weighted average: I_avg = Σ(I_i × t_i) / t_total. | mA, A |
How to Use This Calculator
- 01Select what you want to solve for: Battery Life, Capacity, or Current Draw.
- 02Enter the two known values and choose the correct units for each - mAh and mA for small devices, Ah and A for larger systems.
- 03Click Calculate to see the result with the active formula and substituted values.
- 04Use Load example to pre-fill a real-world device scenario for the selected mode.
- 05If your load is specified in watts instead of amps, convert it first using I = P / V, then apply an efficiency factor for inverters, regulators, or long cable runs.
- 06For a usable planning number, reduce the theoretical runtime by your chemistry's safe depth of discharge and expected system efficiency before selecting a battery.
How the Formula Works
A battery's capacity rating (mAh or Ah) tells you how much charge it can deliver. An mAh rating means the battery can supply that many milliamps for one hour - or proportionally less current for longer, or more current for shorter. Dividing capacity by the current draw gives the runtime directly: t = C / I.
This is the standard theoretical estimate used in datasheets, product specs, and quick engineering checks. In practice, actual runtime is shorter because: (1) batteries should not be fully discharged to zero to avoid damage - typically 80–90% depth of discharge is used; (2) higher discharge rates reduce effective capacity due to internal resistance losses (Peukert effect, most significant in lead-acid batteries); (3) cold temperatures reduce capacity; and (4) aging degrades rated capacity over charge cycles.
For a rough real-world estimate, multiply the theoretical result by 0.70–0.85. For precision work, use battery datasheets with discharge curves at the actual C-rate and temperature.
Worked Example 01
Smartphone idle runtime
Known
- Battery Capacity (C): 4,000 mAh
- Current Draw (I): 80 mA
Formula
t = C / I
Substitution
t = 4,000 / 80
Result
t = 50 hours
A 4,000 mAh phone battery drawing 80 mA in idle/standby gives a theoretical runtime of 50 hours - or roughly 2 days. Real-world standby is typically 30–40 hours after accounting for partial discharge limits and background tasks.
Worked Example 02
Sizing a battery for an IoT sensor
Known
- Target Life (t): 720 h (30 days)
- Average Current Draw (I): 5 mA
Formula
C = t × I
Substitution
C = 720 × 5
Result
C = 3,600 mAh
A remote sensor drawing 5 mA on average needs at least 3,600 mAh for 30 days of operation. In practice, select a 4,500–5,000 mAh cell to account for temperature derating and depth-of-discharge limits.
Worked Example 03
Maximum current draw for a wearable
Known
- Battery Capacity (C): 300 mAh
- Target Life (t): 24 h
Formula
I = C / t
Substitution
I = 300 / 24
Result
I = 12.5 mA
A smartwatch with a 300 mAh battery must average no more than 12.5 mA to last 24 hours. This becomes the system-level current budget: display, processor, sensors, and radio combined must average ≤ 12.5 mA.
Applications
- 01Estimating how long a phone, tablet, or portable device will last on a charge
- 02Sizing a battery pack for an IoT sensor, Arduino, or embedded system
- 03Calculating the minimum battery capacity for a target unattended runtime (e.g., 7-day wildlife camera, 24-hour remote monitor)
- 04Finding the maximum allowable current draw to meet a product battery-life specification
- 05Rough-checking solar battery storage sizing for overnight loads
Assumptions
- 01Current draw is constant - the device does not switch between power modes (sleep, active, transmit, etc.).
- 02The battery is fully discharged - no depth-of-discharge safety margin is applied.
- 03Ideal battery behavior - no Peukert derating, no internal resistance, no temperature effect, no aging.
- 04Capacity is the full rated value - battery is new and at room temperature.
Where This Model Stops
- 01Does not account for depth-of-discharge limits - for Li-ion, typically stop at 20% remaining; for lead-acid, stop at 50%.
- 02Does not model the Peukert effect - at high discharge rates (above C/5), especially in lead-acid batteries, actual capacity is lower than rated.
- 03Temperature effects ignored - battery capacity drops significantly below 0 °C and above 45 °C.
- 04Does not handle variable or duty-cycled loads - calculate an average current manually before entering it.
- 05Does not convert watt-hour labels into amp-hour capacity unless you know the nominal battery voltage.
- 06Does not check maximum discharge current, BMS cutoff, inverter surge capacity, charging limits, or battery cycle-life impact.
References
- [1]Battery Life Estimation for Low-Power Embedded Systems (SLUA140)
Texas Instruments
Derives t = C / I and discusses derating factors for practical embedded applications.
- [2]9.1 Battery Capacity and Discharge - All About Circuits
All About Circuits
Explains mAh, Ah, C-rate, and how battery capacity interacts with discharge current.
Frequently Asked Questions
What is mAh and how does it relate to battery life?
mAh stands for milliamp-hours. It is a measure of electric charge - specifically, how much current a battery can deliver over time. A 2,000 mAh battery can supply 2,000 mA for exactly 1 hour, or 200 mA for 10 hours, or 20 mA for 100 hours. Dividing mAh by your device's average current draw in mA gives battery life in hours.
Why is actual battery life shorter than the calculated value?
The formula t = C / I assumes ideal conditions: constant current, full discharge, no internal resistance, and a new battery at room temperature. In reality, (1) you should stop at 20–50% charge to protect the battery chemistry; (2) high discharge rates reduce available capacity (Peukert effect); (3) cold temperatures can reduce capacity by 20–40%; and (4) an aged battery has lower rated capacity. A 70–85% efficiency factor is a common rule-of-thumb adjustment.
Should I use mAh and mA, or Ah and A?
Either works as long as you are consistent. The milli-prefix cancels: mAh ÷ mA = h, just as Ah ÷ A = h. Consumer electronics (phones, earbuds, power banks) are universally rated in mAh, while automotive, marine, and solar systems use Ah. This calculator accepts both - just make sure you select the matching units for capacity and current.
What is average current draw, and how do I calculate it?
Most devices cycle between power states: a microcontroller might sleep at 10 µA and transmit at 30 mA. Average current is the time-weighted mean: I_avg = (I_sleep × t_sleep + I_active × t_active) / (t_sleep + t_active). For example, if a device sleeps for 9 seconds at 10 µA and transmits for 1 second at 20 mA, I_avg = (10 µA × 9 + 20,000 µA × 1) / 10 = 2,009 µA ≈ 2 mA.
What is C-rate and how does it affect battery life?
C-rate describes discharge current as a multiple of the battery's capacity. A 1C rate for a 1,000 mAh battery means drawing 1,000 mA - fully discharging it in 1 hour (theoretically). At high C-rates (above C/5 for lead-acid, above 2C for Li-ion), internal resistance causes voltage drop and heat, reducing deliverable capacity below the rated value. For most consumer electronics operating at C/10 or lower, this effect is negligible and t = C / I is accurate enough.
How is this different from the Amp Hour Calculator?
Battery Life starts from charge capacity and current to estimate runtime. The Amp Hour Calculator is broader for sizing battery capacity, converting Ah and Wh, and comparing loads; use it when your problem is more about battery pack sizing than simple runtime.
What if my device load is listed in watts?
Convert watts to current with I = P / V using the battery or bus voltage. For example, a 24 W load on a 12 V battery draws about 2 A before inverter or regulator losses. If an inverter is 85% efficient, divide by 0.85 to get a more realistic battery current.