Battery life calculator projects runtime from capacity and current draw, then adds C-rate, discharge time, and recharge-cycle estimates for planning IoT sensor field deployments.
Battery Life Calculator: Estimate Device Runtime from Capacity and Current Draw
This calculator divides battery capacity by load current to estimate device runtime, then extends that runtime into C-rate, discharge, duty-cycle, and recharge-frequency figures. Embedded systems engineers, IoT product designers, and electronics hobbyists use it to size batteries and estimate recharge intervals before building or ordering hardware.
How to Use the Runtime and C-Rate Calculator
Enter Battery Capacity in milliamp-hours (mAh) and Load Current in milliamps (mA), then calculate. The tool returns Theoretical Battery Life in hours (plus minutes, days, seconds), the discharge C-rate, time to 50% discharge, projected runtime under partial-duty IoT sleep cycles, and how many recharge cycles that runtime implies per day, month, and year.
The Runtime Formula: Capacity Divided by Load Current
$$\text{Battery Life (h)} = \frac{\text{Battery Capacity (Ah)}}{\text{Load Current (A)}}$$
Per Battery University (BU-402: What Is C-rate?), a fully charged 1 Ah battery discharging at 1 A — a 1C rate — is depleted in exactly one hour; at 0.5 A (0.5C) it lasts two hours, and at 2 A (2C) it lasts thirty minutes. The same relationship, rearranged, gives the C-rate itself:
$$\text{C-rate} = \frac{\text{Load Current (A)}}{\text{Battery Capacity (Ah)}}$$
$P_1$ and $P_2$ don’t apply here — this is a single-ratio calculation, not a comparison between two states. $C$ is the rated capacity; $I$ is the load current drawn continuously.
Three input mistakes that throw off the result:
- Mixing Ah and mAh (or A and mA) between the two fields. Since $1 \text{ Ah} = 1000 \text{ mAh}$, a missed conversion throws the runtime off by a factor of 1,000.
- Treating “Theoretical Battery Life” as real-world runtime. Per Battery University, discharging faster than a battery’s rated C-rate lowers the capacity it can actually deliver, so a simple mAh ÷ mA division is a best-case number, not a lab-measured one.
- Confusing the tool’s “Cycles per Year” — a recharge count derived purely from continuous runtime — with a manufacturer’s rated cycle life (how many full charge/discharge cycles a cell survives before capacity fades). These measure different things and shouldn’t be read interchangeably.
Battery Discharge Curve: Capacity vs. Runtime
C-Rate to Discharge Time: Standard Reference Values
| C-Rate | Discharge Time (1 Ah / 1,000 mAh cell) |
|---|---|
| 5C | 12 min |
| 2C | 30 min |
| 1C | 1 h |
| 0.5C (C/2) | 2 h |
| 0.2C (C/5) | 5 h |
| 0.1C (C/10) | 10 h |
| 0.05C (C/20) | 20 h |
Values per Battery University (BU-402: What Is C-rate?), for a 1 Ah (1,000 mAh) cell. Discharge times scale proportionally for other capacities.
Common Questions About Estimating Battery Runtime
What is the standard formula for battery runtime?
Runtime (h) = Battery Capacity (Ah) ÷ Load Current (A), or in milliamp-hour terms, mAh ÷ mA. Per Battery University’s BU-402, this reflects the same relationship that defines the C-rate.
Why does my device run for less time than the calculator predicts?
The calculator gives a theoretical maximum based strictly on nominal capacity. Per Battery University, drawing current faster than a battery’s rated discharge rate reduces the capacity it can actually deliver.
What is a C-rate, and how is it calculated?
C-rate is discharge current expressed relative to battery capacity: C-rate = Load Current (A) ÷ Battery Capacity (Ah). Per Battery University, a 1C rate discharges the full rated capacity in one hour.
Does “Cycles per Year” mean my battery will wear out that fast?
No. That figure is a recharge count based on continuous runtime, not a rated cycle life. Manufacturer cycle-life ratings — how many charge/discharge cycles before capacity fades — are a separate, chemistry-specific specification.
How does the IoT duty-cycle runtime differ from theoretical battery life?
It divides the base runtime by the fraction of time the device actively draws the specified load, since a device that sleeps most of the time uses far less average current than one running continuously.