Battery Life Calculator

Estimate How Long Your Battery Will Last

This calculator helps you estimate how long a battery can power a device based on capacity, average current draw, reserve percentage, and battery health. It is useful for planning phone usage, sizing power banks, checking if a laptop will last a work session, or estimating runtime for cameras, handheld consoles, IoT devices, and other electronics.

The estimate assumes that the device draws a roughly steady average current in milliamps (mA). Real devices vary their power use from moment to moment, so treat the result as an informed approximation rather than a guarantee.

Key Inputs and What They Mean

Battery capacity (mAh)

Battery capacity describes how much charge the battery can store. In this calculator it is entered in milliamp-hours (mAh). Higher capacity means more energy available and therefore potentially longer runtime.

• Small IoT sensor: 200–1,000 mAh
• Typical smartphone: 3,000–5,500 mAh
• Large phone / small tablet: 5,000–8,000 mAh
• Power bank: 5,000–30,000 mAh (sometimes more)

If your capacity is specified in watt-hours (Wh) instead of mAh, you can convert it approximately using the battery voltage:

Average consumption (mA)

Average consumption is how much current your device draws while it is running, on average. This can change a lot with usage:

• Screen brightness, CPU load, and wireless radios (Wi‑Fi, 5G, Bluetooth) all increase current draw.
• Idle or standby states use far less current than gaming, video streaming, or 3D rendering.

Ways to approximate this value:

• Some devices or apps show average power in mA or in watts. If it is shown in watts, divide by the battery voltage and multiply by 1000 to get mA.
• Manufacturer datasheets sometimes list typical or maximum current draw.
• For a rough estimate, choose a value between light and heavy usage, e.g., halfway between idle and gaming numbers.

Battery voltage (V)

The battery voltage is the nominal voltage of the pack. A single lithium‑ion cell is often rated around 3.6–3.8 V, while multi‑cell packs (like laptop batteries) may be 7.2 V, 11.1 V, or higher. The calculator mainly uses voltage if you are thinking in terms of watt‑hours or comparing different packs, but the core runtime formula is based on mAh and mA.

Reserve capacity (% kept in reserve)

Reserve capacity is the percentage of the battery you do not plan to use. Many people avoid draining a battery to 0% to preserve health or keep an emergency buffer. For example:

• Set 10–20% reserve if you always want some battery left as a safety margin.
• Set 0% reserve if you want to know the theoretical maximum runtime from 100% down to empty.

Battery health (%)

Battery health represents how much of the original capacity is still available. A brand‑new battery is 100%. Over time, batteries lose capacity:

• 90% health → you effectively have 90% of the rated mAh.
• 70% health → run time will be roughly 30% shorter than new.

Many phones and laptops show a battery health figure in their settings; you can enter that value here to get a more realistic estimate.

How the Calculator Computes Battery Life

The basic idea is straightforward: take the usable battery capacity and divide it by the device's average current draw.

Step 1: Adjust capacity for health

First, the rated capacity is scaled by the battery health percentage:

Here, C_rated is the original capacity in mAh and health is the remaining capacity as a percentage.

Step 2: Account for reserve percentage

Next, only the portion of capacity that you intend to use is considered. If you keep some percentage in reserve:

For example, if battery health is 80% and you keep 20% in reserve, you only plan to use 0.8 × 0.8 = 64% of the original rated capacity.

Step 3: Compute runtime in hours

The estimated runtime in hours is then:

where I_avg is the average current draw in mA. The calculator can also express this in minutes by multiplying by 60.

Interpreting the Results

After you enter your values and run the calculation, you will see an estimated runtime, typically displayed in hours and minutes. Use this number as a guide for planning, not as a precise prediction.

• If the estimate seems too high, your consumption value may be unrealistically low, or your reserve/health settings may make more capacity usable than you actually experience.
• If the estimate seems too low, you may have used a worst‑case consumption figure (like gaming or heavy video editing) while your typical use is lighter.
• For safety‑critical cases (e.g., medical devices, network gear, or remote monitoring), consider derating the result further and testing in real conditions.

Worked Example: Smartphone Streaming Video

Imagine a smartphone with these characteristics:

• Battery capacity: 4,500 mAh
• Average consumption while streaming HD video: 600 mA
• Battery voltage: 3.8 V (nominal)
• Reserve capacity: 15% (you do not want to go below 15% charge)
• Battery health: 90% (a one‑year‑old phone)

Step by step:

1. Adjust for health: 4,500 mAh × 0.90 = 4,050 mAh effective capacity.
2. Apply reserve: 4,050 mAh × (1 − 0.15) = 4,050 mAh × 0.85 ≈ 3,442.5 mAh usable.
3. Compute runtime: 3,442.5 mAh ÷ 600 mA ≈ 5.74 hours.

The calculator would therefore show an estimated runtime of about 5 hours and 45 minutes of continuous streaming before you reach your 15% reserve level.

Typical Capacities and Consumption by Device Type

The table below summarizes rough ranges for battery capacities and average current draws for common categories of devices. These are only typical values; individual models can be lower or higher.

*Runtime examples assume 100% health, 0% reserve, and a constant current draw at the heavy‑use figure.

Assumptions and Limitations

Battery life calculations are inherently approximate. This calculator makes several simplifying assumptions that you should be aware of when interpreting the results:

• Constant average current: The calculator assumes your device draws the same average current throughout the entire discharge. Real‑world devices often fluctuate widely between idle, bursts of activity, and sleep states.
• Stable temperature: Battery capacity and efficiency decrease in extreme cold and, to a lesser extent, extreme heat. The estimate assumes normal room‑temperature conditions.
• Healthy internal resistance: As batteries age, internal resistance increases, reducing usable capacity under high load. Battery health percentage only approximates this behavior.
• Linear discharge behavior: The model effectively treats the relationship between state of charge and voltage as linear, while real batteries have a more complex discharge curve.
• No conversion losses: Many devices use voltage regulators, DC‑DC converters, or inverters which introduce efficiency losses. Those losses reduce actual runtime but are not explicitly modeled here.
• No safety margins for critical systems: For mission‑critical or safety‑critical equipment, you should apply additional safety factors and perform real‑world tests instead of relying on a simple online estimate.

Because of these limitations, the result should be viewed as a best‑effort estimate under average conditions. For planning travel, gaming sessions, or daily use, this is often sufficient. For engineering design or professional applications, detailed testing and manufacturer data are recommended.

Tips for Getting More Realistic Estimates

• Use scenario‑specific consumption: If your main interest is gaming, use a current draw measured while gaming, not an all‑day mixed‑use average.
• Adjust battery health honestly: If your device is several years old, its health is probably well below 100%. Entering a lower value often brings estimates closer to reality.
• Include a reserve: Set a reserve percentage that matches how you actually use the device—for example, 10–20% if you usually recharge before reaching very low levels.
• Test and refine: After getting an initial estimate, observe your real‑world battery life under similar conditions and tweak the consumption value until the calculator and reality line up more closely.

When to Expect Bigger Differences from the Estimate

The simple model used here can diverge more noticeably from reality in some situations:

• Very high load currents relative to battery size, where voltage sag and internal resistance reduce usable capacity.
• Very low temperatures, which significantly reduce the effective capacity of lithium‑ion and other chemistries.
• Devices with aggressive power management, where current draw alternates between very high peaks and very low idle values.
• Systems that convert between several voltages (for example, power banks driving laptops) where cumulative efficiency losses can be substantial.

In such cases, consider using more conservative input values or adding an extra safety margin to the calculator's result.