This tool helps you quickly estimate the theoretical runtime of a device based on its battery capacity and current draw. An accurate calculation can help you better understand a device’s performance and assist in project design.
Battery Life
Battery Life Formula
The fundamental principle behind estimating battery life is the relationship between energy stored in the battery (capacity) and the rate at which that energy is consumed (current draw). At its core, this relationship can be expressed by the simple formula: Battery Life (in hours) = Battery Capacity (in mAh) / Current Draw (in mA).
To use this calculator effectively, you need to know two key values: the battery’s capacity and the device’s average current consumption. Battery capacity is typically measured in milliampere-hours (mAh) or ampere-hours (Ah) and is usually printed on the battery itself or listed in its datasheet. Current draw, measured in milliamperes (mA) or amperes (A), is the average amount of current the device consumes while in operation.
Understanding Battery Capacity (mAh)
Battery capacity is a measure of the total charge a battery can deliver at its rated voltage. A 2000mAh battery, for example, is theoretically capable of supplying 2000mA for one hour, 1000mA for two hours, 200mA for ten hours, and so on. The “m” in mAh stands for “milli,” which is one-thousandth, so 1 Ah = 1000 mAh.
It is important to note that a battery’s rated capacity is often measured under specific, controlled conditions (e.g., a constant, low discharge rate at room temperature). In real-world applications, factors like temperature, discharge rate, and battery age can significantly affect the actual usable capacity. For instance, discharging a battery very quickly (at a high C-rate) often results in a lower total delivered capacity compared to a slow discharge. Similarly, extreme cold or hot temperatures can temporarily or permanently reduce a battery’s capacity.
Determining Device Current Consumption (mA)
This is often the most challenging variable to determine accurately. The current draw of a device is rarely constant. It can fluctuate dramatically depending on what the device is doing. A microcontroller in a low-power sleep mode might draw only a few microamperes (µA), while the same device with a Wi-Fi module transmitting data might draw hundreds of milliamperes (mA).
To get a realistic estimate for a complex device, you need to calculate the average current draw over a typical usage cycle. This involves creating a “power profile” of your device’s operation. For example, if a device spends 90% of its time in a sleep state, drawing 10µ,A and 10% of its time in an active state drawing 200mA, the average current consumption would be:
(0.90 * 0.01mA) + (0.10 * 200mA) = 0.009mA + 20mA = 20.009mA
This more advanced calculation provides a much more accurate prediction of battery life than simply using a peak current value. For most simple projects, however, using the typical current draw is a good starting point.
Factors That Influence Real-World Battery Life
While the calculator provides a solid starting point, the real world is full of variables that can change the outcome. Understanding these factors is key to managing expectations and optimizing your device’s performance.
Battery Aging: Batteries degrade over time and with each charge-discharge cycle. A one-year-old battery will not have the same capacity as a brand-new one.
Temperature: Extreme temperatures, both hot and cold, can negatively impact a battery’s performance and capacity. Cold temperatures can temporarily reduce available capacity, while high temperatures can accelerate long-term degradation.
Internal Resistance: As a battery discharges, its internal resistance increases. This can cause the voltage to drop, potentially affecting the device’s ability to function correctly and reducing overall runtime, especially under heavy loads.
Discharge Rate (C-rate): Discharging a battery at a high C-rate (i.e., drawing a large current relative to its capacity) can lower its total usable capacity. For example, a battery may deliver 100% of its rated capacity at a 0.2C rate but only 90% at a 1C rate.
State of Charge (SoC) and Depth of Discharge (DoD): Many devices operate best within a specific voltage range, and a battery’s voltage drops as it discharges. The full “mAh” capacity is only usable if the device can operate from the full to empty voltage range. Additionally, deep discharges (100% DoD) can be more stressful for a battery than shallow discharges (e.g., 50% DoD), potentially shortening its cycle life.
Parasitic Drain: Even when “off,” many devices still draw a small amount of current to maintain state, run real-time clocks, or listen for user input. This small, continuous current draw, known as parasitic drain, can significantly impact battery life over long periods of inactivity.
FAQ
This calculator provides a theoretical estimate. Actual battery life is influenced by many factors, including battery aging, temperature, load fluctuations, and device sleep modes.
This is common. The actual current draw may not be constant, but varies with device activity. Also, the usable battery capacity can diminish with use and over time.
The most accurate method is to use a digital multimeter (DMM) or a professional power analyzer to measure the current under typical operating conditions. You can measure the average current for different operating modes (e.g., idle, active, peak).
A 20000mAh power bank (with an actual output of around 13,300mAh) can typically charge tablets and laptops about 1.5 times.
In normal situations, a 10,000mAh battery usually does not last three days on a smartphone. This is true unless the phone is used very lightly or it has low power use. However, if it’s used to power other low-power devices or as a power bank, then three days is more achievable.
