A battery's printed capacity almost never matches how long it actually lasts in real use, and the gap gets bigger the harder you push it. Draining a battery fast, running it in the cold, or comparing two batteries rated in different units all throw off a simple mental estimate.

The **battery life calculator** turns a battery's rated capacity and a device's current draw into a realistic runtime estimate, correcting for real-world factors like discharge rate and efficiency losses. This article covers exactly how the math works, why fast discharge costs you more capacity than it seems like it should, and where people most often get the estimate wrong.

What Is a Battery Life Calculator and Why Does It Matter

A **battery life calculator** estimates how long a battery will power a device, based on the battery's capacity and the device's current draw. Consumers use it to compare power banks and flashlights, while off-grid solar users and hobbyists use it to size deep-cycle batteries for inverters and electric equipment. The core method: runtime equals capacity divided by current draw, adjusted for real-world efficiency losses. Getting it wrong means an undersized battery bank, a power bank that dies mid-charge, or a flashlight that goes dark sooner than expected.

What You Need Before You Start

Have this information ready before you enter anything into the calculator.

- The battery's rated capacity, in milliamp-hours (mAh), amp-hours (Ah), or watt-hours (Wh).

- The battery's nominal voltage, needed if you're converting between amp-hours and watt-hours.

- The device's current draw, in milliamps or amps, or its power draw in watts if that's how it's rated.

- The battery chemistry, such as lithium-ion, LiFePO4, or lead-acid, since this changes both the efficient discharge rate and the safe depth of discharge.

- Whether the load is constant, like a flashlight, or variable, like a phone with a screen that turns on and off.

- The maximum safe depth of discharge for your battery type, since fully draining some batteries shortens their lifespan or isn't physically possible.

How to Use This Battery Life Calculator Step by Step

Follow these steps to get a realistic runtime estimate.

1. Enter your battery's rated capacity and select its unit, mAh, Ah, or Wh.

2. Enter the nominal voltage if you're working in amp-hours and need a watt-hour comparison.

3. Enter the device's current or power draw, matching the unit your device's specification sheet uses.

4. Select the battery chemistry so the calculator can apply the right Peukert exponent and depth of discharge limit.

5. Click Calculate to see both the ideal runtime and the corrected, real-world runtime estimate.

6. Click Show Steps to see the depth of discharge and efficiency adjustments applied separately.

7. Compare the corrected estimate to any real-world reviews or your own past experience with a similar battery before relying on it for critical planning.

The Battery Life Calculator Formula Explained Simply

The simplest version of battery life is just the rated capacity divided by the current being drawn, similar to dividing a tank of gas by a car's fuel consumption rate. Real batteries don't behave quite this simply, though, since pulling current out faster actually removes more total capacity than a straight division would suggest, an effect first described by German scientist Wilhelm Peukert.

Basic Runtime = Capacity (Ah) / Current Draw (A)

Peukert-corrected Runtime = Capacity (Ah) / (Current Draw (A) raised to the power n)

- Capacity: the battery's rated amp-hours, typically measured by the manufacturer at a specific, standardized discharge rate.

- Current Draw: the actual current the device pulls from the battery, in amps.

- n (Peukert exponent): a battery-specific constant describing how much capacity is lost at higher discharge rates, typically 1.0 to 1.05 for lithium-ion and 1.1 to 1.3 for lead-acid.

The Peukert exponent works because internal resistance and chemical reaction speed inside a battery both get less efficient at higher currents, so the battery effectively delivers less total energy the harder it's pushed. This method is a widely accepted formula in battery engineering, known as Peukert's Law, developed in 1897 and still used today as the standard way to correct simple runtime math for real-world discharge rates.

Battery Life Calculator Worked Examples

Example1:Beginner Case - Estimating Flashlight Runtime From a Simple Battery Pack

A camper wants to know how long a 3,000 mAh battery pack will run an LED flashlight that draws a steady 300 mA.

1. Identify the values: capacity equals 3,000 mAh, current draw equals 300 mA.

2. Apply the basic runtime formula: 3,000 divided by 300 equals 10 hours.

3. Since lithium-ion batteries have a Peukert exponent close to 1.0 at this low, steady discharge rate, little to no correction is needed.

4. Apply a typical real-world efficiency factor of about 85 to 90 percent to account for conversion losses: 10 hours times 0.87 equals approximately 8.7 hours.

The camper can plan on roughly 8 to 9 hours of real-world flashlight use from this battery pack, and should treat the simple 10-hour figure as an upper bound rather than an expected result.

Example2:Advanced Case - Sizing an Off-Grid Lead-Acid Battery Bank Under Heavy Load

A solar off-grid user has a 100 amp-hour lead-acid battery, rated at the standard 20-hour discharge rate, and wants to run a power inverter that draws 20 amps continuously, well above that rated rate.

1. Identify the values: capacity equals 100 Ah, current draw equals 20 A, Peukert exponent for lead-acid is approximately 1.2.

2. Apply the basic formula first, for comparison: 100 divided by 20 equals 5 hours, the naive estimate.

3. Apply Peukert's Law instead: corrected runtime equals 100 divided by 20 raised to the power 1.2, which works out to approximately 3.2 hours.

4. Apply the lead-acid depth of discharge limit, typically 50 percent to preserve battery lifespan: 3.2 hours times 0.5 equals approximately 1.6 hours of safely usable runtime.

The off-grid user should plan for roughly 1.6 hours of safe runtime at this heavy 20-amp draw, far less than the naive 5-hour estimate, and should consider a larger battery bank or a lower-draw appliance if longer runtime is needed.

Example3:Edge Case - Comparing Two Batteries With the Same mAh Rating but Different Voltage

A shopper is comparing two power banks, both rated at 10,000 mAh, but one runs at 3.7 volts and the other at 7.4 volts, and wants to know if they actually store the same amount of energy.

1. Convert both to watt-hours to make a fair comparison: watt-hours equals amp-hours times voltage.

2. Convert the first power bank: 10,000 mAh equals 10 Ah; 10 Ah times 3.7 volts equals 37 watt-hours.

3. Convert the second power bank: 10 Ah times 7.4 volts equals 74 watt-hours.

4. Compare the results: the 7.4 volt power bank actually stores twice the usable energy of the 3.7 volt one, despite having the identical mAh rating.

This is a common and costly mistake, since mAh alone doesn't reveal total energy storage unless the voltage is also known, and a shopper comparing two products purely by mAh rating could easily choose the lower-capacity option without realizing it.

Example4:Regional Variant - Converting a European Ah-Rated Deep-Cycle Battery for a US mAh-Rated Comparison

A US hobbyist is comparing a European deep-cycle marine battery, rated at 120 Ah at 12 volts, against a US consumer power station rated at 50,000 mAh at 12 volts, and wants both figures on the same basis.

1. Recognize that Ah and mAh are the same unit at different scales: 1 Ah equals 1,000 mAh.

2. Convert the European battery's rating: 120 Ah times 1,000 equals 120,000 mAh.

3. Compare directly, since both are now in mAh at the same 12-volt rating: 120,000 mAh versus 50,000 mAh.

4. Confirm the energy comparison in watt-hours as well: 120 Ah times 12 volts equals 1,440 Wh, versus 50 Ah times 12 volts equals 600 Wh.

The European deep-cycle battery stores nearly two and a half times the usable energy of the US power station once both are converted to the same unit, showing why comparing raw Ah and mAh figures across product listings from different regions requires a quick unit conversion before any real comparison is valid.

Battery Life Calculator Results What the Numbers Mean

The basic runtime figure, capacity divided by current, is always the highest possible estimate, and real-world runtime is essentially always lower than this number. A Peukert-corrected estimate that's close to the basic figure means the discharge rate is low relative to the battery's rated capacity, common in low-draw devices like sensors or flashlights. A Peukert-corrected estimate that's noticeably lower than the basic figure means the device is drawing current fast relative to the battery's design, common in high-power tools, inverters, or e-bikes.

The depth of discharge adjustment matters just as much as the discharge rate correction, especially for lead-acid batteries, where regularly draining below 50 percent shortens the battery's overall lifespan even if a single discharge cycle seems fine. Lithium-based batteries, particularly LiFePO4, tolerate a much deeper discharge, often 80 to 95 percent, which is a major reason they've become the preferred choice for high-drain or long-runtime applications.

The most common misunderstanding is assuming a battery with double the mAh rating always lasts twice as long, without checking whether the voltage or discharge rate is also different. As the edge case above shows, mAh alone can be a misleading number, and watt-hours or a Peukert-corrected runtime give a much more honest comparison between two different batteries or devices.

How Your Inputs Change the Result

The discharge rate relative to a battery's rated test rate is the single biggest factor separating a naive estimate from a realistic one. A lead-acid battery discharged near its rated 20-hour rate performs close to its naive estimate, while the same battery discharged in under an hour, as in the off-grid example, can lose 30 to 40 percent of its effective capacity to the Peukert effect alone.

Battery chemistry changes both the Peukert exponent and the safe depth of discharge, and the two together compound significantly. Lithium-ion and LiFePO4 batteries barely lose capacity at high discharge rates and can be drained much deeper, while lead-acid batteries lose capacity quickly at high rates and should be drained much less deeply, meaning two batteries with an identical Ah rating can deliver dramatically different real-world runtimes.

Temperature, while not part of the core formula, shifts the whole calculation in cold conditions. A lithium-ion battery operating near freezing can lose 20 to 40 percent of its usable capacity compared to room temperature, an effect worth budgeting for separately from the discharge-rate correction covered here.

Battery Type | Peukert Exponent | Typical Safe Depth of Discharge

Lithium-ion / LiFePO4 | 1.0 to 1.05 | 80 to 95 percent

Lead-acid (flooded or AGM) | 1.1 to 1.3 | 50 percent, less for car starter batteries

Nickel-metal hydride | 1.05 to 1.1 | 70 to 80 percent

Battery Life Calculator Common Mistakes to Avoid

- Ignoring the Peukert effect entirely for lead-acid batteries, which overestimates runtime by 20 to 40 percent at high discharge rates compared to a simple capacity-divided-by-current calculation.

- Comparing two batteries by mAh rating alone without checking voltage, which can hide a large difference in actual stored energy, as shown in the edge case above.

- Forgetting that a battery's rated capacity is measured at a specific, standardized discharge rate, usually a 20-hour rate for lead-acid, pulling more current than that rate reduces effective capacity significantly.

- Draining a lead-acid battery below its safe depth of discharge, typically 50 percent, which shortens its usable lifespan even if a single discharge seems to work fine.

- Applying a lead-acid Peukert exponent to a lithium battery, or the reverse, which produces a runtime estimate that's significantly off in either direction.

- Ignoring temperature effects on lithium-ion batteries, especially in cold outdoor use, where 20 to 40 percent of usable capacity can disappear compared to a room-temperature estimate.

- Mixing up Ah and mAh during a unit conversion, since a factor-of-1,000 error is easy to introduce when comparing a European Ah-rated product to a US mAh-rated one.

- Assuming a device's average power draw applies the whole time, when many real devices, like phones and sensors, spend most of their time in a low-power sleep state that dramatically extends real-world battery life beyond a worst-case active-use estimate.

Key Terms Explained

Amp-hour

An amp-hour is a unit of battery capacity representing one amp of current delivered continuously for one hour.

Watt-hour

A watt-hour is a unit of total stored energy, calculated by multiplying amp-hours by voltage, useful for comparing batteries at different voltages.

Peukert exponent

The Peukert exponent is a battery-specific constant describing how much usable capacity is lost as discharge current increases.

Depth of discharge

Depth of discharge is the percentage of a battery's total capacity that can be safely used before recharging, without shortening its lifespan.

C-rate

C-rate describes a battery's charge or discharge current relative to its rated capacity, where 1C means discharging the full capacity in one hour.

Nominal voltage

Nominal voltage is a battery's standard rated voltage, used as the reference point for converting between amp-hours and watt-hours.

Self-discharge

Self-discharge is the gradual loss of stored energy a battery experiences over time even when not connected to any device.

Deep-cycle battery

A deep-cycle battery is designed to be regularly discharged to a lower level and recharged, unlike a starter battery designed for brief, high-current bursts.

How This Calculator Works Methodology

This calculator applies the basic runtime formula, capacity divided by current draw, then corrects it using Peukert's Law, developed by Wilhelm Peukert in 1897 and still the standard method for adjusting battery runtime estimates for discharge rate. The Peukert exponent defaults to typical published values for each battery chemistry, approximately 1.0 to 1.05 for lithium-ion and LiFePO4, and 1.1 to 1.3 for lead-acid, though these can vary by specific manufacturer and model. A depth of discharge limit is applied based on the selected chemistry to reflect the manufacturer-recommended safe usable capacity. This calculator does not independently model temperature effects or battery age-related capacity loss, both of which can further reduce real-world runtime beyond the figures shown here.

Frequently Asked Questions About Battery Life Calculator

1. How do you calculate battery life?

Divide the battery's rated capacity by the device's current draw, both in the same unit, to get a basic runtime estimate. For high discharge rates or lead-acid batteries, apply Peukert's Law to correct for the extra capacity lost at faster discharge speeds.

2. How accurate is a battery life calculator?

It's an estimate, not an exact figure, since real-world runtime also depends on temperature, battery age, and how consistent the actual current draw is. A Peukert-corrected estimate that also accounts for depth of discharge is typically accurate within about 10 to 15 percent of real-world results for steady loads.

3. What is Peukert's Law in simple terms?

Peukert's Law explains that discharging a battery faster removes disproportionately more of its total capacity than a simple division would suggest. A 100 Ah lead-acid battery discharged in one hour might only deliver 60 to 70 usable Ah, not the full 100.

4. Is mAh or Ah the better unit for comparing batteries?

Neither is inherently better, but they must be at the same voltage to compare fairly, or converted to watt-hours for a true energy comparison. As the edge case above shows, two batteries with the same mAh rating can store very different amounts of actual energy if their voltages differ.

5. When does a lead-acid battery calculation not apply to my lithium battery?

Lead-acid Peukert exponents, typically 1.1 to 1.3, and lead-acid depth of discharge limits, typically 50 percent, don't apply to lithium-ion or LiFePO4 batteries, which have exponents near 1.0 and can often be discharged to 80 to 95 percent. Always select the correct chemistry in the calculator before relying on the result.

6. How can I get a more precise battery life estimate?

Use the actual measured current draw of your specific device rather than a nameplate maximum figure, since many devices draw significantly less than their rated peak during normal use. Also select the correct battery chemistry and account for temperature if you're operating outside of typical room-temperature conditions.

7. What is a typical result people check with this tool?

Consumers most often check power bank or flashlight runtime for a known current draw. Off-grid solar users and hobbyists most often check whether a deep-cycle battery can support an inverter or high-draw tool for a target number of hours, which usually requires the full Peukert correction.

8. Should I consult a professional for a large battery bank installation?

Yes. For whole-home solar backup systems, RV power systems, or any installation involving multiple large batteries, consult a qualified solar or electrical professional to confirm sizing, wiring, and safety requirements beyond what a runtime calculator alone can provide.

Related Calculators Worth Bookmarking

- Solar Panel Sizing Calculator: use this alongside battery life to size a complete off-grid solar system rather than just the battery bank alone.

- Amp-Hour to Watt-Hour Converter: helpful for quickly converting between the two most common battery capacity units without a full runtime calculation.

- Inverter Sizing Calculator: relevant for confirming an inverter can handle the peak load before checking how long the battery bank will sustain it.

- Charging Time Calculator: useful for estimating how long a depleted battery will take to recharge from a given charger or solar input.

- Ohm's Law Calculator: pairs with battery calculations when working out current draw from a device's voltage and resistance instead of a nameplate rating.

- Electric Vehicle Range Calculator: relevant since EV range estimation uses the same underlying battery capacity and discharge-rate principles.

- Power Consumption Calculator: helpful for adding up multiple devices' current draw before running a single combined battery life estimate.

- Generator Fuel Consumption Calculator: useful for off-grid users comparing battery-only runtime against a backup generator's fuel-based runtime.

References and Further Reading

1. Peukert W. Ueber die Abhaengigkeit der Kapazitaet von der Entladestromstaerke bei Bleiakkumulatoren. Elektrotechnische Zeitschrift, 1897.

2. Battery University. How to Define Battery Life. Cadex Electronics, 2023.

3. Battery University. Discharge Methods. Cadex Electronics, 2023.

4. Linden D, Reddy TB. Handbook of Batteries. McGraw-Hill, 2019.

Important Disclaimer

This calculator is for educational and informational purposes and provides an estimate only. For safety-critical, off-grid, marine, or large-scale battery installations, consult a qualified electrical or solar professional before finalizing system design.