Battery mAh Calculator

Understanding the Battery mAh Calculator

A battery runtime calculator divides capacity by current draw and applies an efficiency factor. The efficiency factor is doing considerable work, and even with it the result is an upper estimate, because real devices draw variable current and batteries do not deliver their rated capacity under all conditions.

How it actually works

Enter device current draw in milliamps, battery capacity in milliamp-hours, and an efficiency percentage. The calculator divides capacity by draw and multiplies by efficiency. A 5,000 mAh battery powering a 200 mA load at 80% efficiency gives 20 hours, or 0.83 days.

The deeper context most people miss

Milliamp-hours describe charge, not energy, which is why comparing batteries across voltages using mAh alone is misleading. A 5,000 mAh battery at 3.7 volts holds 18.5 watt-hours while a 5,000 mAh battery at 5 volts holds 25, and the second stores 35% more energy despite the identical mAh figure.

Why real runtime falls short of the calculation

Several effects compound, which is what the efficiency factor crudely represents. Voltage conversion is the largest for most devices: a lithium cell delivers around 3.7 volts nominal, while USB output requires 5 volts, and the boost conversion between them loses energy as heat, typically 10 to 20%. Power banks are the clearest case, which is why a 10,000 mAh power bank delivers considerably less than 10,000 mAh to a phone, usually somewhere around 6,000 to 7,000 mAh of usable charge at the device. Peukert's law describes a second effect: battery capacity falls as discharge rate rises, so a cell rated at a modest discharge current delivers less total charge when drawn down quickly. The effect is pronounced in lead-acid batteries and modest in lithium-ion, but present. Temperature matters considerably, with capacity dropping substantially in cold conditions, which is why phone battery life falls noticeably in winter and why electric vehicle range drops in cold weather. Age degrades capacity, with lithium-ion cells typically retaining around 80% of original capacity after several hundred full charge cycles, and calendar ageing occurring regardless of use. Self-discharge removes a small amount continuously. And the rated capacity itself is measured under specific laboratory conditions that real use rarely matches.

A worked example: why devices are harder to predict than the formula suggests

The 20-hour figure assumes a constant 200 mA draw, and almost nothing draws constant current. A phone alternates between deep sleep at a few milliamps, idle with the screen off at tens of milliamps, screen on at hundreds, and peaks of over a thousand during heavy processing, gaming, or camera use. Cellular radio draw varies enormously with signal strength, since a phone in a weak signal area transmits at higher power and drains substantially faster, which is why battery life collapses in areas with poor coverage. Display is typically the largest single consumer on a phone, and brightness scales draw dramatically. This means a realistic estimate requires a duty cycle model: estimating time spent in each state and the draw in each, then computing a weighted average. For a device with a genuinely constant load, such as a sensor logging at fixed intervals or an LED strip, the calculation is far more reliable, and this is where the formula genuinely earns its keep. For designing a low-power embedded device, the calculation is central, and the design work consists largely of minimising time spent in high-draw states, since a microcontroller sleeping at microamps between brief active periods can run for years on a coin cell while the same chip running continuously would last days.

Deciding how to extend battery life

The levers depend on where the energy goes, and measuring rather than guessing matters. On phones and laptops, screen brightness and screen-on time typically dominate, so reducing both has more effect than most other interventions combined. Background activity, location services, and push notifications all consume energy through radio use, and both major mobile platforms provide per-app battery breakdowns that identify the actual consumers. Cellular signal strength matters, and switching to WiFi in poor coverage areas saves substantially. For laptops, display, processor state, and discrete graphics use dominate. On the battery health side, lithium-ion cells last longest when kept in a moderate state of charge rather than repeatedly cycled between empty and full, and both heat and time at high charge accelerate degradation. This is why several manufacturers now offer optimised charging that holds at around 80% until shortly before you need it, and why leaving a device plugged in at full charge in a hot environment is among the worst things for long-term capacity. Deep discharge to zero should be avoided, and storing a lithium battery long-term is best done at around half charge in a cool place. The old advice to fully discharge periodically applied to nickel-based chemistries and does not apply to lithium-ion.

Reading capacity specifications critically

Battery specifications are frequently presented in ways that flatter the product, and knowing what to check helps. Power bank capacity is almost always stated as cell capacity at 3.7 volts rather than deliverable capacity at 5 volts, so the usable figure is roughly 60 to 70% of the headline number once conversion losses are counted, and reputable manufacturers state both. Watt-hours is the honest unit for comparison, since it accounts for voltage, and it is also the figure airlines use for carry-on restrictions, with limits commonly around 100 Wh without approval. Converting is straightforward: watt-hours equals milliamp-hours times voltage divided by 1,000. Cycle life ratings specify the number of full charge cycles before capacity falls to a stated percentage, usually 80%, and partial cycles count proportionally, so two half-discharges equal one cycle. Charge rate is expressed in C, where 1C is the current that would fully charge or discharge in an hour, and faster charging generates more heat and accelerates degradation, which is the trade-off behind fast charging. Devices increasingly report battery health as a percentage of original capacity, which is more informative than cycle count alone. For any application where runtime matters, measuring actual current draw with a USB power meter or multimeter produces a figure far more useful than any specification.

Variations: chemistries, watt-hours, and load types

Battery chemistry affects nearly everything discussed here. Lithium-ion and lithium-polymer dominate portable electronics, offering high energy density with modest Peukert effects and no memory effect. Lithium iron phosphate trades some energy density for longer cycle life and better thermal safety, which is why it appears in storage and increasingly in vehicles. Nickel-metal hydride retains a niche in rechargeable AA cells, with higher self-discharge unless low-self-discharge variants are used. Lead-acid remains standard for vehicle starting and backup power, with pronounced Peukert effects making runtime calculations at high draw substantially optimistic. Alkaline primary cells have high capacity at low draw and poor performance at high draw. For load types, constant-current loads suit the calculation well, constant-power loads such as switching regulators draw more current as voltage falls during discharge, and pulsed loads such as radio transmissions have peak requirements that can exceed what a small cell can deliver even when average draw is modest, which is a common design failure with coin cells driving radio modules.

Estimating battery runtime realistically

Treat the result as an upper bound, since voltage conversion losses, temperature, discharge rate, and cell age all reduce real capacity below the rated figure. Use watt-hours rather than milliamp-hours when comparing batteries across different voltages, since mAh describes charge rather than energy and a 5,000 mAh cell at 3.7 volts holds far less than one at 5 volts. Expect a power bank to deliver roughly 60 to 70% of its stated capacity to a device, since the headline figure is cell capacity before conversion losses. Build a duty cycle model for devices with variable draw, estimating time in each state rather than assuming a constant load. Measure actual draw with a USB power meter or multimeter where runtime matters, since specifications rarely reflect real use. Keep lithium-ion cells in a moderate charge range rather than cycling between empty and full, and avoid heat, since both accelerate capacity loss. And ignore advice about periodic full discharge, which applied to older nickel chemistries rather than lithium-ion.

What people get wrong

• Comparing batteries by milliamp-hours across different voltages, when mAh measures charge rather than energy and watt-hours is the meaningful comparison.
• Expecting a power bank to deliver its stated capacity, when voltage conversion from 3.7 to 5 volts typically leaves 60 to 70% usable at the device.
• Assuming constant current draw for a phone or laptop, when display, signal strength, and processing state cause draw to vary by more than an order of magnitude.
• Fully discharging lithium-ion batteries periodically, which applied to older nickel chemistries and actually accelerates degradation in lithium cells.

Where the math comes from

Runtime in hours = (Battery Capacity in mAh / Device Draw in mA) × (Efficiency % / 100). The efficiency factor crudely accounts for voltage conversion losses, discharge rate effects, and the gap between rated and deliverable capacity. Note that milliamp-hours measure charge rather than energy; watt-hours, calculated as mAh × voltage / 1,000, is the meaningful unit for comparing across voltages.

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