Battery Life Calculator

What mAh measures, and what it leaves out

Milliamp hours is a count of charge, not a count of energy. One mAh is one thousandth of an amp flowing for one hour. A 5,000 mAh battery can, in theory, supply 5,000 mA for one hour, or 500 mA for ten hours, or 250 mA for twenty.

That ratio is the whole idea behind runtime math, and it is why the same 5,000 mAh pack behaves differently in a phone and in a power bank. The phone asks for a few hundred milliamps. A laptop asks for a few thousand. The battery does not know or care which one is attached.

To compare packs of different voltages you need watt-hours, which is charge times voltage. A 5,000 mAh cell at 3.7 volts stores 18.5 Wh. A 10,000 mAh power bank at 3.7 volts stores 37 Wh. A 5,000 mAh pack rated at 5 volts stores 25 Wh, which is more energy in the same milliamp hours, because the voltage is higher.

This is where most shopping comparisons go wrong. Two power banks both labeled 20,000 mAh can differ by 20% in real energy if one is measured at 3.7 volts and the other at 5 volts, and neither label is lying.

Runtime is one division: usable mAh over draw

Hours of runtime equals usable capacity in mAh divided by the current draw in mA. Nothing else is needed for a first estimate, and it gets you within about 15% of what a meter would show.

Take a 5,000 mAh phone battery with 4,250 mAh usable in practice, powering a device that pulls 400 mA. That is 4,250 divided by 400, or 10.6 hours, which is 10 hours 38 minutes. Halve the draw to 200 mA and the same battery runs 21.3 hours.

The draw is rarely constant. A phone idles at 50 mA and spikes past 1,000 mA when the camera opens, so the honest way to use this math is with an average. A phone that lasts 12 hours of mixed use is averaging about 350 mA from a 4,250 mAh usable pack.

If you know the wattage instead of the current, convert first. Watts divided by volts gives amps, and amps times 1,000 gives milliamps. A 5 W device on a 5 volt rail pulls 1,000 mA.

Why the label is optimistic: the 80 to 90% rule

A battery rated at 5,000 mAh does not deliver 5,000 mAh to your device. Three things eat the difference: the discharge cut-off voltage, the conversion losses in the power path, and the fact that a cell is rated at a slow discharge at room temperature.

In practice, plan on 85% of the label reaching the device. For a phone that is a fair average. For a power bank, which has to boost 3.7 volts up to 5 volts and then the phone converts it back down, 75% to 85% is more realistic. For a laptop pack, 90% is achievable because the pack voltage already matches the system.

Cold makes it worse in a hurry. A lithium-ion cell at 0°C (32°F) gives up roughly 10% to 20% of its capacity, and at minus 10°C (14°F) the loss can reach 30%. That is why a phone that lasts all day in July dies at 4 pm on a ski lift.

Heat costs capacity in the other direction and does permanent damage. Running a cell above 35°C (95°F) accelerates the chemical side reactions that consume lithium, so a phone left on a dashboard in summer loses capacity that never comes back.

Worked example: a 5,000 mAh phone

A mid-range phone with a 5,000 mAh battery at 3.7 volts, used in a way that averages 400 mA.

• Nominal energy: 5,000 x 3.7 / 1,000 = 18.5 Wh
• Usable capacity at 85%: 4,250 mAh
• Runtime at 400 mA: 10.6 hours, or 10 hours 38 minutes
• Runtime at 200 mA, light use: 21.3 hours
• After 500 full cycles: about 80% of the original, or 3,400 mAh usable
• Runtime after those cycles: 8.5 hours at the same 400 mA

That last line is what people notice before they look at a battery health screen. The phone is not slower, the apps are not heavier, the battery holds less. A drop from 10.6 to 8.5 hours is 20%, and it happens over 500 cycles, which at one full cycle a day is about 16 months.

Charging twice a day at 50% each time is one cycle, not two. A cycle is a full battery's worth of charge, counted cumulatively. That is why a phone charged in small top-ups lasts years longer than one run from 100% to 0% every day.

Cycles, not years, are what wear a battery out

A charge cycle is one full battery's worth of energy in and out, however it is split. Two half charges count as one cycle. Five 20% top-ups count as one cycle. The counter is cumulative charge, not the number of times you plugged in.

Manufacturers rate a cell by the number of full cycles it survives before capacity falls to 80% of new. That 80% mark is the industry convention, and it is also roughly where a phone stops lasting a full day.

A laptop looks terrible in that table and is not, because most people do not run a laptop battery through a full cycle a day. A laptop plugged in at a desk might see 100 cycles a year, which puts 400 cycles at four years. An electric car at 12,000 miles a year uses about 40 full cycles, so 1,200 cycles is 30 years of driving, which is why car packs outlast the cars.

Calendar age also matters, independent of cycles. A cell loses 2% to 3% of capacity a year just sitting at a high state of charge, so a battery that is never used still ages. Storage at 40% to 50% charge in a cool room is the slowest way to age a cell.

Where your battery sits after two years

A simple model gets close enough for planning: capacity falls by 20 percentage points over the rated cycle count, then more slowly. Two years of one cycle a day is 730 cycles, which is past a 500 cycle rating.

Run the arithmetic on a 500 cycle cell: 730 divided by 500 is 1.46, and 20 points times 1.46 is 29 points, so capacity is about 71% of new. On an 800 cycle cell the same two years leaves 82%, because 730 divided by 800 is 0.91 and 20 times 0.91 is 18 points.

That is why the same phone model gets widely different battery complaints from two owners. The one who charges overnight from 20% to 100% every day hits 730 cycles in two years. The one who tops up at a desk and rarely drops below 40% might hit 300 cycles in the same time and still be above 90%.

Signs that the capacity number is already low, regardless of what a settings screen says:

• The phone shuts down at 15% or 20% instead of running to 1%.
• It gets warm during a charge that used to be cool.
• Battery percentage drops in visible jumps, such as 40% to 32% in a minute.
• The case bulges or the screen lifts at one edge, which means a swollen cell and a fire risk. Stop using it.

A replacement cell is usually $50 to $120 installed on a phone and $80 to $150 on a laptop. Weigh that against the cost of a new device before deciding the whole thing is worn out.

Habits that shorten battery life

Chemistry sets the ceiling, and habits decide how fast you reach it. The four that matter most, in order of damage:

• Heat. A cell held at 40°C (104°F) loses capacity about twice as fast as one at 25°C (77°F). Fast charging in a hot car or under a pillow is the worst combination.
• Full charge and full discharge. Sitting at 100% stresses the cathode, and dropping to 0% stresses the anode. The 20% to 80% band is where a lithium-ion cell is happiest.
• Fast charging every day. Not fatal on its own, since phones manage the current, but the heat it generates is. A 65 W charge on a phone that accepts 25 W just makes heat.
• Deep storage at full charge. A spare power bank left at 100% in a drawer for a year can lose 10% to 20% of its capacity without a single cycle.

The practical routine is simple. Charge to 80% when you do not need the range, use a slower charger overnight, keep the phone out of direct sun, and store spare packs at about half charge. None of that is exotic and together it can double the cycles you get before 80%.

What does not matter much: leaving the phone on the charger after it hits 100%, since modern phones stop charging and run from the cable. And wireless charging is fine on its own, though it generates more heat than a cable for the same energy.

Watt-hours, airlines and picking a power bank

Once you work in watt-hours, airline rules and power bank shopping both get easier. The US limit for lithium-ion batteries in carry-on baggage is 100 Wh without approval, and up to 160 Wh with airline approval. Spare batteries cannot go in checked bags at all.

Convert the label before you fly. A 20,000 mAh power bank at 3.7 volts is 74 Wh, which is under the limit. A 27,000 mAh pack is 99.9 Wh, right at it. A 30,000 mAh pack is 111 Wh, which needs approval and will get pulled at some security lanes. The FAA lithium battery guidance spells out the tiers, and the Department of Energy's energy storage pages explain how cells are tested and rated in the first place.

Two shopping rules follow from the math. First, compare packs in watt-hours, not milliamp hours, because that is the number that predicts how many phone charges you get. Second, subtract 15% for conversion losses: a 74 Wh pack gives about 63 Wh to your phone, which at 18.5 Wh per phone charge is 3.4 charges, not 4.

Capacity also fades, so a three year old pack is not the pack you bought. A 20,000 mAh pack used twice a week hits 300 cycles in about three years, which is 80% of its rating, so it holds closer to 16,000 mAh. That is the honest reason a pack that used to charge a phone four times now manages three.

Runtime figures are estimates for planning. Real draw varies with screen brightness, temperature, signal strength and the age of the cell. Check the device maker's rated watt-hours before you fly, and replace any battery that is swollen.