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Battery life calculator
Put your battery capacity, current draw and use per day in: a 4,000 mAh battery at 200 mA lasts 20 hours a charge — 1.67 days a charge at 12 hours a day, 4.2 charges a week.
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Battery Life Calculator: 20 Hours From a 4,000 mAh Battery
On the defaults — a 4,000 mAh battery at a 200 mA draw, used 12 hours a day — battery life is 20 hours a charge, 1.67 days a charge, and 4.2 charges a week. This is the battery life tool: divide capacity by draw for the hours, divide by daily use for the days a charge lasts, and divide the week’s use by the hours for the charges — a battery quoted in milliamp-hours alone tells you nothing until the draw is known.
What does 20 hours a charge, 4.2 charges a week mean?
It means a 4,000 mAh battery feeding a device that pulls 200 mA runs for 20 hours before it is empty, and used 12 hours a day that is 1.67 days per charge — so the week needs 4.2 charges. The capacity sets the tank, the draw sets how fast it drains, and the daily use sets only how the hours spread across days: using the device less never stretches a charge past 20 hours, it only stretches the days those hours cover. The only judgement is the draw — 200 mA suits a modest device, while a bright screen, a radio, or a motor can double it, and a label’s milliamp-hours without a draw is a tank without a leak rate.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Hours a charge | capacity ÷ draw | 20 hours — 4,000 ÷ 200. |
| Days a charge | hours ÷ use per day | 1.67 days — 20 ÷ 12. |
| Charges a week | use × 7 ÷ hours | 4.2 charges — 12 × 7 ÷ 20. |
Three things to check before you trust the number
First, check the capacity honestly: a printed 4,000 mAh on an old battery is a new battery’s figure, and an aged cell can hold far less — a battery that dies early is usually a smaller tank than the label claims, not a hungrier device. Second, check the draw against the real use: 200 mA is an average, and a device that idles at 50 mA and peaks at 500 mA drains at neither figure — measure the average over a real day, not the peak on the spec sheet. Third, check the use against the day: 12 hours means the device is on for 12 hours, not merely owned for 12 — standby hours barely count, and counting them flatters the days a charge.
Worked example
4,000 mAh, 200 mA draw, 12 hours a day. Hours: 4,000 ÷ 200 = 20. Days a charge: 20 ÷ 12 = 1.67. Charges a week: 12 × 7 ÷ 20 = 4.2. In practice that means charging every day and a half, roughly four to five times a week — the number to plan around when packing a charger, since a 20-hour battery is a daily-charge battery for anyone who uses the device all day.
Every branch, checked
With a 5,000 mAh battery instead of 4,000, the charge lasts 25 hours — 2.08 days a charge, 3.36 charges a week. With a 100 mA draw instead of 200, the charge lasts 40 hours — 3.33 days a charge, 2.1 charges a week. With 8 hours a day instead of 12, the charge still lasts 20 hours but stretches to 2.5 days a charge, and the week needs 2.8 charges. With 16 hours a day instead of 12, the charge still lasts 20 hours but shrinks to 1.25 days a charge, and the week needs 5.6 charges.
The reverses, worked backwards
The capacity behind 20 hours is 4,000 mAh: at 3,000 mAh the charge is 15 hours (1.25 days a charge, 5.6 charges a week); at 5,000 mAh it is 25 hours (2.08 days a charge, 3.36 charges a week). The draw behind 20 hours is 200 mA: at 100 mA the charge is 40 hours (3.33 days a charge, 2.1 charges a week); at 400 mA it is 10 hours (0.83 days a charge, 8.4 charges a week). The use behind 20 hours is 12 hours: at 8 hours the charge is still 20 hours but lasts 2.5 days a charge (2.8 charges a week); at 16 hours it is still 20 hours but lasts 1.25 days a charge (5.6 charges a week) — the use never touches the hours, only the days.
What the defaults assume
The defaults assume a 4,000 mAh battery, a steady 200 mA draw, and 12 hours of use a day. They do not assume cold weather, which shrinks usable capacity; an aged battery, which holds less than its label; or a spiky load, where peaks drain faster than the average suggests — a winter night, an old cell, and a motor all shorten the real charge below the calculated one.
Who uses this
- A camper packing for a weekend: the 1.67 days a charge settles how many spare batteries to carry.
- A parent buying a tablet for a child: the 4.2 charges a week off the calculator is the charging-routine line.
- A hobbyist sizing a project battery: the 20-hour figure is the line for choosing the cell, not the label.
- A traveller choosing a power bank: the charges-a-week figure translates the bank into days away from a socket.
Accuracy & limitations
The arithmetic is exact for the inputs entered; the judgement is the draw and the true capacity. Real batteries lose capacity with age, cold, and fast charging, and real devices draw unevenly — so treat 20 hours as the planning charge for a new 4,000 mAh battery at a steady 200 mA, and expect a used or cold battery to fall short of it.
Cite this calculator
Bright Side Kit, “Battery Life Calculator: 20 Hours From a 4,000 mAh Battery”, https://brightoolbox.com/battery-life-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (4,000 mAh, 200 mA draw, 12 hours a day) so the result can be checked.
Frequently asked questions
How long will a 4,000 mAh battery last at 200 mA?
20 hours: 4,000 ÷ 200. At 12 hours of use a day that is 1.67 days a charge, or 4.2 charges a week.
Does using the device less make the battery last longer?
It stretches the days, not the hours: 8 hours a day gives 2.5 days a charge, but the charge itself is still 20 hours. Only a bigger battery or a smaller draw stretches the charge.
Why does my battery die faster than the calculator says?
Usually an aged battery holding less than its label, a draw higher than the average entered, or cold weather shrinking usable capacity. Re-measure the draw over a real day and the true capacity, and the gap usually closes.
What if the device draws 400 mA?
Then the same battery lasts 10 hours — 0.83 days a charge at 12 hours a day, 8.4 charges a week. Doubling the draw halves the charge, exactly.
Can use per day be more than 24 hours?
No: a day holds 24 hours, and the calculator refuses anything above it. If the device runs around the clock, enter 24.
Is this the same as a phone’s battery percentage?
No: percentages hide the draw and the capacity behind them. Milliamp-hours and milliamps are the honest units, and this calculator uses only those.