Battery Life Calculator estimates how long a battery runs a load from its mAh, Ah or Wh capacity, with a discharge reserve, sleep cycles, converter loss and self-discharge counted.
Capacity is printed on the battery label. Voltage converts between mAh, Wh and watts.
Sleep Mode and Duty Cycle
Reserve, Efficiency and Self-Discharge
Typical self-discharge per month: Li-ion 1 to 2%, lead-acid about 5%, nickel-based 10 to 15%.
Battery Life by Charge Level
| Charge | Runtime | Clock Time |
|---|---|---|
| 100% | 24.00 h | 24 h 0 min |
| 90% | 21.00 h | 21 h 0 min |
| 80% | 18.00 h | 18 h 0 min |
| 70% | 15.00 h | 15 h 0 min |
| 60% | 12.00 h | 12 h 0 min |
| 50% | 9.000 h | 9 h 0 min |
| 40% | 6.000 h | 6 h 0 min |
| 30% | 3.000 h | 3 h 0 min |
Step-by-Step Working
- Capacity: 3,000 mAh = 3 Ah.
- Load current: 100 mA = 0.1 A.
- Usable share: 100% charge - 20% reserve = 80%, or 2,400 mAh.
- Battery life = 2,400 mAh ÷ 100.0 mA = 24.00 h (24 h 0 min).
Battery Life Calculator Formula in Hours
The Battery Life Calculator divides what a battery holds by what the device draws. To calculate battery life by hand, use the capacity printed on the label, the load current and the share of the battery you plan to keep in reserve.
Capacity is in amp-hours or milliamp-hours, load is in amps or milliamps and the answer comes out in hours. Reserve is the part of the charge that is never used, set to 20% when the tool opens.
The opening example is a 3,000 mAh cell feeding a 100 mA load. With nothing held back that is 30 hours, shown on the Ideal Runtime card. Keeping the 20% reserve leaves 2,400 mAh for the load, so the result is 24 hours.
For how many years of battery life a small load gets, the result changes its unit to suit the answer, moving from minutes to hours, days, months and years. The line under it repeats the time in clock form and prints the numbers that went into the formula.
Load can be entered as current or as power. If you only know the resistance of the load, the Ohm's Law Calculator or the Current Calculator will turn it into amps first. Working backward from a target run time to a capacity is the job of the Battery Size Calculator.
How Long a 3,000 to 20,000 mAh Battery Lasts
How many hours a 3,000 mAh battery will last depends only on the load once the reserve is fixed. The table below gives battery life in hours at four loads, with the 20% reserve the Battery Life Calculator starts on.
| Capacity | 100 mA | 500 mA | 1 A | 2 A |
|---|---|---|---|---|
| 3,000 mAh | 24 h | 4.8 h | 2.4 h | 1.2 h |
| 5,000 mAh | 40 h | 8 h | 4 h | 2 h |
| 7,000 mAh | 56 h | 11.2 h | 5.6 h | 2.8 h |
| 10,000 mAh | 80 h | 16 h | 8 h | 4 h |
| 20,000 mAh | 160 h | 32 h | 16 h | 8 h |
So 5000mAh battery life in hours is 8 at a 500 mA load, 7000 mAh battery life in hours is 11.2 and 20,000 mAh battery life in hours is 32. Set the reserve to 0% and every figure rises by a quarter, which makes the 5,000 mAh row 10 hours at 500 mA.
How long you can use a 10,000 mAh battery as a power bank takes one more step in the Battery Life Calculator. Its cells are rated at about 3.7 V but the output is boosted to 5 V or more, and the converter wastes some energy. Enter the load in watts and set Converter Efficiency for that loss.
A 10,000 mAh pack at 3.7 V stores 37 Wh. Feeding a 10 W load through a converter entered at 85%, with no reserve, it runs for 3.15 hours. Typed as 2 A against 10,000 mAh with no voltage step, the same pack would look like 5 hours.
Wh, 12V, Laptop and Car Batteries
Capacity in watt-hours is amp-hours times voltage, which is why the tool asks for Battery Voltage. Switch the capacity unit to Wh and the load to watts to work as a battery life calculator in Wh, the way laptop and power station labels are printed.
For a battery life calculator laptop check, a 60 Wh pack under an average 15 W draw gives 4 hours with no reserve and 3.2 hours with 20% held back. When capacity and load are both in watt terms, the voltage entry does not change the run time.
As a battery life calculator for 12V systems, take a 100 Ah battery and a 120 W load. The load draws 10 A, so the run time is 8 hours with a 20% reserve and 5 hours with 50%. An inverter entered at 90% efficiency cuts the 50% case to 4.5 hours.
A car battery life calculator question is usually about a parked or engine-off drain. A 60 Ah battery under a 10 A load lasts 3 hours down to half charge. A 50 mA standby drain takes 25 days to reach the same point, or about 23.5 days once lead-acid self-discharge of 5% a month is added.
To find the watts or watt-hours of a load before entering it, the Electrical Power Calculator and the Energy Consumption Calculator cover both. If the supply is a switching stage, such as the one sized in the Flyback Switch Mode Regulator Calculator, its efficiency goes in the Converter Efficiency field.
Sleep Mode, Reserve and Self-Discharge
Many battery devices are awake for a moment and asleep the rest of the time. Under Sleep Mode and Duty Cycle, choose Awake and Sleep Cycle and the tool averages the two currents by the time spent in each.
In the Battery Life Calculator, take the 3,000 mAh cell with a sensor that draws 100 mA for 2 seconds and then sleeps at 10 µA for the rest of a 10 minute cycle. The average current is 343.3 µA, awake 0.333% of the time, and the run time grows from 24 hours to 9.57 months.
Over that many months the battery also drains itself. Battery University lists self-discharge at 1 to 2% a month for lithium-ion, about 5% for lead-acid and 10 to 15% for nickel-based cells, with a lithium-ion protection circuit adding about 3% more.
Used as a battery life calculator for lithium ion cells, enter 2% under Self-Discharge per Month and the sensor example drops from 9.57 to 8.38 months. The tool treats the leak as proportional to the charge left, so the load and the leak drain the cell together.
Here I is the battery current, Q0 is the starting charge, Qr is the reserve and k is the hourly self-discharge rate worked out from the monthly percentage. With k at zero it falls back to the first formula.
The reserve depends on the chemistry and the cut-off voltage of the device. A LiPo pack in a drone should not be taken below 20%, and lead-acid batteries are commonly held to half their capacity. The note under the result marks answers below 1 hour or beyond 1 year, the range outside which a capacity over load estimate drifts from real cells.
Drain per Hour and Partial Charge
How long will a 72 percent battery last?
Enter 72 under Starting Charge. For the 3,000 mAh cell at 100 mA, a 72 percent battery lasts 15.6 hours down to the 20% reserve, or 21.6 hours if it is run to empty. A 100% battery lasts 24 hours and 30 hours on the same terms.
The fourth card answers this without retyping. It shows the run time from 75%, 50% and 25% charge, which is 16.5, 9 and 1.5 hours here, and the Battery Life by Charge Level table lists every 10% step.
How much battery should drain in 1 hour?
The first card in the Battery Life Calculator gives the percent drained per hour for your load. A 5,000 mAh phone battery loses 5% an hour at 250 mA, 10% at 500 mA and 20% at 1 A. There is no single correct figure, because the drain follows what the device is doing.
Is 1 battery drain in 3 minutes normal?
One percent every 3 minutes is 20% an hour, or 5 hours from full to empty. On a 5,000 mAh battery that takes a 1 A draw, which fits gaming, navigation or video at high brightness. The same rate with the screen off is not normal and points to an app or radio that is keeping the phone awake.
The Time per 1% of Charge row makes the check direct. Time how long your device takes to lose 1%, then adjust the load until the row matches, and the load shown is what the device is really drawing.
Can a battery drain overnight?
It can, and the Used in an 8 h Night row shows by how much. A 5,000 mAh phone idling at 50 mA uses 8% overnight. The 3,000 mAh cell at 100 mA uses 26.67%, and a load large enough to empty the usable charge in 8 hours reads All of It.
Battery Health and What Shortens It
Is 80% battery health ok?
Battery health is the capacity left compared with new, which is different from the charge level. Apple states that iPhone batteries are designed to retain 80% of their original capacity at 500 complete charge cycles, and at 1,000 cycles for iPhone 15 models and later.
So 80% battery health is the design point of a used battery, and 70% is past it. To see what it costs, multiply the capacity by the health. Enter that in the Battery Life Calculator, and a 5,000 mAh battery at 80% health becomes 4,000 mAh, with its run time at 500 mA falling from 8 hours to 6.4.
Is 50% battery life good?
As a charge level, 50% is a healthy place for a lithium-ion battery to sit. Battery University notes that electric vehicles typically limit charge to 85% and discharge to 25% to prolong battery life, so a good battery percentage for daily use is the middle of the range.
As a health reading it is the opposite. A battery at 50% or 70% health holds only that share of its original capacity, and the run time from the Battery Life Calculator shrinks by the same share.
How do I check my battery health?
On an iPhone, open Settings, tap Battery, then Battery Health. Android phones keep it under their own battery menus, which differ by maker. To test a phone battery life yourself, note the percentage, use the phone for an hour and compare the drop with the Drained per Hour card.
Battery health cannot be improved or reset back to 100%, because the loss is chemical wear inside the cell. A replacement battery is the only way to return to full capacity, and good habits only slow the decline.
What kills my battery life?
Two different things go by that name. Run time on one charge is killed by load, and the screen is the load you control most. A Purdue University study found that dark mode on OLED phones saves only 3 to 9% of power at 30 to 50% brightness, and 39 to 47% at full brightness.
So dark mode does save battery life, but lowering brightness does more. To improve battery life on a phone and stop it dying so fast, cut the average load, because halving it doubles the hours. The 3,000 mAh cell goes from 9.6 hours at 250 mA to 19.2 hours at 125 mA.
The battery itself is worn out faster by heat, by sitting at full charge and by deep discharges. Battery University treats anything above 86°F (30°C) as elevated temperature for lithium-ion and notes that a partial discharge and a partial charge both reduce stress.
Is fast charging bad for my battery?
It adds some wear. Battery University reports that charging and discharging lithium-ion above 1C reduces service life, where 1C is the current that would fill or empty the battery in one hour. The Discharge Rate row on the second card shows the C rate of your own load.
Can a battery last 7 years?
As run time on one charge it can, if the average current is tiny. A 3,000 mAh battery with a 20% reserve needs an average draw of about 39 µA to last 7 years, which only a sleeping sensor reaches. The chemistry must also hold its charge that long, and Battery University lists primary lithium-metal cells at 10% self-discharge in 5 years.
As years of service for a rechargeable battery, the count is in charge cycles and not in hours. Which battery lasts the longest between charges is the one with the most watt-hours for the same load, so compare labels in Wh and not by brand. A good battery capacity is the load times the hours you need, divided by the usable share.