Battery Life Calculator
How long a battery lasts from capacity in mAh or Wh, draw in mA or watts and an efficiency factor — plus the capacity a target runtime would need.
Battery Life Calculator: with the default inputs, estimated runtime is 12 hours, 45 minutes.
mAh and Ah are charge, not energy — they only become energy once you know the voltage.
3.7 V for a lithium-ion cell, 1.5 V alkaline, 12 V lead-acid. Needed to turn mAh into watt-hours.
Covers regulator losses, the charge you can't drain, and capacity lost to temperature and age. 80–90% is typical.
How long the battery holds up at this draw, after the efficiency factor.
- Runtime (hours)
- 12.75
- Battery energy (Wh)
- 11.1
- Usable energy after efficiency (Wh)
- 9.44
- Device draw (W)
- 0.74
- Device draw (mA)
- 200
- Capacity needed for the target (Wh)
- 20.89
- Capacity needed for the target (mAh)
- 5,647
Assumptions
- Constant draw at the nominal battery voltage; a real cell's voltage falls as it discharges.
- The efficiency factor stands in for regulator losses, unusable charge, temperature and age.
- Self-discharge is not modelled, so very long predicted runtimes are optimistic.
- Peukert's effect (capacity loss at high discharge rates) is not modelled separately.
| Draw (mA) | Draw (W) | Runtime (hours) | Runtime |
|---|---|---|---|
| 50 | 0.185 | 51 | 2.1 days |
| 100 | 0.37 | 25.5 | 1.1 days |
| 200 | 0.74 | 12.75 | 12.7 hours |
| 400 | 1.48 | 6.37 | 6.4 hours |
| 800 | 2.96 | 3.19 | 3.2 hours |
| 2,000 | 7.4 | 1.28 | 1.3 hours |
Runtime is inversely proportional to draw: halve the current and you double the time, up to the point where the battery's own self-discharge takes over.
How this is worked out
The formula
Energy (Wh) = mAh ÷ 1000 × nominal voltage (or Ah × V, or Wh directly) Power (W) = mA ÷ 1000 × nominal voltage (or A × V, or W directly) Runtime (h) = Energy × efficiency ÷ Power Capacity needed = Power × target hours ÷ efficiency
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Battery capacity
- A number.0 or more · defaults to 3000
- Capacity is in
- mAh and Ah are charge, not energy — they only become energy once you know the voltage.mAh (milliamp-hours) · Ah (amp-hours) · Wh (watt-hours)
- Battery nominal voltage
- 3.7 V for a lithium-ion cell, 1.5 V alkaline, 12 V lead-acid. Needed to turn mAh into watt-hours.0 or more · defaults to 3.7
- Device draw
- A number.0 or more · defaults to 200
- Draw is in
- Choose one of 3 options.mA (milliamps) · A (amps) · W (watts)
- Usable capacity / conversion efficiency
- Covers regulator losses, the charge you can't drain, and capacity lost to temperature and age. 80–90% is typical.a percentage · from 1 to 100 · defaults to 85
- Runtime you want(under More options)
- Used only for the 'capacity needed' output.0 or more · defaults to 24
What you get back
- Estimated runtimemain answer
- How long the battery holds up at this draw, after the efficiency factor.
- Runtime (hours)
- Battery energy (Wh)
- Usable energy after efficiency (Wh)
- Device draw (W)
- Device draw (mA)
- Capacity needed for the target (Wh)
- Capacity needed for the target (mAh)
What this assumes
- Constant draw at the nominal battery voltage; a real cell's voltage falls as it discharges.
- The efficiency factor stands in for regulator losses, unusable charge, temperature and age.
- Self-discharge is not modelled, so very long predicted runtimes are optimistic.
- Peukert's effect (capacity loss at high discharge rates) is not modelled separately.
About this calculator
Battery runtime is one division — energy in the pack divided by power out of it — wrapped in two things people get wrong: the units, and the fact that a battery never delivers its label.
mAh is not energy
A capacity in mAh or Ah is charge, not energy. 3,000 mAh means the cell can push 3,000 mA for an hour, or 300 mA for ten hours, but it tells you nothing about how much work that represents until you multiply by voltage. That is why a 10,000 mAh power bank does not charge a 4,000 mAh phone twice: the bank's cells are 3.7 V, the USB output is 5 V, and 37 Wh going in at 5 V is only about 7,400 mAh of output charge before conversion losses.
Wh is energy and is directly comparable across chemistries and voltages. When a spec sheet gives you both, use the Wh. Airlines regulate lithium batteries in Wh for the same reason.
Note the useful shortcut this calculator uses under the hood: if capacity and draw are both expressed in the same charge units (mAh and mA), the voltage cancels out completely and runtime is simply mAh ÷ mA × efficiency. 3,000 mAh at 200 mA is 15 hours before losses, 12.75 hours at 85%.
Why the efficiency factor is not optional
The nameplate number assumes a fresh cell at 20 °C discharged slowly to its cutoff voltage. Reality subtracts:
- Regulator losses. A boost or buck converter is 85–95% efficient, and the loss is real heat.
- Unusable charge. Devices cut off before the cell is flat, to protect it. You never get the last 5–15%.
- Peukert's effect. Draw hard and you get less. It is mild on lithium and severe on lead-acid, where a 20-hour rating can be 40% optimistic at a 1-hour draw.
- Temperature. At 0 °C a lithium cell delivers roughly 80% of its room-temperature capacity; below −10 °C it falls off a cliff.
- Age. Most chemistries are "end of life" at 80% of original capacity, and they reach it around 500 cycles.
85% is a sane default for a device with a regulator, at room temperature, at moderate current. Drop to 70% for cold, hard-worked or elderly cells.
Reading the result
The main number is the runtime with your efficiency factor applied. The table shows how it moves with draw — the relationship is a simple inverse, which is why halving a sleep-mode current doubles the standby time of an IoT device. The capacity needed output runs the sum backwards: give it the runtime you want and it sizes the pack.
For duty-cycled devices (a sensor that wakes for 200 ms a minute), do not enter the peak current. Compute the average — active current × duty cycle + sleep current × the rest — and enter that. Quiescent current in sleep almost always dominates the annual budget, not the wake-ups.
Frequently asked questions
▸How do I convert mAh to hours?
Divide capacity by draw when both are in the same charge units: 3,000 mAh ÷ 200 mA = 15 hours, then multiply by an efficiency factor. If the draw is in watts you must go through the voltage first.
▸How do I convert mAh to Wh?
Wh = mAh ÷ 1000 × nominal voltage. A 3,000 mAh 3.7 V cell holds 11.1 Wh. Going the other way, mAh = Wh ÷ V × 1000.
▸Why does my power bank charge my phone less than twice?
The bank's mAh rating is at its internal 3.7 V, but it outputs 5 V, and the boost converter plus your phone's charging circuit lose 20–35% between them. Compare watt-hours, not milliamp-hours.
▸What efficiency should I use?
80–90% for a typical regulated device at room temperature. Use 70% for cold conditions, an old battery, or a high discharge rate; use 95% only for a simple resistive load straight off the cell.
▸Does drawing more current reduce total capacity?
Yes — Peukert's effect. It is small for lithium-ion at sensible rates but large for lead-acid, where a battery rated at a 20-hour discharge may deliver 40% less when drained in an hour.
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