A battery life calculator answers a question every datasheet dodges: how long will this actually run? The number printed on the cell is a laboratory figure measured under gentle, controlled discharge. The number you care about is what happens when a real load pulls current through real wiring at whatever temperature the device happens to be sitting in. The gap between those two figures is routinely 20% or more, and this tool exists to make that gap visible instead of letting it surprise you halfway through a flight.
Arb Digital maintains a free tools library covering the everyday maths people actually need, and battery sizing keeps coming up — for camera rigs, portable displays, off-grid sensors, e-bike range questions and the endless argument about which power bank is genuinely bigger. The calculation itself is simple division. Getting the inputs into the same units, and then being honest about the losses, is the part that goes wrong.
What This Battery Life Calculator Does
You give it four things: how much energy the battery stores, the voltage it delivers, how hard the load pulls, and how much of the stored energy you can realistically reach. It converts everything into watt-hours and watts, applies your depth-of-discharge limit and your efficiency figure, then divides. The result is a runtime in hours and minutes, with the intermediate numbers shown so you can see exactly where the energy went.
The supporting figures are deliberately chosen. Stored energy in watt-hours is the only fair way to compare two batteries. Load power in watts is the only fair way to compare two devices. Usable energy shows what survives after the cut-off and the losses. The discharge rate in C tells you whether you are treating the cell gently or working it hard, which is the single best predictor of whether your efficiency estimate is generous or pessimistic.
The three bars underneath show the same runtime three ways: the nameplate answer you would get by naive division, the answer after applying the depth-of-discharge limit, and the answer after efficiency losses. Watching the third bar shrink is the whole point of the tool.
How to Use It
- Enter the capacity and pick its unit. Phone and power-bank cells are labelled in mAh, larger packs in Ah, and laptop and tool batteries increasingly in Wh. If your figure is already in Wh, the voltage field stops mattering.
- Set the nominal voltage. Use the cell chemistry's nominal figure, not its fully charged figure. A lithium-ion cell charged to 4.2 V spends most of its discharge nearer 3.7 V, which is why 3.7 V is the number printed on the pack.
- Describe the load. Choose milliamps, amps or watts depending on what your device's specification gives you. If you only know the wall-adapter rating, that is the maximum, not the average — measure or estimate the average draw instead.
- Set depth of discharge. 90% suits consumer electronics that shut down near empty. Use 50% for a lead-acid battery you intend to cycle repeatedly, and 80% for a lithium pack you want to last years.
- Set efficiency. Start at 85%. Drop it toward 75% for high current draws, cold conditions or an older pack, and raise it toward 95% only for a gentle, steady load on a new cell.
The Formula / How It's Calculated
Everything reduces to energy divided by power. The conversions to get there are:
Stored energy (Wh) = capacity (Ah) × nominal voltage (V), with mAh divided by 1,000 first. Load power (W) = load current (A) × voltage (V) when you enter current, or taken directly when you enter watts. Usable energy (Wh) = stored energy × depth of discharge × efficiency. Runtime (hours) = usable energy ÷ load power.
Worked example, matching the values the page loads with. A 5,000 mAh cell at 3.7 V stores 5.0 × 3.7 = 18.5 Wh. A 500 mA load at the same voltage draws 0.5 × 3.7 = 1.85 W. Naive division gives 18.5 ÷ 1.85 = 10.00 hours. Applying a 90% depth of discharge leaves 16.65 Wh, or 9.00 hours. Applying 85% efficiency leaves 18.5 × 0.90 × 0.85 = 14.15 Wh, and 14.15 ÷ 1.85 = 7.65 hours, which the tool reports as 7 h 39 min. The discharge rate is 0.5 A drawn from a 5 Ah pack, or 0.10 C — a very gentle load, which is why 85% is a reasonable efficiency assumption here.
Notice that voltage appears on both sides when the load is given as current. That is why a current-based estimate at 3.7 V produces the same runtime as one at 12 V for a proportionally larger pack: the volts cancel. They stop cancelling the moment a voltage converter sits between the battery and the load, which is the subject of the next section.
Why Milliamp-Hours Are a Misleading Unit
Milliamp-hours measure charge, not energy, and charge only becomes energy when you multiply it by a voltage. Two packs advertised as 10,000 mAh can hold completely different amounts of usable energy depending on their cell arrangement, and the marketing rarely makes that clear.
The most common version of this confusion involves USB power banks. A 20,000 mAh power bank is almost always 20,000 mAh at the internal cell voltage of about 3.7 V, which is 74 Wh. To charge a phone it must step that up to 5 V, and the conversion is not free. After boost-converter losses and the phone's own charging losses, the delivered energy is typically well under 60 Wh — which is why a 20,000 mAh bank does not charge a 5,000 mAh phone four times. If you want that comparison to work, convert both to watt-hours first, then apply an efficiency figure to the conversion step. The University of Washington's Clean Energy Institute overview of lithium-ion batteries sets out the cell voltage and energy-density figures that make this arithmetic possible.
Airline carry-on limits are also written in watt-hours precisely because milliamp-hours are ambiguous. If you have ever had to work out whether a pack qualifies, the unit converter handles the general case and the energy converter moves between watt-hours, joules and calories directly.
Discharge Rate: The Reason Fast Draws Cost You Capacity
C-rate expresses the load as a multiple of the battery's capacity. Drawing 5 A from a 5 Ah pack is 1C; drawing 0.5 A from the same pack is 0.1C. It matters because a battery's effective capacity falls as the C-rate rises. Internal resistance turns part of the stored energy into heat, and the cell's voltage sags under load, so the device hits its low-voltage cut-off while chemical energy still remains.
The practical consequence is that the efficiency figure in this calculator is not a constant. At 0.1C, 85–95% is realistic. At 1C, 80% is a fair guess for a healthy lithium cell. At 3C or above — think power tools, drones and RC vehicles — effective capacity can fall much further, and a manufacturer's discharge curves become the only reliable guide. Lead-acid chemistry is the extreme case: its capacity rating assumes a 20-hour discharge, and pulling the same pack down in two hours can leave you with barely half the rated amp-hours.
This is why the C-rate appears in the results grid. If the tool shows something above 1C, treat the runtime as an optimistic ceiling and lower the efficiency input until it reflects the discharge curve for your specific cell.
Temperature, Age and the Losses Nobody Budgets For
Cold is the loss most people meet first. Lithium-ion chemistry slows down as temperature drops, internal resistance climbs, and a phone that runs eight hours indoors can shut down in two on a winter hillside. The energy is not gone — warming the device back up usually restores it — but it was unavailable when you needed it. If the device will be used below freezing, model it by cutting the efficiency input substantially rather than by changing the capacity.
Age is the loss that creeps. Every full charge cycle degrades a cell slightly, and calendar ageing continues even in storage, faster when the pack is kept full and hot. A three-year-old laptop battery holding 80% of its original capacity is entirely normal, not a defect. Model it by reducing the capacity input to the measured present figure, which most operating systems will report, rather than by pretending the efficiency is worse.
Self-discharge is the loss that surprises people who store equipment. A lithium-ion cell loses a small percentage of its charge each month sitting idle, so a pack charged in spring is not full in autumn. And at end of life, none of this arithmetic applies any more — batteries removed from service need proper handling, and the EPA guidance on used lithium-ion batteries explains why they must never go into household bins.
Sizing a Battery Backwards From a Target Runtime
The calculator runs forward, but the more useful question is often the reverse: what capacity do I need for eight hours? Rearranging gives required Wh = target hours × load watts ÷ (depth of discharge × efficiency). For an 8-hour target at 1.85 W with the same 90% and 85% assumptions, that is 8 × 1.85 ÷ 0.765 = 19.3 Wh, or roughly 5,230 mAh at 3.7 V. You can get to the same answer by raising the capacity in the tool until the runtime hits your target.
Two habits make this estimate hold up in practice. First, size the load on average draw rather than peak, but confirm the battery can supply the peak without its voltage collapsing — a sensor that sleeps at 5 mA and transmits at 200 mA is an average-draw problem with a peak-current constraint. Second, add margin deliberately rather than accidentally: choose the runtime you need, then add 20–30% on top, instead of quietly using an optimistic efficiency number to get the answer you wanted. For related device maths, the watts to horsepower converter and the power converter cover unit changes on the load side, and the electricity bill calculator prices the mains energy used to recharge the pack.
Arb Digital builds fast, self-contained interactive tools that attract search traffic and keep visitors on the page. Browse the free library, or tell us what you need built.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Comparing packs by milliamp-hours — charge is not energy. Multiply by voltage and compare watt-hours, or the comparison is meaningless.
- Using the charged voltage instead of the nominal voltage — a lithium cell reads 4.2 V full but averages nearer 3.7 V, and using 4.2 V inflates stored energy by around 13%.
- Taking the adapter rating as the load — that figure is the maximum the supply can deliver, not what the device averages. Use measured or typical draw.
- Assuming 100% of capacity is usable — devices cut off before empty, and cycling a pack to true zero shortens its life sharply.
- Ignoring the discharge rate — a high C-rate reduces effective capacity, so a runtime calculated with a gentle-load efficiency figure will overshoot badly.
Related Free Tools From Arb Digital
Use the energy converter to move between watt-hours, joules and BTU, the power converter for watts, horsepower and kilocalories per hour, and the electric charge converter when you need coulombs and amp-hours side by side. The electricity bill calculator prices the grid energy that goes back into the pack, and the EV charging cost calculator does the same job for a vehicle battery. Everything else sits in the free online tools hub.
Frequently Asked Questions
Usually because the efficiency input is too generous. Cold temperatures, a high discharge rate, an aged cell and voltage-conversion losses all reduce usable energy. Lower the efficiency figure until the tool matches a runtime you have actually measured, then keep that number for future estimates on the same device.
Divide the milliamp-hours by 1,000 to get amp-hours, then multiply by the nominal voltage. A 5,000 mAh cell at 3.7 V is 5.0 × 3.7 = 18.5 Wh. Watt-hours are the only figure that lets you compare batteries of different voltages fairly.
Around 90% for consumer electronics that shut down near empty, about 80% if you want a lithium pack to last many years, and about 50% for lead-acid batteries that will be cycled repeatedly. Deeper discharge always means more runtime now and fewer cycles later.
It is the load expressed as a multiple of capacity. Drawing 5 A from a 5 Ah pack is 1C. Below about 0.2C a battery delivers close to its rated capacity; above 1C the effective capacity falls because internal resistance wastes energy as heat and the voltage sags toward the cut-off.
Yes, as long as you set the correct nominal voltage — 12 V for a lead-acid block, 2 V per lead-acid cell, 1.2 V per NiMH cell. Lead-acid ratings assume a 20-hour discharge, so if you plan to empty the pack faster, reduce the efficiency figure accordingly.
Because the rating is measured at the internal cell voltage of about 3.7 V, and the bank must step that up to 5 V to charge anything. Boost conversion and the phone's own charging losses both consume energy, so delivered energy is typically well below the nameplate figure.
Yes. Multiply your target hours by the load in watts, then divide by the depth of discharge and efficiency as decimals, to get the watt-hours required. Or simply increase the capacity input until the reported runtime reaches the figure you need, then add margin.
This tool provides engineering estimates only. Battery behaviour varies with chemistry, age, temperature and load profile, and the figures here are not a substitute for the manufacturer's discharge data or for safety guidance on handling, charging and disposing of cells.
