Battery Life Calculator
Estimate battery runtime from capacity and load, with real-world derating and Peukert's law for lead-acid
Enter the battery capacity and the average load current. Runtime ≈ capacity ÷ load × efficiency.
Real batteries deliver less than their rated capacity due to conversion losses, self-discharge and the cutoff voltage. ~85% is typical.
This is an estimate
Real runtime varies with the discharge curve, temperature, age, the cutoff voltage and the actual load profile. Use this as a planning figure, not a guarantee.
About this tool
This battery life calculator estimates how long a battery will power a device. The current mode uses runtime ≈ capacity ÷ load current, scaled by an efficiency (derating) factor because real cells never deliver their full rated capacity: regulator and inverter losses, self-discharge, ageing and the unusable charge below the cutoff voltage all eat into it, so ~85% is a sensible default. The power mode works from watts instead of amps — handy for power banks and UPS sizing — using energy (Wh) = capacity (Ah) × voltage. The Peukert mode applies Peukert's law, t = H·(C/(I·H))ⁿ, which captures the real behaviour of lead-acid batteries where high discharge currents deliver noticeably less capacity. Li-ion chemistries are close to ideal (n ≈ 1.05), so their Peukert losses are small.
How to use
- 1 Pick a mode: Current, Power (W), or Peukert for lead-acid.
- 2 Enter the battery capacity and choose mAh or Ah; in power mode also enter the voltage and the power draw in watts.
- 3 Set the efficiency (derating) slider — about 85% reflects typical real-world losses.
- 4 Read the estimated runtime, the energy in watt-hours, and (in Peukert mode) the effective capacity delivered at that current.
How it works
The simplest model divides capacity by load: a 2000 mAh battery at 100 mA lasts about 2000 ÷ 100 = 20 hours in theory. Multiplying by an efficiency factor (say 0.85) drops that to ~17 hours to reflect losses you cannot avoid. In power terms, energy in watt-hours equals capacity in amp-hours times the pack voltage, and runtime is that energy divided by the wattage. Lead-acid batteries break the simple rule: their effective capacity shrinks as you draw more current. Peukert's law, t = H·(C/(I·H))ⁿ, models this — with the rated capacity C measured over a rated time H (typically the 20-hour rate) and an exponent n of about 1.1–1.3 for lead-acid. At the rated current the law returns exactly the rated runtime; pull harder and you get proportionally less.
Frequently asked questions
How do I calculate battery life from mAh?
Divide the capacity by the load current in the same units, then multiply by an efficiency factor. A 4000 mAh battery feeding a 200 mA load lasts about 4000 ÷ 200 = 20 hours ideally, or roughly 17 hours at 85% efficiency. Always use a realistic average current, not the peak.
Why is the real runtime shorter than capacity ÷ load?
Because a battery never delivers 100% of its rated capacity to your device. Voltage regulators and inverters waste some energy, the cell self-discharges, ageing reduces capacity, and the charge below the cutoff voltage is unusable. The efficiency (derating) factor — about 85% by default — folds these losses into one number.
What is Peukert's law and when do I need it?
Peukert's law describes how a lead-acid battery's effective capacity falls as the discharge current rises: t = H·(C/(I·H))ⁿ, where n is the Peukert exponent (about 1.1–1.3 for lead-acid). Use it whenever you discharge a lead-acid battery faster than its rated rate. Lithium-ion is close to ideal (n ≈ 1.05), so the effect is small there.
Can I use this for a drone, power bank or UPS?
Yes. For a drone, enter the pack capacity and the average current in amps. For a power bank or UPS, the power mode is easier: enter capacity, voltage and the device's wattage to get runtime from watt-hours. Remember the result is an estimate — real runtime depends on temperature, the load profile and the discharge curve.
Related tools and uses
Pair this with the Ohm's law calculator to turn voltage and power into the current draw, the series & parallel calculator when combining cells into a pack, and the wire gauge calculator to size the wiring for that current safely.