Battery Life Calculator
Estimate how long a battery lasts from its capacity, the load it drives, efficiency, and an optional Peukert correction.
Enter battery capacity, active and standby load, nominal voltage, and explicit derating factors. Apply Peukert correction only with a manufacturer-supported exponent and rated capacity period.
Use the battery datasheet for the temperature factor, Peukert exponent, and rated capacity period. Leave Peukert correction off when those battery-specific values are unavailable.
- Iavg = Iactive ร duty + Istandby ร (1 โ duty)
- Ibatt,avg = Iavg / ฮท
- Cusable = Crated ร DoD ร f_temp
- t(h) = Cusable / Ibatt,avg
- E(Wh) = Crated ร Vnom
How It Works
- 1
Enter battery capacity and load
Pick mAh, Ah, or Wh for capacity and mA, A, or W for load. If you use Wh or W, the calculator uses the nominal voltage to convert to amperes.
- 2
Set the data-sheet correction factors
Enter the pack's nominal voltage, conversion efficiency, permitted depth of discharge, and temperature capacity factor. Use values from the battery, charger, inverter, or device data sheets rather than treating the starting values as universal defaults.
- 3
Optional: add Peukert correction
Enable the correction only when the manufacturer supplies a Peukert exponent and the capacity rating's reference time. Otherwise leave the exponent at 1. The calculator shows ideal and corrected runtime, average load, usable capacity, and sensitivity to a 10% load change.
How long a battery really lasts
Alessandro Volta built the first battery in 1800 from copper and zinc disks separated by brine-soaked cloth. The starting runtime equation is still capacity divided by average load, but a useful estimate keeps four inputs separate: conversion efficiency, permitted depth of discharge, temperature-adjusted capacity, and any discharge-rate correction supported by the battery data sheet. The optional Peukert relation models the loss of effective capacity as discharge current rises. It was developed for lead-acid batteries, so the exponent and reference rating should come from the manufacturer; leave the correction at 1 when no supported value is available. mAh and Ah measure charge, while Wh measures energy. Multiplying 3000 mAh by 3.7 V gives 11.1 Wh, whereas 3000 mAh at 1.2 V gives 3.6 Wh. FAA PackSafe guidance uses Wh for lithium battery limits: batteries up to 100 Wh are generally permitted in carry-on baggage, while 101โ160 Wh batteries require airline approval. Real runtime can still change with cutoff voltage, battery-management behavior, aging, and load transients, so the result is an engineering estimate rather than a guarantee.
Common pitfalls
Confusing mAh with Wh. FAA limits are stated per lithium battery in watt-hours. A 27 000 mAh pack at 3.7 V is 99.9 Wh, but the same Ah rating at 12 V is 324 Wh. Check the marked battery rating and the airline's rules before travel.
Applying an unsupported Peukert exponent. The correction is sensitive to the exponent and the reference discharge rate. Use the battery manufacturer's values, or leave the exponent at 1 and treat the simple result as an uncorrected estimate.
Guessing a universal cold-weather percentage. Temperature effects vary by chemistry, cell design, discharge rate, and cutoff voltage. Enter the manufacturer's temperature capacity factor for the intended operating point.
Combining efficiency, depth of discharge, and temperature into one vague derating. These constraints describe different losses and operating limits, so the calculator exposes them separately and shows the resulting usable capacity.
Mixing unmatched cells or modules. A series or parallel bank is constrained by its weakest member and by its battery-management rules. Follow the pack manufacturer's matching, balancing, and replacement instructions.
Frequently Asked Questions
What does Peukert's law actually say?
Peukert's relation models how the effective capacity of a lead-acid battery falls as discharge current rises. The calculator uses C_eff = C_rated ร (I_ref / I)^(k โ 1). An ideal battery has k = 1.00; Victron documents 1.25 as an acceptable default average for many lead-acid batteries, but the correct exponent and rated test current should come from the battery manufacturer. Leave k at 1.00 when you do not have a supported Peukert value.
Why does my phone never last as long as the mAh rating suggests?
A capacity label is measured under specified test conditions, while a phone's screen, radios, processor, power conversion, temperature, cutoff voltage, and battery age all change the usable energy. Model an intermittent device with active load, standby load, and duty cycle. Use the separate efficiency and temperature fields only when you have a measured value or a defensible datasheet estimate.
What's the difference between mAh and Wh?
mAh is charge; Wh is energy. Two 3000 mAh batteries store different energy at different nominal voltages: 3000 mAh at 3.7 V is 11.1 Wh, while 3000 mAh at 1.2 V is 3.6 Wh. Convert with Wh = V ร Ah. The FAA uses watt-hours for passenger battery limits: up to 100 Wh is generally allowed, while 101โ160 Wh requires airline approval and is subject to quantity and packing rules.
How does temperature affect battery life?
Temperature changes usable capacity, internal resistance, voltage sag, and aging, but the size of the effect depends on cell chemistry, discharge rate, and cutoff voltage. Read the capacity-versus-temperature curve in the battery datasheet and enter its remaining-capacity ratio in the temperature factor. Keep this separate from power-conversion efficiency.
What does depth of discharge mean for runtime?
Depth of discharge (DoD) is the fraction of rated capacity you plan to use before stopping. A 50% DoD setting makes 50 Ah of a 100 Ah rating available to the runtime model. The appropriate limit depends on chemistry, cycle-life target, battery-management settings, and manufacturer guidance, so DoD has its own input instead of being hidden inside efficiency.
Why is my battery worse after a year in the drawer?
Battery capacity can fall with calendar age, charge state during storage, temperature, and cycle history. The rate varies widely by chemistry and product. Measure the pack or use its current tested capacity as the calculator input; do not assume the original label still describes an older battery.
Can I just divide mAh by mA to get hours?
For an ideal constant-current estimate, yes: 2500 mAh รท 250 mA = 10 hours. The calculator shows that simple result alongside a corrected estimate. Duty cycle, conversion loss, usable depth of discharge, temperature, cutoff voltage, battery age, and a supported Peukert exponent can all move the real result.
Calcflux / battery-life-1.2.0
Calculation record
- Formula version
- battery-life-1.2.0
- Verified
Formula
t = [C ร I_ref^(kโ1) ร DoD ร f_temp] / [D ร (I_active/ฮท)^k + (1โD) ร (I_standby/ฮท)^k]Assumptions
- Rated capacity, voltage, and correction factors come from the battery or cell datasheet.
- Use the battery datasheet for the temperature factor, Peukert exponent, and rated capacity period. Leave Peukert correction off when those battery-specific values are unavailable.
Limits
- Real cutoff voltage, battery management behavior, aging, and load transients can change runtime.
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