Battery Discharge Calculator

Estimate battery discharge time, remaining usable capacity, or a runtime-based average discharge current from rated capacity, usable capacity factor, state of charge, current, and duration.

This V1 calculator uses a constant average current model. It is intended for SOC window and usable-capacity analysis, not detailed discharge curve integration, Peukert modeling, voltage sag, or dynamic load-profile simulation.

Engineering tool

Battery Discharge Calculator

Estimate discharge time, remaining capacity, or runtime-based average current from rated capacity, usable factor, SOC window, and average current.

Battery Rated Capacity

Manufacturer rated capacity. The calculator converts µAh and mAh to Ah internally.

Usable Capacity Factor

Estimated usable portion of rated capacity. This is not discharge efficiency.

Initial State of Charge

SOC at the start of discharge. Must be greater than 0%.

Target State of Charge

Selected stop or cutoff SOC for the discharge window.

Average Discharge Current

Equivalent average load current over the selected interval.

Estimated Discharge Time

21.6 h

21 h 36 min

Result console

Rated capacity: 3Ah
Usable full-charge capacity: 2.7Ah
SOC window: 80%
Capacity available: 2.16Ah
Capacity available: 2,160mAh
Estimated discharge time: 21.6h
Estimated discharge time decimal hours: 21.6h
Estimated discharge time human-readable: 21 h 36 min
Average discharge current used: 100mA
Usable capacity factor: 90%
Formula used: tdischarge = Crated × Fu × ΔSOC / Iavg

Model note

Usable Capacity Factor approximates available rated capacity under real conditions. It is not the same as discharge efficiency, energy efficiency, or a battery chemistry model.

This calculator uses a constant average current model. It does not simulate Peukert's law, voltage sag, cutoff voltage behavior, temperature compensation, battery aging, or dynamic load profiles.

Formula reference

Battery Discharge Formulas

Battery discharge estimates use Ah, A, hours, SOC as a 0 to 1 fraction, and Fu as the usable capacity factor. Ah divided by A gives hours.

Cusable = Crated × FuΔSOC = (SOCinitial − SOCtarget) / 100Cavailable = Crated × Fu × ΔSOCtdischarge = Cavailable / IavgCremoved = Iavg × tSOCconsumed = Cremoved / (Crated × Fu)SOCend = SOCinitial − Cremoved / (Crated × Fu)Cremaining = Crated × Fu × max(SOCend, 0)Irequired = Crated × Fu × ΔSOC / ttarget

Variable definitions

Crated: Rated battery capacity in Ah
Cusable: Usable full-charge capacity in Ah
Cavailable: Capacity available in the selected SOC window in Ah
Cremoved: Charge removed during discharge in Ah
Cremaining: Remaining usable capacity in Ah
Fu: Usable capacity factor from 0 to 1
SOCinitial: Initial state of charge
SOCtarget: Target state of charge
SOCend: Ending state of charge
SOCconsumed: SOC consumed during the entered duration
ΔSOC: SOC window as a fraction
Iavg: Average discharge current in A
Irequired: Runtime-based average current in A
tdischarge: Estimated discharge time in hours
ttarget: Target discharge duration in hours

Variable Description

Rated Capacity: Manufacturer rated battery capacity measured under specified temperature, discharge rate, and cutoff-voltage conditions.
Usable Capacity Factor: Estimated usable fraction of rated capacity. It is not discharge efficiency or a precise chemistry model.
Initial SOC: Battery state of charge at the beginning of discharge.
Target SOC: Selected stop or cutoff state of charge for discharge time and required-current modes.
SOC Window: Initial SOC minus target SOC for the selected discharge interval.
Average Discharge Current: Time-equivalent average load current. Variable loads should be averaged over time.
Remaining Capacity: Estimated usable capacity left after the entered discharge interval, clamped to zero if over-discharged.
Depth of Discharge: Absolute DoD is 100% minus SOC; incremental DoD for this calculation is initial SOC minus target SOC.

Worked Examples

Ideal Full Discharge

3000 mAh, usable factor = 100%, SOC = 100% to 0%, current = 150 mA

Available capacity = 3000 mAh; discharge time = 20 h

Protected SOC Window

3000 mAh, usable factor = 90%, SOC = 100% to 20%, current = 100 mA

Available capacity = 2160 mAh; discharge time = 21.6 h = 21 h 36 min

Remaining Capacity

5000 mAh, usable factor = 90%, initial SOC = 100%, current = 500 mA, duration = 4 h

Charge removed = 2000 mAh; ending SOC ≈ 55.556%; remaining usable capacity = 2500 mAh

Partial Initial SOC

10 Ah, usable factor = 80%, initial SOC = 75%, current = 2 A, duration = 2 h

Usable full capacity = 8 Ah; ending SOC = 25%; remaining usable capacity = 2 Ah

Required Average Current

100 Ah, usable factor = 80%, SOC = 100% to 50%, target duration = 10 h

Available capacity = 40 Ah; maximum average current = 4 A

Low-Power Sensor

2400 mAh, usable factor = 85%, SOC = 100% to 10%, current = 250 µA

Available capacity = 1.836 Ah; time = 7344 h = 306 days

Discharge Beyond Available Capacity

2 Ah, usable factor = 80%, initial SOC = 50%, current = 1 A, duration = 2 h

Time to estimated empty = 0.8 h; ending SOC displays 0%; remaining capacity displays 0

Engineering Notes

How Battery Discharge Is Calculated

This calculator estimates usable charge in an SOC window and divides by constant average current.

Rated Capacity

Rated capacity is usually measured under manufacturer-defined test conditions.

Usable Battery Capacity

Usable capacity can be lower than rated capacity because of cutoff voltage, temperature, discharge rate, and aging.

State of Charge

SOC describes remaining charge level as a percentage of usable full-charge capacity.

Depth of Discharge

Absolute DoD is 100% minus SOC; the calculation window is the change from initial SOC to target SOC.

SOC Window

The SOC window controls how much of the usable full-charge capacity is available.

Average Discharge Current

Average current should reflect the time-weighted load, not just peak current.

Constant-Current Discharge

The model is most accurate when the load behaves like a constant average current sink.

Variable Loads

For active, idle, sleep, and pulsed loads, compute a time-weighted average or use a future load-profile runtime calculator.

Discharge Rate

High discharge rates can reduce usable capacity for some batteries.

C-Rate

C-rate relates current to battery capacity, such as 1C for a current equal to rated capacity in amperes.

Cutoff Voltage

A system may stop before the battery reaches true empty because of undervoltage limits.

Voltage Sag

Internal resistance can reduce terminal voltage under load and trigger early cutoff.

Internal Resistance

Internal resistance causes heat and voltage drop during discharge.

Temperature Effects

Low temperature can reduce usable capacity and voltage performance.

Battery Aging

Aged batteries usually deliver less capacity than new cells.

Self-Discharge

Long-duration low-power systems may be affected by self-discharge as much as load current.

Battery Chemistry Differences

Lithium-ion, lead-acid, NiMH, alkaline, and other chemistries have different discharge behavior.

Lithium-Ion Discharge

Lithium-ion systems require cutoff voltage and protection limits to avoid unsafe deep discharge.

Lead-Acid Discharge

Lead-acid runtime at high current may need Peukert's law for better estimates.

NiMH Discharge

NiMH voltage curves and cutoff behavior differ from lithium-ion cells.

Peukert's Law

Peukert's law is commonly used for lead-acid discharge-rate capacity correction, but it is not implemented in this V1 calculator.

Battery Maximum Discharge Current

The calculated current is runtime-based and is not a maximum safe discharge rating.

BMS Discharge Limits

A BMS can limit, interrupt, or stop discharge based on current, voltage, temperature, or protection rules.