What is the discharge time formula?

Discharge time (hours) = (Capacity × Efficiency) / Load current. For example, a 3000 mAh battery with an 85% efficiency factor powering a 500 mA load runs for (3000 × 0.85) / 500 = 5.1 hours.

C-rate is the ratio of discharge current to battery capacity. A 1C rate fully discharges the battery in one hour; 0.5C takes two hours; 2C takes 30 minutes. Exceeding the maximum rated C-rate shortens battery life and can cause overheating.

Why use an efficiency factor?

Real batteries lose energy to internal resistance, heat, and chemistry losses. Efficiency typically ranges from 80–95%: 85% is a reasonable default for most Li-ion and lead-acid cells. Alkaline primaries may drop below 80% at high currents.

Does discharge rate affect actual capacity?

Yes. At high C-rates, Peukert's effect reduces usable capacity — a battery discharged in 1 hour delivers less total energy than the same battery discharged over 10 hours. This calculator uses a fixed efficiency factor as a simplified proxy; for precise lead-acid sizing, apply Peukert's equation separately.

What efficiency should I use for my battery?

Use 90–95% for fresh Li-ion/LiPo cells at moderate loads, 85% for typical Li-ion at medium loads, 80% for lead-acid, and 70–80% for NiMH or alkaline at moderate to high currents. Reduce efficiency further for aged cells or extreme temperatures.

Why does the runtime table show different values per charge level?

The table multiplies the full-charge runtime by the fraction of charge remaining (75%, 50%, 25%). This gives a quick reference for how long the battery will last if you start at a partial state of charge.

Battery Discharge Calculator

Name: Battery Discharge Calculator
Availability: InStock
Author: Nham Vu

Enter battery capacity, load current, and efficiency to instantly get discharge time, C-rate, and runtime at multiple state-of-charge levels.

Battery Parameters

Battery Capacity

Load Current

Efficiency: 85%

50% (lossy) 100% (ideal)

Depth of Discharge (DoD)

% (100% = fully discharged)

Enter battery parameters and click Calculate

Discharge time, C-rate, and runtime table will appear here

Summary

Enter battery capacity, load current, and efficiency to instantly get discharge time, C-rate, and runtime at multiple state-of-charge levels.

How it works

Enter the battery capacity in mAh or Ah and select the unit.
Enter the load current (the current your device draws) in mA or A.
Adjust efficiency to account for real-world losses (default 85% covers most battery types).
Click Calculate — the tool computes total discharge time, C-rate, and a runtime breakdown by charge level.
Use the results table to see how long the battery lasts from full, three-quarter, half, and quarter charge.

Use cases

Estimate how long a 18650 Li-ion cell will power a flashlight or sensor node.
Size a LiPo battery pack for a drone or RC vehicle based on flight/drive time.
Verify a UPS lead-acid battery can sustain a load for a required backup window.
Calculate C-rate to confirm a battery is not being discharged faster than its rated limit.
Plan battery swaps for IoT devices deployed in the field.
Compare runtime of different battery capacities for a given load.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu