Battery Discharge Time

Estimate runtime from battery capacity + voltage + load (amps or watts). Includes efficiency, cutoff, and Show Work.

How to Use

  1. Enter battery capacity (Ah or Wh) and battery voltage.
  2. Enter your load as either amps or watts.
  3. Set optional assumptions: efficiency, cutoff % (usable depth), and Peukert (advanced).
  4. Open Show Work to see the math used to estimate runtime.
Battery Lab View
Visual feedback: usable capacity + estimated runtime based on your inputs.
Runtime
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Usable Wh
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Load
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Avg Current
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Status: —
Usable Capacity Fill level reflects usable % after cutoff and efficiency assumptions. Runtime — Load — Usable Wh —
Inputs & Settings
Provide capacity + voltage + load. Use Advanced settings for more realistic estimates.
Ah is common for lead-acid; Wh is common for power banks.
Examples: 3.7V, 5V, 12V, 24V, 48V
Use if you know the device current draw.
Assumptions
Adjust for real-world losses and usable portion of the battery.
Example: 85–95% for DC-DC conversion losses.
Portion of capacity you plan to use (e.g., 80% usable).
Advanced: lead-acid often ~1.05–1.30. Leave blank to disable.
If using Peukert, set the rated hour (commonly 20hr for lead-acid).
Show Work (step-by-step)
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Work is shown using base units (Ah, Wh, V, A, W, hours). Peukert is applied only if enabled and valid.

Reference

Basic runtime (Ah model): t_hours = Ah_usable ÷ A_load

Convert capacity: Wh = Ah × V and Ah = Wh ÷ V

Convert load: W = V × A and A = W ÷ V

Usable energy (with assumptions): usable = rated × (cutoff%) × (efficiency%)

Notes: Real batteries sag under load, and capacity depends on discharge rate and temperature. Use Advanced settings for closer estimates.

FAQ

Why is my real runtime lower than the estimate?

Losses from converters, wiring, inverter inefficiency, battery voltage sag, and high discharge rates can reduce usable capacity. Try lowering efficiency and enabling Peukert for lead-acid.

Should I enter Ah or Wh?

Either works. If your battery is labeled in Ah, use Ah. If your device or pack is labeled in Wh, use Wh.

What cutoff % should I use?

For longer battery life, you might only use 50–80%. For emergency/short-term use you may go higher, but it depends on chemistry and your minimum voltage requirement.

What is Peukert and when should I use it?

Peukert models how lead-acid capacity drops at higher currents. If you’re using a lead-acid battery and pulling significant amps, enabling Peukert can improve realism.

Tool Info

Last updated:

Updates may include unit support, improved edge-case handling, and clearer show-work output.