Battery Life Calculator

Estimate runtime from battery capacity and load. Supports mAh/Ah/Wh, efficiency losses, and optional Peukert effect.

How to Use

  1. Enter a battery capacity (mAh, Ah, or Wh).
  2. Enter your load as current (mA/A) or power (W).
  3. If using mAh/Ah and load in W, provide voltage (nominal).
  4. Set efficiency for converters/regulators (optional).
  5. Open “Show Work” to see the exact math and assumptions.
Battery Lab View
Visual feedback: load, efficiency loss, and estimated runtime.
Runtime
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Avg Current
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Avg Power
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Usable Energy
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Status: —
Battery Green = usable energy • Red = losses (efficiency / Peukert) Est. Runtime — Load —
Inputs & Settings
Enter capacity + load. Results update instantly (no URL changes while typing).
mAh/Ah are charge; Wh is energy. If using mAh/Ah with W, voltage is required.
Examples: Li-ion 3.7V, USB 5V, lead-acid 12.0V, 24V systems.
Use for electronics draw (mA/A). Leave blank if you enter Watts instead.
Use for device power (W). Leave blank if you enter Current instead.
Optional. For converters/regulators. 100% = ideal (no losses).
Optional. Example: lead-acid often planned at ~50%; Li-ion often ~80–100% depending on goals.
Optional. Common lead-acid rating is 20-hour rate (C/20). Used for Peukert.
Optional. Typical: ~1.05–1.3 for lead-acid. Leave blank to disable Peukert.
Show Work (step-by-step)
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Work is shown in base units (Wh, W, V, A) where applicable.

Reference

Runtime is estimated from usable energy divided by load power: hours ≈ Wh_usable ÷ W_load.

  • mAh → Wh: Wh = (mAh ÷ 1000) × V
  • Efficiency: usable energy reduced by converter loss (e.g., 90% efficiency)
  • Depth of discharge: usable fraction of capacity (e.g., 80%)
  • Peukert (optional): lead-acid capacity decreases at higher discharge rates
This tool provides estimates. Real-world runtime varies with temperature, cutoff voltage, aging, internal resistance, and pulsed loads.

FAQ

Why does my power bank say 10,000mAh but I get less at 5V?

Many power banks rate mAh at the internal cell voltage (~3.7V). Boosting to 5V plus converter losses reduces delivered Wh.

Should I enter nominal voltage or full-charge voltage?

Use nominal voltage for energy conversions (3.7V Li-ion, 12.0V lead-acid). It keeps estimates consistent across a discharge curve.

What is Peukert and when should I enable it?

Peukert describes how lead-acid batteries deliver less capacity at higher currents. Enable it for lead-acid under moderate/high loads.

Can this tool handle loads entered as watts?

Yes. If your capacity is in mAh/Ah, the tool converts to Wh using voltage first.

Tool Info

Last updated:

Updates may include unit support, UI improvements, and calculation edge-case handling.