Battery Runtime Estimator
Runtime
10.0 hours
How it works
Knowing how long a battery will power your device before recharging is essential for field equipment, off-grid setups, emergency power systems, and electronics projects. The Battery Runtime Estimator calculates runtime from battery capacity (Wh or Ah) and device power consumption, with adjustments for battery efficiency, temperature, and depth-of-discharge limits.
**Core formula** Runtime (hours) = Battery capacity (Wh) / Device power (W). For a 100Wh battery powering a 25W device: Runtime = 100 / 25 = 4 hours.
**Ah to Wh conversion** Battery capacity is often rated in Amp-hours (Ah) at a specified voltage. Wh = Ah × Voltage. A 12V 100Ah battery: 100 × 12 = 1200 Wh. A 3.7V 5000mAh phone battery: 5 × 3.7 = 18.5 Wh.
**Efficiency factors** Inverter efficiency (for AC loads from DC batteries): typically 85–92%. A 100Wh battery running a 25W AC device through a 90% efficient inverter: Runtime = (100 × 0.90) / 25 = 3.6 hours (not 4). Peukert's law: battery capacity decreases at high discharge rates (a 100Ah battery discharged in 1 hour may only yield 80Ah). The estimator applies Peukert correction for high-drain scenarios.
**Depth of discharge (DoD)** Lithium-ion batteries: safe DoD is typically 80% (don't discharge below 20% state of charge). Lead-acid: 50% DoD recommended. AGM: 60% DoD. Usable capacity = rated capacity × DoD limit.
**Temperature effects** Cold reduces battery capacity: a 100Wh Li-ion battery at 0°C may deliver only 80Wh; at −20°C, 60–70Wh. The estimator includes temperature correction curves for common battery chemistries.
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Frequently Asked Questions
- Power bank capacity is rated at the battery's internal voltage (typically 3.7V Li-ion cell voltage). Your device charges at 5V USB. The internal boost converter that steps 3.7V up to 5V is typically 85–92% efficient. A 20,000mAh power bank at 3.7V = 74Wh. At 5V output with 88% efficiency: usable energy = 74 × 0.88 / 5 = 13,024mAh at 5V. Additionally, the battery itself can't be discharged to 0% (typically 80% DoD limit). Real usable capacity: ~10,400mAh at 5V — significantly less than the advertised 20,000mAh. This is why '10,000mAh' phone batteries typically charge a 4000mAh smartphone about 2 times, not 2.5.
- Battery capacity decreases significantly in cold: Li-ion at 0°C: ~80% rated capacity. At −10°C: ~70%. At −20°C: ~60%. This is why phones 'die' faster in winter. The effect is reversible — warm the battery up and it recovers capacity. High temperatures (>40°C) degrade batteries permanently over time: for every 10°C above 20°C, Li-ion degradation rate approximately doubles. Optimal Li-ion storage temperature: 15–25°C at 40–60% charge. Lead-acid batteries are even more temperature-sensitive: a fully charged lead-acid loses ~33% capacity at −17°C and can freeze if discharged.
- C-rate describes discharge speed relative to battery capacity. 1C = discharge the full capacity in 1 hour. 0.5C = discharge in 2 hours. 2C = discharge in 30 minutes. High C-rate reduces effective capacity (Peukert's law). A 100Ah battery discharged at 1C (100A) might only deliver 80–85Ah. At 0.1C (10A): may deliver 105–110Ah (slightly above rated capacity). This matters for high-drain applications: power tools, electric vehicles, inverters with heavy loads. For low-drain applications (overnight security cameras, IoT sensors): actual capacity slightly exceeds the rated value. The estimator applies Peukert correction automatically.
- Depth of discharge (DoD) is the percentage of capacity used in a cycle. DoD significantly affects battery cycle life: Li-ion at 100% DoD: ~300–500 cycles. At 80% DoD: ~600–900 cycles. At 50% DoD: ~1200–2000 cycles. At 20% DoD: 3000–5000+ cycles. For maximum lifespan: limit Li-ion to 80% DoD (never discharge below 20% state of charge) and charge to 80–90% rather than 100% for daily use. Electric vehicle batteries use battery management systems to enforce these limits automatically. For off-grid solar storage: size your battery bank so daily usage represents only 50% DoD for lead-acid, 80% for LiFePO4.