Battery Life Calculator
Estimate battery runtime from capacity and load. Supports mAh/Ah/Wh, efficiency losses, and the Peukert effect for lead-acid batteries. Visualizes discharge profile over time.
How to Use
- Enter battery capacity (mAh) and nominal voltage.
- Enter average load current (mA) OR total load power (W).
- Set efficiency (typical 80-95% for DC-DC converters, 95-99% for direct drive).
- Optionally enable Peukert effect for lead-acid batteries (1.1-1.3 typical).
- Read estimated runtime and derated capacity. Shorter runtimes under high load.
Show Work
Formulas
Common Chemistries (reference)
| Chemistry | Nom. V | Peukert | Typical DoD |
|---|---|---|---|
| Li-ion (single cell) | 3.7 V | 1.02–1.05 | 80% |
| LiFePO4 | 3.2 V | 1.02–1.04 | 90% |
| Lead-acid (flooded) | 12 V | 1.15–1.30 | 50% |
| Lead-acid (AGM) | 12 V | 1.10–1.15 | 70% |
| NiMH | 1.2 V | 1.05–1.10 | 80% |
| Alkaline (primary) | 1.5 V | 1.10–1.20 | 90% |
| CR2032 (lithium coin) | 3.0 V | 1.02 | 95% |
History of Battery Capacity Rating
Alessandro Volta built the first true battery — the "voltaic pile" — in 1800, stacking zinc and copper discs separated by brine-soaked cardboard. Early 19th-century chemists (Daniell, Grove, Planté, Leclanché) invented primary and rechargeable cells that powered telegraph networks, early electroplating industries, and eventually automobile starters. The ampere-hour as a capacity unit came into use with Planté's lead-acid cell in 1859, and the practice of reporting nominal capacity at a specified discharge rate (usually C/10 or C/20) dates from battery standardization work in the early 1900s.
Wilhelm Peukert published his now-famous empirical correction in 1897, showing that usable capacity of a lead-acid cell drops nonlinearly as discharge current rises. His equation Cactual = Cnominal × (Inominal/Iactual)^(k−1) with k ≈ 1.15–1.30 for lead-acid is still used unchanged in off-grid solar and marine battery sizing. Modern lithium chemistries have much lower Peukert exponents (1.02–1.05) because their internal resistance is far smaller, but the equation still applies.
Depth-of-discharge (DoD) guidelines became formal practice when battery manufacturers realized that cycle life is logarithmic in DoD: limiting lead-acid to 50% DoD gives roughly 4× the number of usable cycles compared to 100% DoD; lithium-ion to 80% DoD roughly doubles it over 100%. Every battery management system today — from cell phones to Tesla Powerwalls — implements a DoD cutoff in firmware to preserve cycle life.
About This Calculator
Enter battery capacity (Ah or mAh), nominal voltage, and load (watts or amps). Optionally enter conversion efficiency (for inverters and DC-DC converters), depth of discharge percentage, and a Peukert exponent for lead-acid accuracy. The tool returns realistic runtime, total energy, usable capacity after DoD, and current draw.
For quick estimates leave efficiency at 90% and DoD at 100%; for serious design work, pick chemistry-appropriate values from the reference table (Li-ion 80%, LiFePO4 90%, lead-acid 50%, etc.). Always measure actual runtime under your real load before committing to a final battery size — published capacities are under ideal conditions that rarely match field use. All math runs client-side.
Frequently Asked Questions
Why can't I just divide capacity by current?
That ideal calculation assumes perfect efficiency and constant-current discharge. Real batteries lose capacity at high discharge rates (Peukert effect), converters waste energy as heat, and cell chemistry limits usable voltage. The straight math is useful as a ceiling — actual runtime is usually 70-90% of that.
What is the Peukert effect?
Lead-acid and some other chemistries deliver less total capacity at higher discharge rates. A 100Ah battery might give 100Ah at a 20-hour rate but only 70Ah at a 1-hour rate. The Peukert exponent (typically 1.1-1.3 for lead-acid) models this. Lithium cells have much smaller Peukert loss (exponent ≈ 1.02-1.05).
How does efficiency affect runtime?
If you power your load through a DC-DC converter, efficiency (typ 85-95%) means part of the battery's energy becomes heat in the converter instead of useful work. A 90% efficient boost converter means your 100Wh battery effectively delivers 90Wh to the load.
Can I trust the rated capacity on the label?
For quality brands, within ±5-10%. Counterfeit or no-name cells are often 30-70% of rated. Test under a known load before committing to a design. A 1-hour discharge test with a real load is worth more than any datasheet number.
How do I convert between mAh, Ah, and Wh?
mAh × Voltage / 1000 = Wh. Ah × Voltage = Wh. Example: 2500mAh at 3.7V = 2.5Ah × 3.7V = 9.25Wh. Wh is the true energy; mAh only makes sense at a specified voltage.
What's a safe depth of discharge?
Lithium-ion: 80% DoD for best lifetime (100% shortens cycle life). Lead-acid: 50% DoD for deep-cycle longevity. LiFePO4: 90-100% safely. NiMH: 80% typical. Always factor DoD into runtime — usable capacity is rated × DoD.
Common Use Cases
IoT Sensor Node
Low-duty-cycle sensor running on a CR2032 coin cell (~220mAh at 3V). Estimate years of life from the average current — sleep + brief radio bursts.
Drone Flight Time
Quadcopter battery (5Ah, 4S LiPo, 14.8V = 74Wh) with ~300W average draw → ~15 min. Factor in descent reserve + safety margin.
Solar Off-Grid System
100Ah 12V lead-acid bank with 15A load at 50% DoD → 3.3h realistic runtime. Peukert derating reduces it further at high loads.
Portable Electronics
Laptop (56Wh battery, 15W avg consumption) → ~3.7h screen-on time. Real-world: 3h with backlight + WiFi active.
UPS Battery Sizing
Calculate how long a UPS can support a server at known wattage. Typical UPS: 720Wh → 15min at 300W load (realistic, with inverter loss).
E-Bike / Scooter Range
48V 15Ah = 720Wh battery at 20Wh/mile gives 36 miles nominal. Hills, wind, and rider weight can cut that to 20-25 miles.
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