Battery Life Calculator — Runtime & Charge Cycles
Estimate battery runtime from capacity (mAh/Wh) and current draw. Calculate charge cycles to 80% capacity, find the right battery size for your device, and compare battery packs.
Device Presets
mA = current at known voltage · W = power (voltage-independent)
What Is the Battery Life Calculator — Runtime & Charge Cycles?
The Battery Life Calculator estimates how long a battery will power a device, what capacity you need for a target runtime, and how battery degradation affects capacity over charge cycles. It handles both mAh-based calculations (for fixed-voltage devices like phones and IoT sensors) and Wh-based calculations (for variable-voltage devices like laptops and power tools).
- ›Three calculation modes — Runtime Calculator (how long will it last), Battery Sizing (how big a battery do you need), and Charge Cycles (how much capacity remains after N cycles).
- ›mAh and Wh support — enter capacity in either unit with automatic conversion. Enter current draw in mA or power draw in W.
- ›Device presets — smartphone, laptop, tablet, electric scooter, AA battery, and 18650 cell with realistic default specs.
- ›Battery degradation model — Li-ion cycle life estimates show projected capacity at 300, 500, and 1000 cycles to help plan replacement schedules.
- ›Power bank calculator — enter power bank capacity and device battery size to see how many full charges you can get, accounting for the 80% conversion efficiency of power banks.
- ›Temperature impact note — explains how cold temperatures reduce effective capacity by up to 20% at 0°C and 40% at −20°C.
Formula
Runtime from mAh
Runtime (h) = (Capacity_mAh × Efficiency) / Current_draw_mA
Runtime from Wh
Runtime (h) = (Capacity_Wh × Efficiency) / Power_draw_W
Convert mAh ↔ Wh
Wh = mAh × V / 1000
mAh = Wh × 1000 / V
Required Battery Capacity
Capacity_mAh = (Runtime_h × Current_mA) / Efficiency
Power Bank Charges
Charges = (Power_bank_mAh × 0.80) / Device_mAh
| Symbol | Name | Description |
|---|---|---|
| mAh | Milliampere-hours | Battery capacity unit. A 3,000 mAh battery delivers 3,000 mA (3 A) for 1 hour, or 1 A for 3 hours. |
| Wh | Watt-hours | Energy unit that accounts for voltage. Wh = mAh × V / 1000. Required for comparing batteries at different voltages. |
| V | Voltage (volts) | Battery nominal voltage. Li-ion cells are nominally 3.7V. LFP cells: 3.2V. Lead-acid: 2V/cell. |
| η (efficiency) | Efficiency factor | Accounts for converter losses, heat, and internal resistance. Default: 0.85 (85%). Range: 0.70–0.95. |
| I | Current draw (mA) | Average current the device draws from the battery. Check device spec or measure with a USB ammeter. |
| P | Power draw (W) | Average power consumption. P = V × I. Use this when voltage varies (e.g., laptop batteries). |
How to Use
- 1Select a mode: Runtime Calculator estimates battery life, Battery Sizing finds the capacity you need, Charge Cycles projects degradation over time.
- 2Use a preset or enter custom values: Click a device preset (Smartphone, Laptop, etc.) to pre-fill typical specs, then adjust any value.
- 3Enter capacity and voltage: For mAh-based calculations: enter mAh and nominal voltage (V). For Wh-based: enter Wh directly. The calculator converts between them automatically.
- 4Enter current or power draw: Enter average current draw in mA, or average power in W. Check your device spec sheet or measure with a USB power meter for accuracy.
- 5Adjust the efficiency factor: The default 0.85 (85%) is a good starting point for most Li-ion devices. Use 0.90–0.95 for direct battery connections, 0.75–0.80 for boost/buck converters.
- 6Read the runtime result: Runtime is shown in hours and minutes. The charge cycles panel shows how capacity degrades over the battery lifetime.
Example Calculation
Scenario: Smartphone — 3,500 mAh battery, 3.7V nominal, 600 mA average draw.
Temperature Effect on Battery Life
Lithium-ion batteries lose effective capacity in cold conditions because lithium-ion mobility slows as temperature drops. At 0°C (32°F), a battery may deliver only 80% of its rated capacity. At −20°C (−4°F), effective capacity can fall to 60%. This is why EV range drops in winter and your phone dies faster on a cold day. Keeping the battery warm (in a pocket, in a case) mitigates this significantly. At high temperatures (above 40°C), capacity is temporarily increased but long-term degradation accelerates substantially.
Understanding Battery Life — Runtime & Charge Cycles
Understanding Battery Capacity Units
Battery capacity is most commonly specified in milliampere-hours (mAh) for consumer devices, but this unit alone is insufficient for comparing batteries at different voltages. A 10,000 mAh powerbank with a 3.7V internal cell stores 37 Wh of energy, while a 10,000 mAh lead-acid battery at 12V stores 120 Wh — more than three times as much energy despite the same mAh rating.
Watt-hours (Wh) is the correct unit for energy comparison and is used on airline carry-on limits (typically 100 Wh per battery without approval, 160 Wh with airline permission). For practical device calculations, mAh is convenient when voltage is fixed; convert to Wh when comparing across voltage levels.
Battery Degradation and Cycle Life
Lithium-ion batteries degrade through two primary mechanisms: calendar aging (degradation over time even without use, driven by temperature and state of charge) and cycle aging (degradation from charge/discharge cycles, driven by depth of discharge and C-rate). For most users, cycle aging dominates because high usage devices are rarely left sitting at high temperatures.
The "80% at 500 cycles" figure commonly cited for smartphones represents a moderate degradation scenario. Users who consistently charge overnight (keeping the battery at high state of charge for hours) and frequently use fast charging may see faster degradation. Users who charge to 80% and never let the battery drain below 20% can often achieve 800–1000 cycles to 80% capacity with the same cell chemistry.
Choosing the Right Battery for a Project
- ›IoT sensors (ultra-low current, months of runtime): Use high-capacity primary lithium cells (e.g., ER26500 with 9,000 mAh) or rechargeable LFP with solar charging. Calculate average daily current carefully — a 10 µA sensor with 2× weekly transmit spikes at 50 mA is very different from a 10 mA continuous load.
- ›Portable electronics (days of runtime): Standard Li-ion 18650 cells (3,400–3,600 mAh) in parallel/series configurations. Balance cycle life with energy density.
- ›Power tools and robotics (high current, short runtime): High-drain Li-ion cells (rated 10–30C). Never use standard consumer cells above 2C — thermal runaway risk.
- ›Backup power systems: LiFePO4 for 2,000+ cycle life. Pair with a proper BMS. Size to 1.25× your calculated need to avoid deep discharge cycles that accelerate degradation.
- ›EV and e-bike applications: Use Wh for sizing — range depends on Wh/km (typically 10–20 Wh/km for e-bikes, 150–200 Wh/km for electric cars). Factor in 15–20% buffer for cold weather and hilly terrain.
Maximising Battery Lifespan
- ›Storage charge level: Store Li-ion at 40–60% charge, not fully charged or fully discharged. A fully charged Li-ion left unused for weeks degrades faster than one stored at 50%.
- ›Temperature: Store and operate between 15–25°C for maximum longevity. Avoid leaving devices in hot cars or freezing conditions.
- ›Charge limits: Many modern phones offer an 80% charge limit setting — enable it if you typically have access to charging throughout the day.
- ›Depth of discharge: Shallow cycles (20–80%) cause less stress than full 0–100% cycles. Each 10% reduction in charge swing roughly doubles cycle life for Li-ion.
Frequently Asked Questions
What is the difference between mAh and Wh?
They measure different things — and the distinction matters when comparing batteries:
- ›mAh (milliampere-hours) measures charge capacity but ignores voltage. It only makes sense at a fixed voltage.
- ›Wh (watt-hours) measures energy: charge × voltage. It is the correct unit for cross-voltage comparisons.
- ›A 3,000 mAh battery at 3.7V stores 11.1 Wh.
- ›A 3,000 mAh battery at 7.4V stores 22.2 Wh — twice the energy, same mAh rating.
When comparing a phone battery (3.7V) to a laptop battery (11.1V), always convert both to Wh first.
What efficiency factor should I use?
The efficiency factor accounts for heat, internal resistance, and regulator losses. Guidelines by circuit type:
- ›Direct connection (no regulator): 0.90–0.95
- ›Linear regulator (e.g., LDO): 0.80–0.90 — efficiency depends on voltage headroom.
- ›Buck/boost switching regulator: 0.85–0.92 — more efficient than linear for large voltage steps.
- ›Power bank → device: 0.80 — two conversion stages (bank output + device charging circuit).
When in doubt, use 0.85 (85%) as a conservative starting point for most consumer devices.
How many charge cycles does a lithium-ion battery last?
Cycle life varies significantly by chemistry and usage pattern:
- ›Standard Li-ion (smartphones, laptops): ~300–500 cycles to 80% capacity.
- ›Premium Li-ion (Tesla, high-quality cells): up to 1,000 cycles to 80%.
- ›Lithium Iron Phosphate (LiFePO4): 2,000–4,000 cycles — preferred for EVs and home storage.
A "charge cycle" is one full 0–100% charge. Partial charges count proportionally — charging from 50% to 100% counts as 0.5 cycles. Avoiding deep discharges and high-temperature charging significantly extends cycle life.
Why does my phone battery percentage seem inaccurate?
Li-ion voltage curves are non-linear, making state-of-charge estimation inherently difficult:
- ›The voltage is nearly flat between 30–80% SOC, so small voltage changes map to large swings in the estimated percentage.
- ›The Battery Management System (BMS) also factors in temperature, charge history, and internal resistance — but inherent uncertainty of ±5% is normal.
- ›As the battery ages and internal resistance increases, the BMS estimate drifts further from reality.
- ›Full discharge to 0% followed by a full charge to 100% can recalibrate the BMS's capacity estimate.
How do I measure my device's actual current draw?
The measurement method depends on how the device is powered:
- ›USB-powered devices: Use a USB power meter (inline USB multimeter) between the charger and device — it shows live watts and amps.
- ›Direct battery-powered devices: Connect a DC ammeter in series with the battery positive lead.
- ›Smartphones and laptops: Enable developer options or use apps (AccuBattery on Android, CoconutBattery on Mac) that report mA draw from the BMS.
Always average current over a representative usage period — peak current during startup or screen-on can be 3–5× the idle average.
How efficient are power banks at charging devices?
Power banks lose energy through two conversion stages:
- ›Stage 1 — Power bank output: Converts internal Li-ion voltage (~3.7V) to 5V USB output. Typically ~90% efficient.
- ›Stage 2 — Device charging circuit: Converts 5V input to the battery charge voltage. Typically ~92% efficient.
- ›Combined efficiency: 0.90 × 0.92 ≈ 0.83 (83%).
- ›Practical rule: a 10,000 mAh power bank delivers ~8,000–8,300 mAh at the device battery level.
USB-C Power Delivery (PD) chargers reduce Stage 2 losses by delivering a voltage closer to the device's optimal charging voltage, pushing combined efficiency above 88% in some cases.
Does fast charging harm battery health?
Fast charging does accelerate degradation, but the risk is manageable:
- ›High current increases heat, which speeds up electrolyte decomposition and lithium plating — both reducing capacity.
- ›The damage is most significant above 80% state of charge, where internal resistance rises sharply.
- ›Most modern phones automatically throttle fast charging above 80% to limit heat.
- ›Overnight charging keeps the battery at 100% for hours — more damaging than a fast charge that quickly drops back to active use.
For maximum longevity: charge to 80%, avoid discharging below 20%, and use fast charging selectively rather than habitually.
What is C-rate and why does it matter?
C-rate describes charge or discharge current relative to the battery's capacity:
- ›1C = discharge the full capacity in 1 hour. A 3,000 mAh battery at 1C draws 3,000 mA.
- ›2C = full discharge in 30 minutes. Doubles the heat generated.
- ›0.5C = full discharge in 2 hours. Gentler and more efficient.
- ›Most consumer Li-ion cells are rated for 1C continuous discharge.
- ›Power tools and EV acceleration use cells rated for 10–30C.
- ›Exceeding rated C-rate causes rapid capacity fade and, in extreme cases, thermal runaway.
Higher C-rates also reduce effective capacity due to internal resistance losses — a cell rated 3,000 mAh at 0.2C may only deliver 2,600 mAh at 2C.