Charging Time Calculator: Battery & Power Bank
Estimate battery charging time, runtime and capacity using watt-hours, charging power, starting and target charge levels, and charging losses.
Estimate charging time, runtime and battery capacity realistically
The charging time calculator helps you estimate values for phones, laptops, power banks, tool batteries, e-bike batteries and USB-C chargers. It uses watt-hours, charging power, charge level and losses to provide practical guidance rather than an idealized laboratory result.
The key quantity is the energy stored in the battery: watt-hours (Wh) describe the actual amount of stored energy. Values in mAh are comparable only when voltage is also known, so the calculator can first convert mAh and volts into Wh.
A battery rarely charges at its maximum power throughout the entire process. Charging electronics often reduce power near full charge. The calculator therefore works with adjustable charging losses and an average charging power.
- Estimate charging time: Starting and target charge levels determine how much energy must actually be added.
- Understand charging power: Compare whether 5 W, 20 W, 65 W or USB-C Power Delivery is plausible for your battery.
- Plan runtime: Battery energy in Wh and average power consumption provide a transparent runtime estimate.
Calculator Description
The charging time calculator estimates battery charging time, runtime and capacity. It uses watt-hours, charging power, starting and target charge levels, and charging losses, and it can convert mAh to Wh using voltage.
Formulas for charging time, runtime and battery capacity
Stored energy in Wh is the basis for charging-time and runtime calculations. Values in mAh are therefore converted to Wh using voltage first.
Charging time
Charging-time formula: Time = Wh x charge share x (1 + charging loss / 100) / charging power in W
Charging-time example
Charging a 50 Wh battery from 20% to 80% adds 30 Wh. At 20 W with 10% charging loss: 30 x 1.10 / 20 = 1.65 hours.
Charging from 20% to 80% adds 60% of the battery capacity.
Runtime
Runtime formula: Runtime = Wh / power consumption in W
Runtime example
A 50 Wh battery powering a device that uses 10 W has a theoretical runtime of about 5 hours.
For devices with changing power use, the average consumption is what matters.
mAh to Wh
Capacity formula: Wh = mAh x V / 1000
Capacity-conversion example
10,000 mAh at 3.7 V equals 10,000 x 3.7 / 1000 = 37 Wh.
Without voltage, mAh values cannot be compared reliably.
- Battery capacity: Wh describes stored energy and is more useful for charging-time and runtime comparisons than mAh alone.
- Charging power: A charger can deliver its rated wattage only when the device, cable and charging protocol support it.
- Charge level: Charging from 20% to 80% fills only part of the battery; the final charge range often takes longer.
- Losses: The charger, electronics, cable and heat mean that more energy is drawn than is ultimately stored in the battery.
Calculation basis & transparency
Accuracy The mathematics is exact for the values entered. Real-world differences mainly result from changing charging power, charging profile, temperature, battery aging and energy losses.
Disclaimer This calculator provides non-binding guidance for charging time, runtime and capacity. Check manufacturer specifications, safety instructions and charger compatibility as well.
Formula & logic
The calculator determines the energy to recharge from battery capacity in Wh and the difference between starting and target charge levels. It adds the selected charging loss and divides the result by charging power in watts. Runtime is Wh divided by power consumption in watts; mAh is converted to Wh using voltage.
Assumptions
- The entered charging power represents the average power during the selected charge range.
- Battery capacity in Wh represents the usable or stated energy content.
- The selected charging loss covers typical losses from the charger, electronics, cable and heat.
Known limitations
- The calculator does not model a detailed constant-current/constant-voltage (CC/CV) charging curve and simplifies the final charging stage.
- Temperature, battery age, the battery management system (BMS), fast-charging protocol, cable, firmware and simultaneous device use can significantly change real charging times.
- Safety-critical battery projects, custom-built batteries and high-voltage systems require professional review.
Typical charging times in practice
Phone
18 Wh battery, 20 W charger, 10% loss
Inputs: 18 Wh, 20 W, 10% loss
Calculation: 18 x 1.10 / 20
about 1 hour from 0% to 100%
Fast charging from 20% to 80% is usually quicker than the final charge range.
Laptop
60 Wh battery, 65 W USB-C charger
Inputs: 60 Wh and 65 W
Calculation: 60 / 65
about 1 hour in the ideal calculation
A laptop may reduce charging power when it is hot or being used at the same time.
E-bike
500 Wh battery, 4 A charger at 36 V
Inputs: 500 Wh and about 144 W charging power
Calculation: 500 / 144
about 3.8 hours including an allowance for losses
Actual e-bike charging times are often longer than the ideal calculation.
Power bank
37 Wh stored energy, 18 W input, 15% loss
Inputs: 37 Wh, 18 W, 15% loss
Calculation: 37 x 1.15 / 18
about 2.4 hours
USB protocol, cable quality and heat losses can extend charging time.
Compare chargers and power banks correctly
More watts reduce charging time only when the battery, device, cable and charging protocol support that power.
For phones and power banks, Wh provides a better comparison than mAh alone because voltage and charging losses also matter.
A higher-powered charger is generally suitable when it uses the correct standard; the device limits the power it actually draws.
Typical mistakes
- Comparing mAh values from different devices without considering voltage.
- Treating the charger's maximum wattage as the power delivered continuously.
- Ignoring losses in the charger, cable, electronics and voltage conversion.
- Assuming a constant linear charging speed all the way to 100%.
- Ignoring the fast-charging protocol, cable capability and device settings.
Typical battery values at a glance
These reference values help put mAh, Wh and common uses into context.
| Device type | Typical voltage | Typical capacity | Use case |
|---|---|---|---|
| Phone | 3.7 V - 3.8 V | 3,000 - 5,000 mAh | Everyday mobile use |
| Power bank | 3.7 V | 10,000 - 26,800 mAh | Portable charging |
| E-bike | 36 V - 48 V | 400 - 750 Wh | Electric mobility |
| Laptop | 11.1 V - 15.2 V | 40 - 100 Wh | Mobile work |
Manufacturer specifications, battery aging, temperature and charging profile can significantly change real charging times.
Chargers and power levels
Typical charging power depends on the standard and device.
| Charging standard | Power | Typical devices |
|---|---|---|
| Standard USB | 5 W | Older phones, headphones |
| USB-C Fast Charge | 18 W - 30 W | Modern phones, tablets |
| USB-C PD | 65 W - 140 W | Laptops, large power banks |
| Wireless Qi | 5 W - 15 W | Phones with wireless charging |
Charger comparison
Example for a 60 Wh battery using the existing comparison model with 15% loss.
| Charger | Power | Estimated charging time (incl. 15% loss) |
|---|---|---|
| Standard USB | 5 W | 14 h 7 min |
| USB-C fast charging | 20 W | 3 h 32 min |
| Laptop charger | 65 W | 1 h 5 min |
Frequently asked questions about charging time and battery capacity
How do I calculate battery charging time?
First determine the energy to recharge in Wh. Divide that energy by charging power in watts and include charging losses. A 50 Wh battery charged from 20% to 80% requires 30 Wh. At 20 W with 10% loss, 30 x 1.10 / 20 equals 1.65 hours.
Why does charging often take longer above 80%?
Many lithium-ion batteries do not charge at full power all the way to 100%. Charging electronics reduce current near full charge to protect the battery and manage temperature, so 20% to 80% is often faster than 80% to 100%.
Which is more useful for batteries: mAh or Wh?
Wh is usually more useful for comparisons because it combines voltage and capacity. A value such as 10,000 mAh represents 37 Wh at 3.7 V, but a different amount of energy at another voltage.
Can I use a higher-powered charger?
Generally yes when the charger, cable and device support the same safe charging standard. The device normally draws only the power it can accept. Always follow manufacturer specifications and use a suitable cable.
Why can the actual charging time differ?
Charging profile, temperature, battery age, cable, fast-charging protocol, simultaneous device use and device protection can all affect real charging time. The calculator uses an average charging power and an adjustable loss for practical guidance.
How do I convert mAh to Wh?
Multiply mAh by voltage and divide by 1000. For example, 10,000 mAh x 3.7 V / 1000 = 37 Wh. Voltage is required to convert electric charge into stored energy.
Sources, basis and notes
Practical reference values
- Charging losses
Real charging commonly includes losses from heat, the charger, cable and charging electronics. The calculator lets you enter the loss explicitly. - 20-80% charge range
Lithium-ion batteries generally charge faster through the middle of their range than near full charge.
Technical orientation
- IEC 61960
Standards covering secondary lithium cells and batteries, including terminology and battery-capacity characteristics. - USB-IF USB Power Delivery
Technical orientation for USB-C charging power and negotiation between device, cable and charger. - Battery University
Practical background information on charging behavior, battery care and aging.
Limits
- No charging-profile simulation
The calculator uses average charging power and does not reproduce a complete CC/CV charging curve. - Devices negotiate charging power
Cable capability, temperature, battery age, firmware and fast-charging protocols can substantially change the power delivered.