Recharge Time Calculator

Recharge time calculator for estimating how long it takes to charge a battery

This recharge time calculator estimates how long a battery will take to charge based on the battery size and the charger’s power. The main use case is simple: you want a realistic time estimate for charging a power bank, phone, laptop battery, e-bike pack, UPS battery, or any other rechargeable battery system where the battery energy can be expressed in watt-hours (Wh). If you only know milliamp-hours (mAh), you can still estimate by adding the battery voltage, because mAh on its own does not tell you how much energy the battery actually stores.

The calculator is designed for one decision: “How long will charging take with this charger?” It does not try to predict battery health, cycle life, temperature rise, charger compatibility, or safety limits. It also does not replace the manufacturer’s stated charging times. Instead, it gives a practical estimate using standard energy and power relationships, plus optional real-world corrections for losses and the slower final part of charging.

To use it quickly, enter the battery energy (Wh) and your charger power (W), then calculate. If you do not have Wh, enter battery capacity in mAh and voltage in volts, and the calculator will convert those to Wh for you. If you want a more realistic estimate, open the Advanced section and set your starting charge, target charge, expected efficiency, and taper overhead. Efficiency covers losses in the charger, cable, conversion stages, and the battery itself. Taper overhead accounts for charging that slows down near the end (common with lithium-based batteries), where the “last 10–20%” often takes disproportionately longer than the earlier part.

Assumptions and how to use this calculator

• Battery energy is treated as usable energy in watt-hours; if you enter mAh and voltage, Wh is estimated as (mAh ÷ 1000) × V.
• Charger power is treated as the effective output available for charging; real chargers may deliver less than the advertised maximum depending on device negotiation, cable quality, and temperature.
• Overall efficiency defaults to 85% to represent typical end-to-end losses from wall outlet to stored battery energy.
• Taper overhead defaults to 15% to represent the slower final phase of charging; some devices may have a bigger taper, especially near 100%.
• The estimate assumes normal operating temperature; cold conditions can reduce charging speed and warm conditions may trigger throttling.

Common questions

Why does the calculator prefer watt-hours (Wh) instead of milliamp-hours (mAh)?

Because mAh is not energy by itself. mAh describes charge capacity, but the energy you can store and transfer depends on voltage. A 10,000 mAh battery at 3.7 V stores far less energy than 10,000 mAh at 12 V. Wh combines both in a single number that matches how charger power works (watts).

What should I enter for charger power (W)?

Use the charger’s real output power, not just the adapter label if you suspect the device will not draw the maximum. For example, a “65 W” USB-C adapter may still charge a phone at 15–25 W if the phone cannot negotiate higher power. If you are unsure, use a conservative number. The result is only as good as the power value you provide.

Why are there two times: ideal and typical?

Ideal time ignores charging taper. Typical time adds your taper overhead to reflect the common behavior where charging slows near the end. The typical estimate is usually closer to what people observe for lithium-ion devices charging close to 100%. If you usually stop at 80–90%, set the target charge accordingly and the taper effect becomes smaller.

How do I estimate efficiency and taper if I have no idea?

Leave the defaults. The default efficiency (85%) is a practical midpoint for many consumer setups. The default taper overhead (15%) is a modest correction for the slower end of charge. If your device is known to charge very slowly near the end, increase taper overhead. If you stop charging at 80% or you are charging a chemistry that does not taper as much, reduce it.

When is this calculator not a good fit?

It is not a good fit when charging is limited by something other than power delivery, such as a strict charge-current limit, thermal throttling, proprietary fast-charging protocols, damaged cables, or a battery management system that enforces conservative charge rates. In those cases, the charger wattage you enter does not reflect the actual power going into the battery, and the estimate can be optimistic.