ToolForage.
ENERGY CALCULATOR

Battery Charge Time Calculator

Estimate a constant-current battery charging interval from capacity, state of charge, and charge efficiency.

Calculator

Ah
%
%
A
%
YOUR RESULTS
Constant-current charging estimate6.667 h
Stored charge to add
60 Ah
Charge supplied by charger
66.667 Ah
Charging C-rate
0.1

Page guide

Charge to add and constant-current time

The result is a constant-current planning estimate. Real chargers can reduce current near the voltage limit or under thermal control. A target of 100% can therefore take longer than the displayed time, even when the input current matches the initial charger rating.

For estimating how long the charged battery can power a steady load, the Battery Runtime Calculator uses usable energy and conversion losses.

The formula

Stored charge Ah = capacity Ah × (target% − start%)/100. Supplied charge = stored charge ÷ efficiency fraction. Constant-current hours = supplied Ah ÷ charging A.

Amp-hours describe current integrated over time. Dividing the required supplied charge by a constant battery charging current gives hours. Charge acceptance efficiency here concerns amp-hours retained, not AC energy conversion or inverter efficiency.

Worked example

A 100 Ah battery moving from 20% to 80% needs 60 Ah stored. At 90% charge acceptance, the charger supplies 66.667 Ah. A constant 10 A charging current gives 6.667 hours before any additional taper time.

How to use this calculator

  1. Enter battery capacity, starting state of charge using the units shown.
  2. Set target state of charge, charging current, charge acceptance efficiency.
  3. Calculate and review constant-current charging estimate alongside the supporting quantities.

Use the current reaching the battery

A charger input rating, USB port rating, or AC adapter current is not necessarily the battery charging current. Use the current specified or measured at the battery in the constant-current stage. Loads operating during charging may take some of the available current; estimate the net current left for charging rather than assuming the entire supply enters the battery.

Charging stages and termination

Many rechargeable batteries use charging profiles that include a voltage-limited stage with declining current. Capacity, starting charge estimates, temperature, and age also affect timing. This calculator does not set a safe charge voltage or current and cannot replace the battery and charger instructions. Compare it with observed charge time to understand where taper or other limits matter.

Assumptions & limitations

What this calculation assumes

  • Battery charging current and effective amp-hour acceptance remain constant.

What to keep in mind

  • Excludes voltage taper, balancing, temperature limits, and chemistry-specific charging profiles.

Common questions

Why does a full charge take longer?

Current may taper near the final voltage. The constant-current estimate does not simulate that stage.

Is C-rate an efficiency?

No. It is charging current divided by amp-hour capacity, used to express current relative to the battery size.

What if start and target charge are equal?

No additional charge is required in this model, so the displayed charging interval is zero.

Sources & further reading

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