Battery Charge Time Calculator

How the battery charge time calculation works

The calculator first finds how much charge must be returned to the battery:

charge to replace (Ah) = capacity (Ah) × (target SoC − starting SoC) ÷ 100

The ideal time divides that charge by the charger current. The calculated minimum also accounts for the selected charge-efficiency assumption:

calculated minimum (hours) = charge to replace ÷ (charger current × efficiency)

This is not a guaranteed full-charge time. Lead-acid batteries may enter an absorption phase near the top of charge, when current falls and the remaining charge takes longer. Charger limits, temperature, battery condition and a BMS can also change the actual time. For this reason, the calculator says “at least” when a lead-acid target above 80% is selected without a sourced charger profile or an additional absorption time.

Charging-time examples

60 Ah lead-acid battery, from 20% to 100%, with a 5 A charger

The battery needs 48 Ah: 60 Ah × (100% − 20%) = 48 Ah. The ideal time is 9 hours 36 minutes. With the visible 95% efficiency assumption, the calculated minimum is 10 hours 6 minutes. Because the target is 100%, the variable absorption phase is not included and full charge may take longer.

Open this example in the calculator

100 Ah LiFePO4 battery, from 20% to 90%, with a 20 A charger

The battery needs 70 Ah. The ideal time is 3 hours 30 minutes. With the visible 99% efficiency assumption, the estimate rounds to 3 hours 32 minutes. The charger or BMS may still limit current, so treat the result as an estimate rather than a promise.

Open this example in the calculator

Frequently asked questions

How long does it take to charge a 60 Ah battery with a 5 A charger?

From 0% to 100%, the ideal calculation is 12 hours. With a 95% efficiency assumption, the calculated minimum is about 12 hours 38 minutes. A lead-acid absorption phase can extend the full-charge time.

How long does it take to charge a 100 Ah battery with a 10 A charger?

From 0% to 100%, the ideal result is 10 hours. With a 95% efficiency assumption, the calculated minimum is about 10 hours 32 minutes, excluding variable absorption time.

Does battery voltage change the Ah-based charging-time result?

Not directly when capacity is entered in amp-hours and charger output in amps. System voltage is used here to interpret a sourced resting-voltage profile. Voltage is required when energy is calculated in watt-hours, which belongs to the related capacity and runtime tool.

Can resting voltage determine the starting state of charge?

Only approximately and only with a profile suitable for the battery. Measure after the battery has rested, with charging and loads disconnected. The calculator labels the selected source. It does not offer a generic voltage-to-SoC conversion for LiFePO4.

Why can the final part of lead-acid charging take longer?

During absorption, the charger holds a controlled voltage while current tapers. A simple Ah/A division assumes constant current and therefore cannot predict that variable final phase.

Is a higher charging current always better?

No. The permitted current depends on the battery and charger. The calculator can flag a high C-rate, but the battery and charger documentation must decide the safe limit.

Is the result exact?

No. It is an estimate based on the inputs and visible assumptions. Temperature, battery age and condition, charger behaviour, cable losses and BMS limits can change actual time.

Does the calculator work offline?

Yes, after the application has loaded successfully and its offline files have been cached. Saved configurations remain on the device unless the user shares a parameterised link.

Sources

• Victron BMV-712 / SmartShunt — charge efficiency settings
• Victron Blue Smart IP65 — reference maximum charge-rate guidance
• Trojan AES AGM — resting-voltage / state-of-charge table at 25 °C

Related battery tools