Off-Grid Solar Calculators

Battery storage

Battery Charging Time Calculator

Estimate the hours needed to put the usable amp-hours back into a battery from a shore charger, DC-DC charger or alternator. The result includes charging inefficiency.

Inputs

Units
Ah

Rated amp-hour capacity of the battery or bank being charged.

How empty the battery is, as a fraction of rated capacity. Use 95% after a deep LiFePO4 cycle.

A

Constant current from the shore charger, DC-DC charger or alternator.

Not all of the charge current ends up stored in the battery; some is lost as heat. 90–95% is typical.

Terms of Service

Results

Please tick the box above to display the Result.

Engineering methodology

Amp-hours to replace = capacity × DoD. Hours = (Ah × DoD) ÷ (charge current in A × charging efficiency). Efficiency (often 90–95%) covers the share of charge current lost as heat in the battery and, on lead-acid, gassing. Real LiFePO4 and lead-acid chargers taper in the constant-voltage stage, so wall-clock time is longer than this constant-current estimate.

Frequently asked questions

Why is actual charging time longer than Ah divided by amps?

CC-CV chargers hold current only until absorption voltage, then current tapers. This tool assumes constant current times efficiency. Absorption, temperature compensation and a full 100% top-off add time, especially on lead-acid.

What charge current is reasonable for LiFePO4?

Many drop-in LiFePO4 packs allow about 0.5C continuous (50 A on a 100 Ah pack). The BMS datasheet is the limit, not this formula. Alternators and cheap AC chargers may be far below that.

Does charging efficiency change the amp-hours I must put back?

The usable Ah to replace is capacity × DoD. Efficiency is in the denominator of time: lower efficiency means more hours at the same charger amps because some input energy is lost as heat.

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