Battery Charge Time Calculator

Estimate how long a battery takes to charge from its capacity, charger current, and charging efficiency — plus the energy (Wh) needed to reach your target charge level.

Quick Facts

Formula
Time (h) = Capacity × ΔCharge% ÷ (Charger current × Efficiency)
Charging efficiency typically runs 80-95% due to heat and AC/DC conversion losses.

Your Results

Calculated
Charge time
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Estimated hours to reach target
Charge time (H:M)
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Hours and minutes
Energy to add
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Watt-hours (Ah × voltage)
Effective charging current
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After efficiency losses

Ready

Enter your battery and charger details, then press Calculate.

About the Battery Charge Time Calculator

This calculator estimates how long it takes to charge a battery using the standard capacity-over-current relationship: the energy that must be added is the battery's capacity multiplied by the percentage of charge you still need, and the time to deliver that energy depends on how much current the charger actually pushes in, once real-world charging losses are accounted for.

The formula

  • Charge needed (Ah) = Battery capacity (Ah) × (Target charge % − Current charge %) ÷ 100
  • Effective charging current (A) = Charger output current (A) × Charging efficiency (%) ÷ 100
  • Charge time (hours) = Charge needed (Ah) ÷ Effective charging current (A)

Multiplying the amp-hours needed by the battery's nominal voltage gives the energy in watt-hours (Wh) — useful for comparing against a charger's power rating or estimating the electricity used during charging.

Why efficiency matters

No charger delivers 100% of its rated current into stored battery capacity — some energy is lost as heat and in AC-to-DC conversion. Typical charging efficiency runs 80-95% depending on battery chemistry and charger design, which is why this calculator divides by efficiency rather than treating the charger's rated current as the delivered current.

Common reference points

  • 12V lead-acid car battery (50 Ah): a 10A charger going from 20% to 100% takes roughly 3-4 hours at typical efficiency.
  • Li-ion phone battery (about 4 Ah at 3.85V): a compact charger delivering around 2.5A can take under 2 hours, though most tapers current well before 100%.
  • EV battery pack (roughly 60 kWh at 400V, about 150 Ah): a Level 2 home charger (32A) needs several hours for a full charge from empty.

Assumptions and limits

This calculator assumes a constant charging current for the whole session. Real chargers — especially lithium-ion chargers using constant-current/constant-voltage (CC/CV) charging — taper the current down as the battery nears full, so the last portion of a charge often takes longer than this linear estimate suggests.

Frequently Asked Questions

How do you calculate battery charge time?
Charge time in hours equals the amp-hours you need to add divided by the effective charging current. The amp-hours needed are the battery's capacity (Ah) multiplied by the percentage-point gap between your target and current charge level. The effective charging current is the charger's rated output current multiplied by the charging efficiency, since not all of the rated current reaches the battery's stored capacity.
Why does charging efficiency matter?
Chargers lose some energy as heat and during AC-to-DC conversion, so the current that actually reaches the battery's usable capacity is lower than the charger's rated output. Typical charging efficiency runs 80% to 95% depending on battery chemistry and charger design, which is why dividing by efficiency gives a more realistic charge time than using the rated current alone.
Does this account for charging slowing down near full?
No. This calculator assumes a constant charging current for the full session. Many chargers, especially lithium-ion chargers using constant-current/constant-voltage (CC/CV) charging, taper the current down as the battery approaches full, so the final portion of a charge often takes longer in practice than this linear estimate suggests.
How are amp-hours (Ah) and watt-hours (Wh) related?
Watt-hours equal amp-hours multiplied by voltage (Wh = Ah × V). This calculator uses your battery's nominal voltage to convert the amp-hours needed into watt-hours, which is useful for comparing against a charger's power rating or estimating the electricity used during charging.

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