Quick answer
Charge time = capacity × share to recharge ÷ (charger current × efficiency). Recharging 80% of a 100 Ah battery with a 10 A charger at 85% efficiency takes 80 ÷ 8.5 ≈ 9.41 hours.
Enter the battery capacity, the charger's current, the efficiency and how much of the capacity needs refilling. The result is in hours.
Related tools: EV Charge Time Calculator: Time to charge between two battery levels. Time Converter: Convert seconds, minutes, hours, days, weeks, months and years. Time Duration Calculator: Elapsed time between two clock times, including across midnight.
How to use the Battery Charge Time Calculator
- Enter battery capacity (Ah) (above 0).
- Enter charger current (A) (above 0).
- Enter charging efficiency (%) (between 0 and 100). It starts at 85.
- Enter share of capacity to recharge (%) (between 0 and 100). It starts at 100.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Battery capacity | A number, in Ah | > 0 |
| Charger current | A number, in A | > 0 |
| Charging efficiency | A number, in % | > 0, ≤ 100, starts at 85 |
| Share of capacity to recharge | A number, in % | > 0, ≤ 100, starts at 100 |
Formula and method
Formula
time = capacity × share to recharge ÷ (charger current × efficiency)
IEEE-754 double precision; shown to the decimals indicated.
A constant-current estimate; chargers taper near full, which adds time.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Battery capacity = 100; Charger current = 10; Charging efficiency = 85; Share of capacity to recharge = 80.
| Result | Value |
|---|---|
| Charge time | 9.41 h |
Example 2
Inputs: Battery capacity = 50; Charger current = 5; Charging efficiency = 90; Share of capacity to recharge = 100.
| Result | Value |
|---|---|
| Charge time | 11.11 h |
Example 3
Inputs: Battery capacity = 200; Charger current = 20; Charging efficiency = 80; Share of capacity to recharge = 50.
| Result | Value |
|---|---|
| Charge time | 6.25 h |
The first line is the headline result (charge time). IEEE-754 double precision; shown to the decimals indicated.
Why efficiency matters
Not all the charger's current is stored. At 85% efficiency a 10 A charger delivers an effective 8.5 A, so the 80 Ah that must be put back takes 9.41 hours instead of 8. Lead-acid batteries often have lower efficiency near full charge.
What the simple model omits
Real charging is not constant current: chargers taper as the battery fills, and the last 20% can take as long as the first 80%. Temperature and battery age change both capacity and efficiency. The figure is a planning estimate only.
How the result changes with battery capacity
Holding every other input at the first worked example, this table changes only battery capacity. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Battery capacity | Charge time |
|---|---|
| 50 | 4.71 h |
| 100 | 9.41 h |
| 200 | 18.82 h |
| 400 | 37.65 h |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Battery capacity: entering "0" shows "Battery capacity must be greater than 0." beside that field and no result.
- Charging efficiency: entering "0" shows "Charging efficiency must be greater than 0." beside that field and no result.
Choosing between the Battery Charge Time Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [EV Charge Time Calculator](/automotive/ev-charge-time-calculator/) | usable battery capacity, starting charge and target charge | Charging time |
| [Time Converter](/converters/time-converter/) | value, from and to | Day (d) → Hour (h) |
| [Time Duration Calculator](/time/time-duration-calculator/) | start time and end time | Duration |
| [Time Card Calculator](/time/time-card-calculator/) | start times, end times and unpaid break per day | Total time |
| [Download Time Calculator](/technology/download-time-calculator/) | file size, size unit and connection speed | Time |
Assumptions and limitations
- Real charging slows near full
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for battery capacity and charging efficiency to confirm this.
Frequently asked questions
How do I charge a battery in less time?
Use a higher-current charger within the battery's rated charge current.
Does voltage matter?
Not here: the formula uses amp-hours and amps, so capacity and current must be in the same units, for example Ah and A.
Can I use it for a phone?
Yes, with mAh and mA, but phone chargers taper heavily and will usually be slower than the estimate.
What does the Battery Charge Time Calculator calculate?
Hours to charge a battery from a charger current. It returns Charge time.
What formula does the Battery Charge Time Calculator use?
time = capacity × share to recharge ÷ (charger current × efficiency). A constant-current estimate; chargers taper near full, which adds time.
What do I need to enter in the battery Charge Time Calculator?
You enter Battery capacity (Ah), Charger current (A), Charging efficiency (%) and Share of capacity to recharge (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Real charging slows near full.
What is an example of a battery Charge Time result?
With battery capacity 100, charger current 10, charging efficiency 85 and share of capacity to recharge 80, the charge time is 9.41 h.
Where does the method come from?
The method follows: Charge balance (Q = I × t).
Related calculators
- EV Charge Time Calculator — Time to charge between two battery levels.
- Time Converter — Convert seconds, minutes, hours, days, weeks, months and years.
- Time Duration Calculator — Elapsed time between two clock times, including across midnight.
- Time Card Calculator — Total hours, overtime and gross pay from daily start and end times.
- Download Time Calculator — Time to download a file at a given speed.
- Time From Now Calculator — Clock time after adding or subtracting minutes.
Sources
- Charge balance (Q = I × t)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.