Battery Charge Time Calculator
Estimate how long a charge really takes. Enter battery capacity in mAh or Ah, the charger's output current in amps, your starting and target percentages, and the chemistry — the calculator applies a stated charge-efficiency factor and shows time, energy added, and the C-rate.
Example: with Battery capacity 5000 · Capacity unit mAh (phones, packs) · Charger output current (A) 2 · Starting charge (%) 20 · Target charge (%) 100 → Estimated charge time: 2.22 h (≈ 2 h 13 min) — the last 15-20% tapers, so 100% often runs longer.
- Charge to add4.0 Ah (4,000 mAh) into the battery
- Charge rate (C-rate)0.40C — gentle; standard overnight-style charging
Computed by the calculator below using its default values. Change any input to see your own numbers.
Time = capacity to add ÷ (charger current × charge efficiency). Efficiency assumptions are stated plainly: about 90% for li-ion, 85% for lead-acid, 75% for NiMH — losses go to heat and, near full, to taper/absorption phases.
The honest version of capacity ÷ current
The naive formula — hours = Ah ÷ A — assumes every electron the charger pushes ends up stored. It does not: some becomes heat, and chemistries differ in how much. This calculator divides by an explicit efficiency factor: roughly 90% for lithium-ion, 85% for lead-acid, 75% for NiMH (the standard round numbers from battery-university-style references). So a 5,000 mAh phone at 20% on a 2 A charger needs 4 Ah ÷ (2 A × 0.9) ≈ 2.2 hours to reach 100%.
There is a second, separate effect the single factor cannot fully capture: charge tapering. Li-ion chargers run constant-current until about 80-85%, then hold constant voltage while current falls; lead-acid has a similar absorption phase. That is why 0-80% is quick and the last 20% crawls — for time-critical top-ups, charge to 80% and leave.
Reading the C-rate
C-rate is charging current relative to capacity: 2 A into a 5 Ah battery is 0.4C. Most li-ion cells accept 0.5-1C comfortably; fast-charge phones push beyond that with sophisticated thermal management. Lead-acid prefers 0.1-0.3C — a 100 Ah AGM battery is happiest around 10-25 A. Charging faster than the rating shortens cycle life and, for lead-acid, gasses the electrolyte.
How it’s calculated
Charge to add (Ah) = capacity × (target% − start%) ÷ 100, with mAh ÷ 1000 = Ah. Time (h) = charge to add ÷ (charger current × efficiency), using stated charge efficiencies of 0.90 (li-ion), 0.85 (lead-acid), 0.75 (NiMH). C-rate = charger current ÷ capacity in Ah. Peukert-style rate effects and the constant-voltage taper are not modeled beyond the efficiency factor; the taper note flags charges past 90%.
Assumes the charger actually delivers its rated current the whole time — real li-ion charging tapers after ~80% and battery-management systems throttle on heat, so treat times to 100% as best-case floors.
Typical charge times with these assumptions
| Battery | Charger | 0 → 100% time |
|---|---|---|
| Phone, 5,000 mAh (li-ion, 90%) | 2 A | 2.8 h |
| Tablet, 8,000 mAh (li-ion, 90%) | 3 A | 3.0 h |
| Power bank, 20,000 mAh (li-ion, 90%) | 3 A | 7.4 h |
| Car battery, 60 Ah (lead-acid, 85%) | 10 A | 7.1 h |
| AA NiMH, 2,000 mAh (75%) | 0.5 A | 5.3 h |
Computed with time = Ah ÷ (A × efficiency); rounded to 0.1 h. Real li-ion charges to 100% run somewhat longer due to taper.
Common mistakes
- Using the charger's wattage as its current — a '20 W' USB-C charger delivers current set by voltage negotiation; use the amp rating at the battery's voltage.
- Ignoring efficiency and taper, then wondering why the last 20% takes an hour.
- Mixing mAh and Ah: 5,000 mAh is 5 Ah; slipping a factor of 1,000 gives absurd times.
- Charging lead-acid at li-ion speeds — above ~0.3C it heats, gasses, and loses life fast.
Frequently asked questions
What is the battery charge time formula?
Time (hours) = capacity to add (Ah) ÷ (charger current in amps × charge efficiency). Example: 4 Ah into a phone from a 2 A charger at 90% efficiency takes 4 ÷ 1.8 ≈ 2.2 hours.
Why does my phone charge to 80% fast and then slow down?
Li-ion charging switches from constant current to constant voltage around 80-85%, and the current tapers toward zero as the cell fills. The efficiency factor here covers average losses, but the final 15-20% inherently crawls by design — it protects the cell.
Does a higher-watt charger always charge faster?
Only up to what the device's battery management allows. If a phone caps intake at 25 W, a 100 W charger delivers 25 W. For fixed-current charging (tools, lead-acid chargers), more amps do cut time proportionally — until you exceed the safe C-rate.
What efficiency should I assume for my battery?
Round planning numbers: li-ion about 90%, lead-acid about 85% (AGM at the high end, flooded lower), NiMH about 75%. These fold heat and overhead losses into one factor; cold batteries and fast rates do worse.
Is it bad to charge to 100% every time?
For li-ion longevity, living between roughly 20% and 80% reduces wear; occasional full charges are fine. Lead-acid is the opposite — it wants to be returned to full and sulfates if left partially charged.