Battery charge time: the formula, why the last 20% is slow, and what a C-rate means
The simple rule is capacity over current: a 3,000 mAh cell on a 1 A charger takes three hours. Real chargers take longer. Part of the charge is lost as heat, and lithium and lead-acid chargers hold the current only until the cell reaches its voltage limit, then let it taper, so the last fifth of the charge can take as long as the first four. This calculator works out both phases from the capacity, the charger current and the chemistry, gives the C-rate with the chemistry's limits, the charge pushed in against the charge stored, the energy, and a timeline to every state of charge, with the cell and the charge curve drawn live.
How to use the battery charge time calculator
- Type the capacity in mAh and, if you want the energy, the nominal voltage.
- Type the charger current (1 A, or 500 mA) and pick the chemistry in the menu row; it sets the efficiency and whether there is a constant-voltage tail.
- Leave the charge at 0% to 100% or type a top-up such as 20% to 80%. The Timeline tab lists when each state of charge is reached.
The formula
t = C × ΔSoC / (I × η)
Charge is current times time, so the time is the charge needed over the current. The charge needed is the capacity times the share to fill, divided by the efficiency η, because the cell stores less than the charger pushes in: 99% for lithium, about 75% for NiMH, 85% for lead-acid. That gives the time for a charger that holds its current to the end. Lithium and lead-acid chargers do not, so a second term is added for the taper.
Constant current, then constant voltage
A lithium cell must never exceed 4.2 V. The charger pushes its full current until the cell reaches that voltage, at roughly 80% charge, then holds the voltage and lets the current fall as the cell fills, stopping when the current drops to a termination level, usually 0.05 C. In that tail the current decays exponentially, so its average is (I − Iend) / ln(I / Iend), and the last 20% of the charge take that remaining charge over that average. With a 1 A charger and a 3 Ah cell the average is about 0.45 A, which is why the tail takes nearly as long as the bulk phase. A bigger charger shortens the bulk phase but hardly the tail, and stopping at 80% skips it altogether.
C-rates
The C-rate is the current over the capacity: 1 C is 3 A for a 3 Ah cell, C/10 is 300 mA. Most lithium cells are charged at 0.5 C and tolerate 1 C; fast-charging phones use a cell designed for more. NiMH is safe at C/10 for 14 to 16 hours without any termination, and at 0.5 to 1 C only with −ΔV or temperature cut-off. Lead-acid likes 0.1 to 0.2 C. Above the chemistry's limit the cell heats, lithium plates on the anode, and the cycle life falls.
Chemistries
| Chemistry | Efficiency | CC up to | Stops at | Usual · limit |
|---|---|---|---|---|
| Li-ion / LiPo | 99% | 80% | C/20 | 0.5 C · 1 C |
| LiFePO4 | 99% | 90% | C/20 | 0.5 C · 1 C |
| NiMH | 75% | the end | timer / −ΔV | 0.5 C · 1 C |
| NiCd | 70% | the end | timer / −ΔV | 0.5 C · 2 C |
| Lead-acid (SLA / AGM) | 85% | 80% | C/50 | C/10 · 0.3 C |
| Lithium titanate (LTO) | 99% | 85% | C/20 | 1 C · 5 C |
Your charge, step by step
- Charge to push in: 3,000 mAh × 100% / 99% = 3,030 mAh (stores 3,000 mAh).
- Rate: 1 A / 3,000 mAh = 0.33 C.
- Constant current: 2,424 mAh / 1 A = 2 h 25 min to 80%.
- Taper: average 448 mA, 606 mAh → 1 h 21 min.
- Total: 3 h 47 min, 80% after 2 h 25 min.
Worked example: a 3,000 mAh 18650 on a 1 A charger
The charger runs at 1 A / 3 Ah = 0.33 C, a normal rate. From empty, with 99% efficiency, the first 80% need 2,424 mAh at 1 A: 2 h 25 min. At 4.2 V the current tapers down to 0.05 C = 150 mA; its average over the tail is (1 − 0.15) / ln(1 / 0.15) = 448 mA, so the last 606 mAh take 1 h 21 min. Total: 3 h 47 min, against the three hours of the simple rule, and 80% is reached after 2 h 25 min. A 2 A charger would cut the bulk phase to 1 h 13 min but the whole charge only to 2 h 4 min, because the tail barely changes.
Questions
Why does my phone charge to 80% quickly and then crawl?
That is the constant-voltage tail. The charger cannot raise the voltage any further, so the current falls as the cell fills. Many phones also slow the last 20% on purpose to spare the cell.
Can I charge a battery faster with a bigger charger?
Up to the chemistry's limit, yes, but only the bulk phase shrinks. Beyond the limit the cell heats and ages; a battery with a built-in protection board may also cap the current.
Does the charger's voltage rating matter here?
The charger must match the chemistry and the number of cells in series; the calculator assumes it does and only uses the current. The nominal voltage is used for the energy figure.
How accurate is this?
Within about 10% for a healthy cell at room temperature. A cold or worn cell reaches its voltage limit earlier and spends longer in the tail; a charger that derates when warm takes longer still.