Battery charge time calculator
Enter your battery capacity and charger current to estimate how long charging will take. Choose a target charge and allow for the slowdown as the battery fills.
Battery capacity and nominal voltage were carried over from runtime. Check the charging current and starting and target charge levels before using this estimate.
Estimated charge time from 20% to 100%: 39 hr 4 min.
20% to 100% charge
Based on 78.125 Ah at 2 A. Actual time depends on your battery and charger.
Where the time goes
Constant current23 hr 26 min
Slower finish15 hr 38 min
Assumes 2 A up to 80% and an average of 1 A afterward. How this is calculated
With a faster or slower finish: 33 hr 51 min to 54 hr 41 minThese estimates use 75% and 25% average taper current. Other conditions can put the actual time outside this range.
Time by charge target
From the same 20% starting charge, using the same assumptions.
| Target | Estimated time |
|---|---|
| 80% | 23 hr 26 min |
| 90% | 31 hr 15 min |
| 100% Your target | 39 hr 4 min |
Compare charger currents
Each estimate reaches 100% using the same taper settings. Use these to compare charging times, then check which currents your battery supports. Taper behavior can also change with current.
| Battery current | Estimated time |
|---|---|
| 1 A | 78 hr 8 min |
| 2 A | 39 hr 4 min |
| 4 A | 19 hr 32 min |
Understand the estimate
The last few percent take longer.
Dividing capacity by charger current gives the constant-current baseline. Lithium chargers also have a constant-voltage stage: the charger holds the voltage while the current falls, then ends charging according to its termination rules.
Texas Instruments describes this sequence in its BQ2057 charging documentation. The 80% taper starting point and 50% average current in this calculator are adjustable examples. They are not specifications from TI.
Battery charge time formula
Ideal hours = capacity (Ah) × charge added (%) ÷ 100 ÷ current (A)
For the adjusted estimate, split the charge added at the point where taper begins. Divide the first portion by the charging current and the remainder by the average taper current, then add the two times.
Worked example: a 10 Ah battery charging at 2 A from 20% to 100% needs 8 Ah. The ideal time is 4 hours. With taper beginning at 80% and averaging 1 A, the first 6 Ah take 3 hours and the last 2 Ah take 2 hours: 5 hours total.
Battery charging questions
How do I calculate charging time from mAh or Wh?
Divide mAh by 1,000 to get Ah. For Wh, divide by the battery’s nominal voltage: 360 Wh at 36 V is 10 Ah. Use the voltage associated with that energy rating, then enter the current actually delivered to the battery.
Will doubling the charging current halve the time?
Doubling current halves the constant-current calculation. Actual full-charge time may not halve because the taper behavior, thermal limits, charge controller, and BMS can limit current. Compare only compatible chargers within the battery’s specified charge-current limit; a discharge-current rating is not a charge rating.
Can I use this for LiFePO4, lead-acid, or solar charging?
The adjustable model is for lithium batteries charged using CC/CV, including LiFePO4 with a suitable charger. See the TI BQ25306 documentation for an example supporting these chemistries. It does not model lead-acid absorption/float, NiMH termination, or varying solar output. Different lithium batteries also need different voltage settings; this tool does not select those settings.
Why can my battery take longer than the estimate?
The model assumes a healthy, idle battery and the entered current reaching it. Low-voltage preconditioning, temperature restrictions, aging, cell balancing, charger power limits, and a device running during charging can extend the time. A 0% input represents the reported empty state, not a battery at zero volts. Use a measured charging curve to refine the taper assumptions when available.