How Long to Charge a Battery
Charging time depends on the charger's current output, the battery's capacity, and the charging profile used. A battery does not charge at a constant rate throughout the entire cycle — the last 10–20% takes significantly longer than the first 80%.
The Two-Phase Charging Profile
Most lithium-ion and LFP batteries use a CC-CV (Constant Current, Constant Voltage) charging profile. Understanding these two phases is essential for estimating accurate charge times.
During the CC phase, the charger delivers a constant current (for example, 50A into a 100Ah battery). The battery voltage rises steadily during this phase. This is the fast-charging portion, typically adding 80% of the capacity.
During the CV phase, the charger holds the voltage constant at the battery's maximum (e.g., 3.65V per cell for LFP) while the current tapers exponentially. This phase adds the final 10–20% but takes disproportionately longer because the current is decreasing.
Charging Time Formulas
The 1.25–1.35 multiplier accounts for the CV phase where current tapers. The exact multiplier depends on how deeply the battery was discharged and the chemistry's CV taper characteristics.
Charge Time by C-Rate
C-rate standardizes charging speed regardless of battery size. A 100Ah battery at 0.5C charges at 50A; a 200Ah battery at 0.5C charges at 100A. Both take roughly the same time because the rate is proportional to capacity.
| C-Rate | 100Ah Current | Approx. Time | Notes |
|---|---|---|---|
| 0.1C | 10 A | ~12 hours | Trickle, maximum battery life |
| 0.2C | 20 A | ~6 hours | Slow charge, LFP longevity |
| 0.5C | 50 A | ~2.5 hours | Standard fast charge |
| 1C | 100 A | ~1.25 hours | Aggressive, more heat |
| 2C | 200 A | ~40 minutes | Very fast, reduced cycle life |
Worked Example
Given:
- Battery: 200 Ah at 48V (LFP server rack)
- Charger: 48V 50A (0.25C rate)
- Starting SOC: 20%
- Target SOC: 100%
Step 1: Calculate charge required (Ah):
Step 2: CC phase time:
Step 3: Estimate total time including CV taper:
At a higher 1C charger (100A), the same 80% to 100% charge would take approximately 1 hour 18 minutes. The trade-off is increased cell heating and potential long-term capacity degradation.
Charging Time by Battery Size
The table below shows typical charging times from 20% to 100% SOC for common battery sizes using a 0.2C charger.
| Battery Size | 0.2C Charger | 0.5C Charger | 1C Charger |
|---|---|---|---|
| 50 Ah 12.8V | ~5.2 hrs | ~2.1 hrs | ~1.0 hrs |
| 100 Ah 12.8V | ~5.2 hrs | ~2.1 hrs | ~1.0 hrs |
| 200 Ah 48V | ~5.2 hrs | ~2.1 hrs | ~1.0 hrs |
| 300 Ah 48V | ~5.2 hrs | ~2.1 hrs | ~1.0 hrs |
The times are similar across battery sizes because C-rate is proportional. A larger battery gets proportionally more current from a larger charger, keeping the charge time constant.
Factors That Affect Charge Time
Starting SOC
Charging from 0% takes longer than from 20% because the CC phase covers a larger portion of capacity. The CV taper phase remains roughly constant regardless of starting SOC.
Temperature
Cold batteries accept less current. Below 0°C, lithium BMS units typically block charging entirely. Between 0–10°C, charge rates may be reduced to 0.1C, extending charge time significantly.
Charger Power Limit
The charger's maximum current output is the bottleneck. A 100Ah battery with a 25A charger (0.25C) charges much slower than with a 100A charger (1C), regardless of the battery's capability.
Battery Age
Aging batteries have higher internal resistance, which causes the CV phase to trigger earlier and the taper to be slower. This extends total charge time by 10–20% for batteries at 80% SOH.
Try It
Use the Charging Time Calculator to estimate charge duration for your specific battery and charger configuration.
Open Charging Time CalculatorRelated Articles
What Is Battery C-Rate?
Understand the ratio of current to capacity and how it governs charge speed.
Read Guide →How Long Will a Battery Last?
The complementary guide: estimating discharge runtime from capacity and load.
Read Guide →Frequently Asked Questions
How long does it take to charge a 100Ah battery?
It depends on the charger current. At 0.2C (20A), a 100Ah LFP battery charges in approximately 5 hours. At 0.5C (50A), it charges in about 2 hours. The final CV (constant voltage) phase adds 30–60 minutes as the battery tops off.
Why does charging slow down near full?
Lithium batteries use a CC-CV (constant current, constant voltage) profile. During the CC phase, current is constant and charging is fast. During the CV phase, the charger holds voltage constant while current tapers, taking progressively longer to add each percent of charge.
Can I charge a battery faster than 1C?
Some lithium chemistries support 1C to 2C charging, but faster rates generate more heat and accelerate degradation. LFP chemistry is typically charged at 0.2C to 0.5C for optimal cycle life. Always follow the manufacturer's recommended charge rate.
Does temperature affect charging time?
Yes. Cold batteries accept less current because the BMS limits charge rate to prevent lithium plating. Below 0°C, most lithium batteries should not be charged at all. Charging in cold conditions extends the CV phase, increasing total charge time.