This online Battery Charge Time Calculator helps you estimate the time required to charge a battery pack based on its capacity, charging current (or power), and current state of charge (SoC).
Estimating Battery Charging Time
Electric battery charge time refers to the duration required to replenish a battery’s energy from its current state of charge to full capacity. While the calculation may seem straightforward at first glance, real-world conditions introduce complexities such as energy losses, varying charge rates, and battery health.
There are two common ways to express battery capacity:
• Ampere-hours (Ah), which indicates the amount of charge stored.
• Watt-hours (Wh), which considers both charge and voltage and provides a measure of energy.
Similarly, the charging input may be specified as:
• Current (in Amperes, A), representing the rate of charge flow.
• Power (in Watts, W), which combines current and voltage.
To accurately estimate charge time, both the units and the battery type must be considered.
Basic Formulas for Estimating Charge Time
Our Battery Charge Time Calculator uses the following working formulas related to different ways of representing both battery capacity and charging current strength or power.
When Battery Capacity is Given in Ampere-hours (Ah)
If the capacity is given in Ah and charging current is also provided in Amperes (A), the charging time is:
Where:
• Battery Capacity is in Ah,
• Current SoC is the current (or initial) state of charge expressed as a decimal (e.g., 50% = 0.5),
• Charging Current is in A,
• Charge Efficiency is a dimensionless factor which differs based on battery type.
A few words should be said about the Charge Efficiency parameter. It is also called energy efficiency and is of highest relevance. Energy efficiency is a measure of the amount of energy that can be taken from the battery compared to the amount of energy that was previously charged into the battery.
Example 1.
A 100Ah lead-acid battery at 50% SoC, charged with 10A.
Let’s assume a charging efficiency of 85%. Then we have: Charging Time = (100 × (1 − 0.5)) / (10 × 0.85) = 5.88 hours.
When Battery Capacity is in Watt-hours (Wh) and Charging Power is in Watts (W)
This approach is more common when working with Lithium-ion batteries, especially in consumer electronics and EVs. The corresponding formula in this case looks like this:
Where:
• Battery Energy is battery capacity in Wh,
• Charging Power is in W.
Example 2.
A 500Wh lithium-ion battery at 20% SoC charged at 100W.
Let’s assume a charging efficiency of 95%. Then we have: Charging Time = (500 × (1 − 0.2)) / (100 × 0.95) = 4.21 hours.
Our Battery Charge Time Calculator also covers the cases when the battery capacity is given in Ah and the power in W is given instead of the current, or when the capacity is given in Wh and the current in A. In all these cases, in order to use the above formulas, we need an additional parameter – the nominal voltage (in Volts, V) of the battery in question. These parameters are related by an obvious formula:
We would like to emphasize that in such cases, our calculator automatically adds a field for battery voltage.
Example 3.
A 400Wh nickel cadmium battery at 40% SoC charged with 10A. The battery voltage is 5V.
Let’s assume a charging efficiency of 80%. Then we have: Charging Time = (400 × (1 − 0.4)) / (10 × 5 × 0.8) = 6 hours.
Adjusting for Efficiency
Charging a battery is never 100% efficient. Part of the energy is lost as heat or due to internal resistance. Different battery chemistries exhibit different charge/discharge efficiencies.
To account for this, you can adjust the ideal charging time using the following formula:
This is how the parameter Charge Efficiency appeared in our calculation formulas above.
Below is a table with efficiency ranges of the main types of rechargeable batteries:
Factors Influencing Charging Time
As it was mentioned above, the charging time is affected by more than just capacity and input current or power. Here are key variables that can alter the time needed to fully charge a battery.
• Battery Chemistry. Different chemistries handle charge differently. Lithium-ion batteries charge faster and more efficiently, while lead-acid and nickel-based batteries may require a more staged approach with slower final phases to avoid damage.
• Initial State of Charge (SoC). Charging from 0% takes longer than topping off from 50%. Most batteries also charge faster during the initial phase and slow down significantly toward the end to prevent overcharging.
• Charge Rate (C-rate). The C-rate is a measure of how fast a battery is charged relative to its capacity. A C-rate of 1C means the battery will be fully charged in one hour. Charging a 100Ah battery at 50A corresponds to a 0.5C rate. Higher C-rates can reduce charge time but may lead to heat buildup and reduced lifespan.
• Charger Limitations. The actual output of a charger may be limited by safety features, power source limitations, or charging protocol. Smart chargers may reduce current dynamically to prolong battery life or reduce heat.
• Temperature. Batteries are sensitive to temperature. Charging in cold environments slows down ion movement, while high temperatures can accelerate charging but risk damaging the battery or reducing its life expectancy.
• Battery Age and Health. Over time, batteries degrade. Internal resistance increases, reducing the ability to accept charge efficiently. Older batteries may take longer to charge or not reach full capacity.
Related calculators
Check out our other physics calculators such as Battery Capacity Calculator or Capacitance Calculator.