Use this battery charging calculator to find charging current from C-rate or estimate how long a recharge will take from battery capacity, starting state of charge and target state of charge. Enter only the information you know, calculate, and then expand the result if you want to see the math.
Battery charging calculator
Enter battery capacity and either a C-rate or a known charging current. The calculator returns charge current, capacity to add and estimated charging time.
Advanced options
Show calculation
How to calculate battery charging current
If you know the battery capacity and C-rate, charging current is:
I = Q × C_rate
I is charging current in amperes, Q is battery capacity in ampere-hours and C_rate is the entered C-rate. A 100 Ah battery at 0.1C gives 100 × 0.1 = 10 A. At 0.2C, the same 100 Ah battery gives 20 A.
| Capacity | C-rate | Calculation | Current |
|---|---|---|---|
| 2 Ah | 0.1C | 2 × 0.1 | 0.2 A |
| 2 Ah | 0.2C | 2 × 0.2 | 0.4 A |
| 10 Ah | 0.5C | 10 × 0.5 | 5 A |
| 50 Ah | 0.2C | 50 × 0.2 | 10 A |
| 100 Ah | 0.1C | 100 × 0.1 | 10 A |
| 200 Ah | 0.1C | 200 × 0.1 | 20 A |
These are mathematical conversions only. The calculator does not decide whether a particular charging rate is appropriate for a specific battery; use the battery and charger documentation for that decision.
How to estimate battery charging time
The calculator first works out how much capacity must be added between the starting and target states of charge:
ΔAh = Q × (SoC_target − SoC_start) ÷ 100
For a 100 Ah battery charged from 20% to 80%, the difference is 60 percentage points, so 100 × 60 ÷ 100 = 60 Ah must be added.
The ideal charging-time estimate is then:
t = ΔAh ÷ I
With 60 Ah to add and a charging current of 10 A, the result is 60 ÷ 10 = 6 hours. If you choose an optional time adjustment, the calculator multiplies that ideal time by the selected factor and shows the adjusted estimate separately.
Use C-rate or charging current
Choose the input that matches what you already know. In C-rate mode, enter a value such as 0.1 or 0.2 and the calculator converts it to amperes from battery capacity. In Charging current mode, enter the known charger current directly in amperes.
This distinction matters. For a 100 Ah battery, 0.2C equals 20 A, while 0.2 A is simply two-tenths of an ampere. The calculator keeps these two input modes separate so the result is clear.
NiMH charge-rate calculations with mAh
Small batteries are often labelled in milliamp-hours. The calculator can accept mAh directly and convert it to Ah before applying the C-rate formula. Since 1000 mAh = 1 Ah, a 2000 mAh battery is 2 Ah.
| C-rate | Capacity in Ah | Calculation | Current |
|---|---|---|---|
| 0.1C | 2 Ah | 2 × 0.1 | 0.2 A (200 mA) |
| 0.2C | 2 Ah | 2 × 0.2 | 0.4 A (400 mA) |
| 0.5C | 2 Ah | 2 × 0.5 | 1 A |
| 1C | 2 Ah | 2 × 1 | 2 A |
The arithmetic is the same regardless of chemistry. These examples do not recommend a NiMH charging rate; they show what a chosen C-rate converts to in amperes.
Lead-acid battery charging-current calculation
If you already have a C-rate from the battery or charger documentation, multiply it by capacity. For example, entering 0.1C for a 100 Ah battery gives 10 A, while 0.1C for a 200 Ah battery gives 20 A.
If you already know the charger current, switch to charging-current mode and enter the amperes directly. The calculator then uses that current with the selected start and target SoC to estimate the recharge time.
Partial-charge example
Suppose a 100 Ah battery is at 20% SoC and you want to reach 80% with a 10 A charging current.
- SoC to restore:
80 − 20 = 60% - Capacity to add:
100 × 60 ÷ 100 = 60 Ah - Ideal charging time:
60 ÷ 10 = 6 hours
This is why a partial recharge can take much less time than a full 0% to 100% calculation even when battery capacity and current stay the same.
Nominal energy is optional
Charging current and ideal charge time do not require battery voltage in this model. If you enter nominal voltage under Advanced options, the calculator can also show:
E = ΔAh × V_nom
For 60 Ah to add at 12 V nominal, the nominal-energy result is 60 × 12 = 720 Wh. This is a nominal battery-side calculation from the entered voltage and Ah; it is not a complete model of charger AC input or feeder requirements.
What the result shows
The main result is intentionally short: charging current, capacity to add and estimated charging time. You can then open the calculation details to see the formulas and substituted values. This keeps the common task fast while still making every number checkable.
Frequently asked questions
How do I calculate charging current from C-rate?
Multiply battery capacity in Ah by C-rate. For example, 50 Ah at 0.2C gives 50 × 0.2 = 10 A.
How do I convert mAh to Ah?
Divide mAh by 1000. For example, 2500 mAh equals 2.5 Ah. The improved calculator can also perform this conversion automatically when mAh is selected.
How do I calculate the Ah that must be replaced?
Multiply battery capacity by the difference between target and starting SoC, then divide by 100. A 100 Ah battery moving from 30% to 80% needs 100 × 50 ÷ 100 = 50 Ah.
Does nominal voltage change charging time?
Not in the calculator’s core time equation. Voltage is optional and is used for the nominal-energy result.
Why does entering 0.2 sometimes mean 0.2 A and sometimes 0.2C?
It depends on the selected input mode. C-rate mode treats 0.2 as 0.2C and converts it to current from capacity. Charging-current mode treats 0.2 as 0.2 A.
Does the calculator choose a safe charging rate for my battery?
No. It calculates from the values you enter. Use the battery and charger documentation to choose an appropriate charging rate for the specific product.