Battery C-Rate Calculator

The Battery C-Rate Calculator converts between capacity, C-rate and charge or discharge current in a single step. Enter any two of the three and the calculator returns the third, together with the time the pack takes to empty or fill at that rate, the power and energy involved, and a check of whether the rate is safe for your cell chemistry.

What Is a Battery C-Rate?

The C-rate expresses current as a multiple of a battery's rated capacity, so a single number describes the same relative stress on a 2 Ah phone cell and a 400 Ah storage bank. 1C is by definition the current that would move the entire rated capacity in exactly one hour. On a 100 Ah battery that is 100 A; on a 3,000 mAh 18650 it is 3 A. Halving the C-rate halves the current and doubles the time, and doubling it does the opposite.

Because the number is relative, datasheets can state one safe limit — "1C continuous discharge" — that applies to every size in the range. That is why the C-rate, not the raw amperage, is the figure to check before you connect a load or a charger.

Battery C-Rate Formulas

Three quantities are linked by one relationship, so knowing two always gives the third.

If your capacity is given in milliamp-hours, either divide by 1,000 to get amp-hours or keep everything in milli-units and read the current in milliamps — the ratio is what matters, not the prefix.

C-Rate to Time Reference Table

Typical Safe C-Rates by Chemistry

These are conservative, widely published figures for continuous operation. Individual cells vary enormously between energy grades and power grades, so the datasheet for your exact cell always wins.

Why Real Runtime Is Shorter Than 1 ÷ C-Rate

Charging: Why 1C Does Not Mean One Hour

Lithium chargers use a CC-CV profile. During the constant-current phase the charger holds your chosen C-rate until the cell reaches its maximum voltage, which happens at roughly 80% state of charge. It then holds that voltage while the current tapers away, and this taper adds time without adding much capacity. In practice a 1C charge reaches full in about 1.2 to 1.4 hours, and a 0.5C charge in a little over two. This calculator reports both the constant-current phase and a realistic total.

The Peukert Effect on Lead-Acid

Lead-acid capacity is quoted at the 20-hour rate — a 100 Ah battery means 5 A for 20 hours, which is only 0.05C. Draw it faster and you get fewer amp-hours out. The Peukert equation models this:

where \( H \) is the rating time (20 h), \( C \) the rated capacity, \( I \) the discharge current and \( n \) the Peukert exponent, typically 1.1–1.3 for lead-acid. This calculator uses \( n = 1.15 \), a representative AGM value. Lithium chemistries have an exponent close to 1.0, which is why they are treated as ideal here.

How to Use This Calculator

1. Pick charge or discharge: the safe limits differ, so the verdict changes with the direction.
2. Pick what to solve for: Current, C-Rate or Capacity. The chosen field is greyed out and the other two become your inputs.
3. Enter the pack details: capacity in Ah or mAh, the known C-rate or current, and the cell chemistry. Add the nominal voltage if you want power and energy figures.
4. Click Calculate: read the answer, the time at that rate, the safety gauge against the chemistry limit and the full C-rate ladder.

Worked Example

A 12 V, 100 Ah LiFePO4 battery powering a 0.5C load: the current is \( 0.5 \times 100 = 50\,\text{A} \), the ideal runtime is \( 1 / 0.5 = 2 \) hours, and the power drawn is \( 12 \times 50 = 600\,\text{W} \) from 1,200 Wh of stored energy. Since LiFePO4 is normally rated for 1C continuous, 0.5C sits comfortably at half the limit — a gentle, long-life operating point.

Frequently Asked Questions

What does C-rate mean on a battery?

C-rate expresses current as a multiple of the battery's rated capacity. 1C is the current that would empty the rated capacity in exactly one hour, so a 100 Ah battery at 1C draws 100 A. 0.5C is half that current for twice the time, and 2C is double the current for half the time.

How do I calculate current from C-rate?

Multiply the C-rate by the capacity in amp-hours: Current (A) = C-rate × Capacity (Ah). A 5 Ah pack at 2C draws 10 A. If your capacity is in mAh, divide it by 1,000 first, or work in mAh and read the answer in mA.

How long does a battery last at a given C-rate?

In theory the time is 1 divided by the C-rate, in hours: 0.5C lasts 2 hours, 1C lasts 1 hour, 2C lasts 30 minutes. Real runtime is shorter because of the cut-off voltage, temperature and, on lead-acid, the Peukert effect.

What is a safe C-rate for LiFePO4?

Most LiFePO4 cells are rated for about 1C continuous discharge and 0.5C charge, with short bursts to 2C or 3C. Large prismatic storage cells are often limited to 0.5C. Always confirm against your cell's datasheet, because ratings vary widely between grades.

Does charging at 1C really take one hour?

No. A CC-CV charger only holds the set current to roughly 80% state of charge and then tapers at constant voltage, so a 1C charge typically takes around 1.2 to 1.4 hours to reach full. The constant-current phase is the fast part; the taper adds the rest.

Why does a lead-acid battery deliver less capacity at high C-rate?

Lead-acid capacity is quoted at the 20-hour rate, roughly 0.05C. At higher currents the chemical reaction cannot keep up with demand, so the delivered amp-hours drop. The Peukert equation models this: a 100 Ah battery discharged at 0.2C may only deliver around 80 Ah.

Is a higher C-rating always better?

Not necessarily. A high C-rating means the cell can move current quickly, but high-rate cells usually trade away energy density, and running any cell near its limit produces heat and shortens cycle life. Sizing the pack so your load sits well below the rating usually gives the longest service.

What is the difference between continuous and burst C-rate?

The continuous rating is what the cell can sustain until empty without exceeding its temperature limit. The burst or peak rating applies for a few seconds, typically during acceleration or motor start-up. RC LiPo packs are marketed with burst figures, which is why a "50C" pack should not be run continuously at 50C.

Additional Resources

• C-rate (battery) - Wikipedia
• Peukert's Law - Wikipedia
• Ampere-hour - Wikipedia