Calculate discharge rates and estimated runtime relative to battery capacity.
Ah (Amp-hours)
mAh (Milliamp-hours)
A (Amps)
mA (Milliamps)
Please enter valid positive numbers for both fields.
Calculated C-Rate
0 C
This means the battery is being discharged at 0 times its capacity.
Theoretical Time to Empty/Full
0 Minutes
Assuming 100% efficiency and constant current.
How to Calculate Battery C-Rate
The C-rate is a unit used to measure the speed at which a battery is fully charged or discharged relative to its maximum capacity. Understanding C-rate is crucial for battery health, safety, and estimating how long a device will run.
Formula:
C-Rate = Current (Amps) รท Rated Capacity (Amp-hours)
Why is C-Rate Important?
A C-rate of 1C means the current will discharge the entire battery in exactly 1 hour. Higher C-rates imply faster charging or discharging, which often generates more heat and can reduce the battery's overall lifespan if it exceeds manufacturer specifications.
0.5C: Discharges in 2 hours (gentle usage).
1C: Discharges in 1 hour (standard baseline).
2C: Discharges in 30 minutes (high performance).
5C+: Discharges in minutes (racing drones, power tools).
Calculation Example
Let's say you have a Lithium-ion battery with a capacity of 3000 mAh (which is 3 Ah) and your device draws a current of 6 Amps.
Convert units to match: 3000 mAh = 3 Ah.
Apply the formula: C-Rate = 6 A / 3 Ah.
Result: 2C.
This means your device is drawing power at twice the battery's capacity rate, and the battery will theoretically last for 30 minutes (1 hour divided by 2).
Factors Affecting Real-World Results
While the mathematical calculation provides a theoretical baseline, real-world battery performance is affected by Peukert's Law (for lead-acid) and internal resistance heat losses (for Lithium). As the C-rate increases, the effective capacity often decreases due to voltage sag and heat dissipation. Therefore, a battery discharged at 10C will usually provide less total energy than the same battery discharged at 0.5C.
function calculateBatteryCRate() {
// 1. Get input elements
var capInput = document.getElementById("batteryCapacity");
var currInput = document.getElementById("batteryCurrent");
var capUnitSelect = document.getElementById("capacityUnit");
var currUnitSelect = document.getElementById("currentUnit");
var resultsArea = document.getElementById("resultsArea");
var errorDisplay = document.getElementById("errorDisplay");
var cRateResult = document.getElementById("cRateResult");
var cRateExplanation = document.getElementById("cRateExplanation");
var timeResult = document.getElementById("timeResult");
// 2. Parse values
var capacityVal = parseFloat(capInput.value);
var currentVal = parseFloat(currInput.value);
var capMultiplier = parseFloat(capUnitSelect.value);
var currMultiplier = parseFloat(currUnitSelect.value);
// 3. Validation
if (isNaN(capacityVal) || isNaN(currentVal) || capacityVal <= 0 || currentVal = 1) {
var h = Math.floor(timeInHours);
var m = Math.round((timeInHours – h) * 60);
timeDisplay = h + " Hour" + (h !== 1 ? "s" : "") + " " + m + " Min";
} else {
var totalMinutes = timeInHours * 60;
if (totalMinutes < 1) {
var seconds = Math.round(totalMinutes * 60);
timeDisplay = seconds + " Seconds";
} else {
timeDisplay = Math.round(totalMinutes) + " Minutes";
}
}
// 7. Display Results
resultsArea.style.display = "block";
cRateResult.innerHTML = cRate.toFixed(2) + " C";
cRateExplanation.innerHTML = cRate.toFixed(2);
timeResult.innerHTML = timeDisplay;
}