body {
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
line-height: 1.6;
color: #333;
max-width: 800px;
margin: 0 auto;
padding: 20px;
}
.calculator-container {
background-color: #f8f9fa;
border: 1px solid #e9ecef;
border-radius: 8px;
padding: 30px;
margin-bottom: 40px;
box-shadow: 0 4px 6px rgba(0,0,0,0.05);
}
.calc-header {
text-align: center;
margin-bottom: 25px;
color: #2c3e50;
}
.input-group {
margin-bottom: 20px;
}
.input-group label {
display: block;
margin-bottom: 8px;
font-weight: 600;
color: #495057;
}
.input-group input, .input-group select {
width: 100%;
padding: 12px;
border: 1px solid #ced4da;
border-radius: 4px;
font-size: 16px;
box-sizing: border-box;
}
.input-group .hint {
display: block;
font-size: 0.85em;
color: #6c757d;
margin-top: 5px;
}
.calc-btn {
background-color: #007bff;
color: white;
border: none;
padding: 15px 20px;
font-size: 18px;
border-radius: 4px;
cursor: pointer;
width: 100%;
font-weight: bold;
transition: background-color 0.2s;
}
.calc-btn:hover {
background-color: #0056b3;
}
.results-area {
margin-top: 25px;
background-color: #ffffff;
border: 1px solid #dee2e6;
border-radius: 4px;
padding: 20px;
display: none;
}
.result-row {
display: flex;
justify-content: space-between;
margin-bottom: 15px;
border-bottom: 1px solid #eee;
padding-bottom: 10px;
}
.result-row:last-child {
border-bottom: none;
margin-bottom: 0;
padding-bottom: 0;
}
.result-label {
font-weight: 600;
color: #495057;
}
.result-value {
font-weight: bold;
color: #28a745;
font-size: 1.1em;
}
.article-content h2 {
color: #2c3e50;
margin-top: 30px;
border-bottom: 2px solid #eee;
padding-bottom: 10px;
}
.article-content h3 {
color: #34495e;
margin-top: 20px;
}
.formula-box {
background-color: #e8f4fd;
padding: 15px;
border-left: 4px solid #007bff;
margin: 20px 0;
font-family: "Courier New", Courier, monospace;
}
.warning-box {
background-color: #fff3cd;
border: 1px solid #ffeeba;
color: #856404;
padding: 15px;
border-radius: 4px;
margin-top: 10px;
}
How to Calculate Battery Discharge Rate
Understanding battery discharge rates is crucial for engineers, hobbyists, and anyone relying on battery backup systems. Whether you are sizing a battery bank for a solar setup, calculating flight time for a drone, or estimating the runtime of an inverter, knowing the math behind discharge rates ensures efficiency and longevity for your equipment.
The "discharge rate" can refer to two distinct things: the C-Rate (speed of discharge relative to capacity) or the Current (Amps) drawn over time. This guide covers both.
1. The Basic Discharge Formula
To calculate how long a battery will last under a specific load, you use the relationship between Capacity (Amp-hours) and Current (Amps).
Time (Hours) = Capacity (Ah) / Current (A)
Example:
If you have a 100Ah battery and you connect a load that draws 10 Amps:
100Ah / 10A = 10 Hours
2. Understanding C-Rate
The C-Rate is a standard metric used to describe the speed at which a battery is being discharged relative to its maximum capacity. It allows you to normalize discharge speeds across batteries of different sizes.
C-Rate = Discharge Current (A) / Battery Capacity (Ah)
- 1C Rate: The current will discharge the entire battery in 1 hour. (e.g., 100A draw on a 100Ah battery).
- 0.5C Rate: The current will discharge the battery in 2 hours.
- 2C Rate: The current will discharge the battery in 30 minutes.
Why C-Rate Matters
Battery manufacturers specify maximum discharge C-Ratings. Exceeding this rating can cause overheating, voltage sag, and permanent damage. For example, a standard Li-ion cell might have a max continuous discharge of 1C, while a high-performance LiPo drone battery might handle 50C.
3. Real-World Factors: Depth of Discharge (DoD)
The theoretical calculation ($Capacity / Current$) assumes 100% efficiency and that you can drain the battery to 0%. In reality, this is rarely possible or safe.
- Lead Acid (AGM/Flooded): Should typically not be discharged below 50% DoD to avoid sulfation and reduced cycle life.
- Lithium (LiFePO4/Li-ion): Can typically be discharged to 80-95% DoD safely.
- Peukert's Law: For Lead Acid batteries, discharging at a high rate effectively reduces the total available capacity. A 100Ah battery might only provide 60Ah if discharged very quickly (high C-rate).
Our calculator above includes an "Efficiency Factor" dropdown to account for these real-world limitations. For Lead Acid, we calculate the runtime based on 50% usable capacity to give you a "safe" runtime estimate rather than a theoretical one.
4. Calculating Current from Watts
Often, devices are rated in Watts (Power) rather than Amps. To find the discharge rate in Amps, use the power formula:
Current (A) = Power (W) / Voltage (V)
Example:
You have a 500W inverter running on a 12V battery.
500W / 12V = 41.67 Amps
You would then plug 41.67 into the "Load Current" field in the calculator above.
function calculateBattery() {
// Get Input Values
var capacityStr = document.getElementById('batteryCapacity').value;
var currentStr = document.getElementById('loadCurrent').value;
var efficiencyStr = document.getElementById('batteryType').value;
// Validation
if (capacityStr === "" || currentStr === "") {
alert("Please enter both Battery Capacity and Load Current.");
return;
}
// Parse to Float
var capacity = parseFloat(capacityStr);
var current = parseFloat(currentStr);
var efficiency = parseFloat(efficiencyStr);
// Edge Case Check
if (current <= 0) {
alert("Load Current must be greater than 0.");
return;
}
if (capacity <= 0) {
alert("Capacity must be greater than 0.");
return;
}
// Calculate C-Rate
var cRate = current / capacity;
// Calculate Usable Capacity
var usableCapacity = capacity * efficiency;
// Calculate Time (Hours)
var timeHours = usableCapacity / current;
// Format Time (Hours and Minutes)
var hours = Math.floor(timeHours);
var minutesDecimal = timeHours – hours;
var minutes = Math.round(minutesDecimal * 60);
// Handle minute rollover
if (minutes === 60) {
hours++;
minutes = 0;
}
// Display Strings
var timeString = hours + " Hours " + minutes + " Minutes";
// Very fast discharge warning (under 1 min)
if (hours === 0 && minutes === 0) {
var seconds = Math.round(timeHours * 3600);
timeString = seconds + " Seconds";
}
// Update DOM
document.getElementById('runTimeResult').innerHTML = timeString;
document.getElementById('cRateResult').innerHTML = cRate.toFixed(3) + " C";
document.getElementById('usableCapResult').innerHTML = usableCapacity.toFixed(1) + " Ah";
// Show Results Container
document.getElementById('resultDisplay').style.display = 'block';
// Show Lead Acid Warning if efficiency is 0.5
var warningBox = document.getElementById('leadAcidWarning');
if (efficiency === 0.5) {
warningBox.style.display = 'block';
} else {
warningBox.style.display = 'none';
}
}