Calculate Battery Discharge Rate

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Battery Discharge Rate Calculator

Total Amp-hours of the battery bank.
Nominal voltage (e.g., 12V, 24V, 48V).
Power (Watts) Current (Amps)
Typical range: 85-95% for inverters.
Lithium (LiFePO4/Li-ion) Lead Acid (AGM/Flooded) Affects Peukert effect calculation.
Discharge Current: 0.00 A
C-Rate: 0.00 C
Estimated Runtime (100% Depth): 0h 0m
Safe Runtime (80% Depth): 0h 0m

Understanding Battery Discharge Rates

Calculating the battery discharge rate is essential for designing reliable power systems, whether for solar off-grid setups, RVs, electric vehicles, or backup power. The discharge rate determines how much current is drawn from your battery bank relative to its total capacity, which directly impacts the battery's lifespan, efficiency, and runtime.

What is C-Rate?

The "C-Rate" is a universal measure of the rate at which a battery is discharged relative to its maximum capacity. A C-rate of 1C means the battery will be fully discharged in 1 hour. A C-rate of 0.5C (or C/2) means it will take 2 hours.

  • 1C on a 100Ah battery = 100 Amps discharge current.
  • 0.1C on a 100Ah battery = 10 Amps discharge current.
  • 5C on a 100Ah battery = 500 Amps discharge current (High drain).

Keeping the discharge rate low (typically below 0.5C for Lithium and 0.2C for Lead Acid) ensures better efficiency and longer cycle life.

How to Calculate Discharge Amps

If you know the power load (Watts) of your appliances, you can calculate the discharge current (Amps) using the basic electrical formula:

Current (A) = Power (W) / Voltage (V)

For example, if you are running a 500W microwave on a 12V system:

500W / 12V ≈ 41.67 Amps

However, real-world systems include inverters which are not 100% efficient. If your inverter is 90% efficient, the calculation becomes:

Actual Current = (Power / Voltage) / Efficiency

(500W / 12V) / 0.90 ≈ 46.30 Amps

Peukert's Law and Battery Chemistry

Not all batteries discharge equally. Peukert's Law describes how the capacity of rechargeable lead-acid batteries decreases when discharged at high currents.

  • Lead Acid (AGM, Flooded, Gel): These are heavily affected by Peukert's Law. Drawing a high current significantly reduces the effective capacity. A 100Ah lead-acid battery might only provide 60Ah if discharged very quickly.
  • Lithium (LiFePO4): Lithium batteries have a very low Peukert constant (close to 1.05), meaning their capacity remains stable regardless of whether you discharge them slowly or quickly. This is why Lithium is preferred for high-load applications.

Using This Calculator

Our calculator simplifies these physics by allowing you to input your load in either Watts or Amps. It automatically adjusts for inverter efficiency and estimates the C-Rate. The "Safe Runtime" metric is particularly useful, as it calculates the time until the battery reaches a recommended depth of discharge (80%), preserving battery health.

function toggleLoadInput() { var type = document.getElementById('loadType').value; var label = document.getElementById('loadLabel'); var input = document.getElementById('loadValue'); if (type === 'watts') { label.textContent = 'Load Power (Watts)'; input.placeholder = 'e.g. 200'; } else { label.textContent = 'Load Current (Amps)'; input.placeholder = 'e.g. 10'; } } function calculateDischarge() { // Get Inputs var capacity = parseFloat(document.getElementById('batCapacity').value); var voltage = parseFloat(document.getElementById('batVoltage').value); var loadType = document.getElementById('loadType').value; var loadValue = parseFloat(document.getElementById('loadValue').value); var efficiency = parseFloat(document.getElementById('efficiency').value); var chemistry = document.getElementById('chemistry').value; // Validation if (isNaN(capacity) || capacity <= 0) { alert("Please enter a valid Battery Capacity."); return; } if (isNaN(voltage) || voltage <= 0) { alert("Please enter a valid System Voltage."); return; } if (isNaN(loadValue) || loadValue < 0) { alert("Please enter a valid Load value."); return; } if (isNaN(efficiency) || efficiency 100) { alert("Efficiency must be between 1 and 100."); return; } // Calculate Discharge Current (Amps) var dischargeAmps = 0; var efficiencyDecimal = efficiency / 100; if (loadType === 'watts') { // Power (W) / Voltage (V) = Amps (A) // Account for efficiency: More amps are drawn from battery to compensate for losses dischargeAmps = (loadValue / voltage) / efficiencyDecimal; } else { // If input is Amps, we assume this is the load current // If there is an inverter involved (inefficiency), usually the load IS the output, // so input to inverter is higher. But simpler to assume input is draw from battery // if user selects Amps. Let's assume user measured current at the battery cable. dischargeAmps = loadValue; } // Calculate C-Rate // C-Rate = Discharge Current / Capacity var cRate = 0; if (capacity > 0) { cRate = dischargeAmps / capacity; } // Calculate Runtime (Hours) // Peukert's effect logic var peukertConstant = (chemistry === 'lithium') ? 1.05 : 1.25; // Peukert Formula for time: T = C * (C / I)^(k-1) / I // Simplified: Time = RatedCapacity * ((RatedCapacity/Current) ^ (k-1)) / Current ?? // Standard Approximation for runtime: t = H * (C / (I * H))^k is usually for defining capacity. // Let's use the effective capacity formula: // Effective Capacity = Rated Capacity / ((I / Rated_I)^(k-1))? No. // Practical formula for runtime t (hours) = R * (C / R / I)^k // Where R is the rating hours (usually 20h for lead acid). // Let's stick to a simpler modified Peukert for general web calc usage: // t = Capacity / (Current^k) * Capacity^(k-1) ? Too complex for simple inputs without rating hour. // Let's use Effective Amp Hours = Capacity * (1 / (DischargeCurrent / ReferenceCurrent)^(k-1)) // Reference Current is usually C/20. var estimatedHours = 0; if (dischargeAmps > 0) { if (chemistry === 'lithium') { // Lithium is linear mostly estimatedHours = capacity / dischargeAmps; } else { // Lead Acid Peukert Estimation // Assume reference rating is 20 hours (C20) var referenceHours = 20; var referenceCurrent = capacity / referenceHours; // If discharge is lower than reference, we get more capacity (limit to slightly above) // If discharge is higher, we get less. // Formula: T = 20 * (ReferenceCurrent / DischargeAmps)^k estimatedHours = referenceHours * Math.pow((referenceCurrent / dischargeAmps), peukertConstant); } } // Safe limits var safeHours = estimatedHours * 0.8; // 80% DOD safe limit // Formatting Output document.getElementById('resCurrent').innerHTML = dischargeAmps.toFixed(2) + " A"; document.getElementById('resCRate').innerHTML = cRate.toFixed(3) + " C"; document.getElementById('resTime').innerHTML = formatTime(estimatedHours); document.getElementById('resSafeTime').innerHTML = formatTime(safeHours); // Show results document.getElementById('results').style.display = "block"; } function formatTime(decimalHours) { if (decimalHours === Infinity || isNaN(decimalHours)) return "Infinite"; var hours = Math.floor(decimalHours); var minutes = Math.round((decimalHours – hours) * 60); if (minutes === 60) { hours++; minutes = 0; } return hours + "h " + minutes + "m"; }

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