How Do You Calculate Amp Hours for a Battery

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Battery Amp Hour (Ah) Calculator

Using Amps

Using Watts

12V 24V 48V

Lead Acid / AGM (50% DoD Recommended) Lithium LiFePO4 (80% DoD Recommended) Theoretical Max (100% Discharge)

Note: Discharging batteries fully shortens their lifespan.

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How to Calculate Amp Hours (Ah) for a Battery

Calculating the correct battery capacity is crucial for solar power systems, RVs, marine applications, and off-grid electronics. Amp Hours (Ah) measure the amount of charge a battery can deliver at a specific rate over a specific period of time.

The Basic Amp Hour Formula

The most fundamental calculation for battery capacity is multiplying the current draw by the time it needs to run:

Amp Hours (Ah) = Current (Amps) × Time (Hours)

Calculating Ah from Watts

Since many appliances are rated in Watts, you first need to convert Watts to Amps using the system voltage (Ohm's Law). The formula is:

Amps = Watts / Voltage

Once you have the Amps, you multiply by the hours of runtime to get the total Ah consumed.

Why Depth of Discharge (DoD) Matters

You should never plan to use 100% of a battery's rated capacity. Doing so will permanently damage most battery chemistries.

• Lead Acid/AGM: It is standard practice to only use 50% of the capacity.
• Lithium (LiFePO4): These are more efficient and can typically be discharged to 80-90% safely.

To find the Recommended Capacity, divide the required Ah by the Depth of Discharge (e.g., if you need 50Ah and use Lead Acid, you need a 100Ah battery).

Example Calculation

Suppose you want to run a 60W portable fridge for 12 hours on a 12V system using a Lead Acid battery:

1. Convert Watts to Amps: 60W / 12V = 5 Amps.
2. Calculate Consumption: 5 Amps × 12 Hours = 60 Ah.
3. Apply Safety Factor (50% DoD): 60 Ah / 0.5 = 120 Ah.

In this scenario, you would need a battery rated for at least 120 Amp Hours to run the fridge safely without damaging the battery.

var activeTab = 'amps'; function switchTab(tab) { activeTab = tab; document.getElementById('tab-amps').className = (tab === 'amps' ? 'tab-btn active' : 'tab-btn'); document.getElementById('tab-watts').className = (tab === 'watts' ? 'tab-btn active' : 'tab-btn'); document.getElementById('amps-input-container').style.display = (tab === 'amps' ? 'block' : 'none'); document.getElementById('watts-input-container').style.display = (tab === 'watts' ? 'block' : 'none'); } function calculateBattery() { var hours = parseFloat(document.getElementById('runtimeHours').value); var dod = parseFloat(document.getElementById('batteryType').value); var amps = 0; var voltage = 12; if (isNaN(hours) || hours <= 0) { alert("Please enter a valid number of hours."); return; } if (activeTab === 'amps') { amps = parseFloat(document.getElementById('loadAmps').value); if (isNaN(amps) || amps <= 0) { alert("Please enter a valid current in Amps."); return; } } else { var watts = parseFloat(document.getElementById('loadWatts').value); voltage = parseFloat(document.getElementById('systemVolts').value); if (isNaN(watts) || watts <= 0) { alert("Please enter a valid wattage."); return; } amps = watts / voltage; } // Calculation var netAh = amps * hours; var recAh = netAh / dod; // Display document.getElementById('netAh').innerText = netAh.toFixed(2); document.getElementById('recAh').innerText = recAh.toFixed(2); if (activeTab === 'watts') { document.getElementById('wattSummary').innerText = "Based on " + (amps * voltage).toFixed(0) + " Watts at " + voltage + "V (" + amps.toFixed(2) + " Amps continuous)."; } else { document.getElementById('wattSummary').innerText = "Based on " + amps.toFixed(2) + " Amps continuous draw."; } document.getElementById('battery-result').style.display = 'block'; }