Lead Acid Battery Life Calculator: (SLA, AGM, Gel)

Use our lead-acid battery life calculator to find out how long a Sealed Lead Acid (SLA), AGM, Gel, and Deep cycle lead-acid battery will last running a load.

Lead Acid Battery Life Calculator

Calculator assumptions

• Lead-acid battery discharge efficiency rate: 85%
• Inverter efficiency: 90%

How to use this calculator?

Step 1: Enter the battery capacity and select the unit type. The unit types are amp-hours (Ah), and milliamp-hours (mAh).

Step 2: Enter your battery voltage (V). Is this a 6v, 12v, 24, or 48v battery?

Step 3: Optional: Enter the battery state of charge (SoC). SoC is defined as how much your battery is charged relative to its capacity. (The default value is 100%, meaning a fully charged battery).

Step 4: Optional: Enter the battery depth of discharge limit (DoD). (Default value is 50% because most lead-acid batteries have a DoD limit of 50% in order to maintain their health). Check your battery specs sheet for a more accurate number.

Step 5: Is your load connected through an inverter? Select accordingly. (Inverter is the device that converts the DC current into AC current so we can run our household appliances)

Step 6: Enter the total output load and select the unit type. The unit types are Watts (W), and kilowatts (kW). Note: 1 kW = 1000 watts.

Enter "Calculate" button for the result.

Example

Lead acid battery life (runtime) formula

Formula: Lead acid Battery life = (Battery capacity Wh × (85%) × inverter efficiency (90%), if running AC load) ÷ (Output load in watts).

Example

Let’s suppose,

• Battery capacity: 12v 100Ah (12 × 100 = 1200Wh)
• Battery discharge efficiency – Lead-acid: 85%
• Output load: 100 watts
• inverter efficiency: 90%

Battery life = (1200× 85% × 90%) ÷ (100)

Battery life = (864) ÷ (100)

Battery life = 8.6 hours

why non of the above methods are 100% accurate?

I won't go in-depth about the discharging mechanism of a lead-acid battery. Instead, I'm going to share the key points to remember when discharging your lead-acid battery.

1. The faster you discharge a lead acid battery the less energy you get (C-rating)

Recommended discharge rate (C-rating) for lead acid batteries is between 0.2C (5h) to 0.05C (20h). Look at the manufacturer’s specs sheet to be sure.

Formula to calculate the c-rating: C-rating (hour) = 1 ÷ C  

Discharging your battery at a higher rate will increase the temperature in battery cells which as result will cause power losses. e.g, a 100ah lead-acid battery with a C-rating of 0.05C (20 hours) will last about 20-25 minutes instead of 1 hour while running a 50 amp load (remember the 50% DoD limit).

2. battery age count

A lead-acid battery will lose its 20% storage capacity after 500-900 cycles (Look at the manufacturer’s specs sheet for an accurate value). So if you have an old battery it'll store less power. As a result, it will deplete more quickly than the estimated time.

3. Batteries don’t discharge evenly

Your battery will be discharged at a different rate depending on how heavy the output load is connected. Which makes it really hard to guess the exact runtime of your battery.

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Resources

• Lead-acid battery downsides ^{1}
• Energy Storage with Lead–Acid Batteries ^{2}
• Peukert Equation ^{3}

I hope this guide was helpful to you. Contact us if you need any further information. Thank you!