Discharge Rate Calculation

Battery Discharge & Runtime Calculator

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Understanding Battery Discharge Rates

Whether you are designing a solar power system, outfitting an RV, or calculating backup power for electronics, understanding discharge rate is critical. The discharge rate refers to the speed at which current is drawn from a battery relative to its total capacity.

This calculator helps you determine two vital metrics: the C-Rate (a measure of discharge speed) and the Estimated Runtime (how long your battery will last under a specific load).

What is C-Rate?

The C-Rate is a normalized measure of the current used to discharge a battery. It is calculated by dividing the discharge current (Amps) by the battery's rated capacity (Amp-hours).

• 1C Rate: A current that will discharge the entire battery in exactly 1 hour. (e.g., 100A load on a 100Ah battery).
• 0.5C Rate: A slower discharge that takes 2 hours.
• 2C Rate: A rapid discharge that depletes the battery in 30 minutes.

High C-rates can generate significant heat and reduce the overall lifespan of lead-acid and AGM batteries due to the Peukert Effect.

Calculating Runtime and Depth of Discharge (DoD)

You simply cannot use 100% of a battery's capacity without damaging it, unless it is specifically designed for deep discharge (like Lithium Iron Phosphate). The usable capacity is determined by the recommended Depth of Discharge (DoD).

Common DoD Limits:

• Flooded Lead Acid: 50% DoD recommended to prevent sulfation.
• AGM / Gel: 60% DoD is generally safe for deep cycle variants.
• Lithium (LiFePO4): 80-95% DoD is standard, offering much more usable power for the same rated capacity.

The Math Behind the Calculation

To calculate the runtime manually, follow these steps:

1. Convert Load to Amps: If you know the Watts, divide by Volts ($I = P / V$).
2. Apply Inverter Efficiency: Real-world systems lose energy as heat. We typically assume 85-90% efficiency.
3. Determine Usable Capacity: Multiply Total Ah by the DoD percentage.
4. Calculate Hours: $$ \text{Runtime} = \frac{\text{Usable Capacity (Ah)}}{\text{Load Current (A)}} \times \text{Efficiency} $$

Example Calculation

Suppose you have a 12V, 200Ah Lead Acid battery and you are running a 100 Watt camping fridge.

• Current Draw: $100W / 12V = 8.33 \text{ Amps}$.
• Usable Capacity: Lead Acid is 50% DoD, so $200Ah \times 0.50 = 100 \text{ Usable Ah}$.
• Runtime: $100Ah / 8.33A \approx 12 \text{ hours}$. (Adjusting for 90% efficiency, approx 10.8 hours).

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