Battery Discharge Rate Calculator
Calculate C-Rating, Runtime, and Effective Capacity
About Battery Discharge Rates
Understanding the discharge rate of a battery is crucial for designing reliable power systems, whether for solar setups, electric vehicles, or consumer electronics. This calculator determines the C-Rate (a measure of the rate at which a battery is discharged relative to its maximum capacity) and the expected runtime based on your electrical load.
How It Works
The core logic of battery discharge relies on the relationship between Capacity (measured in Amp-hours or Ah) and Current (measured in Amperes or Amps).
1. C-Rate Formula:
The C-Rate describes how fast a battery is being discharged. A 1C rate means the discharge current will drain the entire battery in 1 hour.
Formula: C-Rate = Load Current (A) / Battery Capacity (Ah)
2. Runtime Calculation:
The theoretical time a battery lasts is calculated by dividing capacity by the current draw. However, real-world inefficiencies (internal resistance, heat, Peukert's Law for lead-acid) reduce this time.
Formula: Runtime (Hours) = (Capacity (Ah) / Current (A)) × Efficiency Factor
Example Calculation
Let's say you have a deep-cycle battery with the following specifications:
- Battery Capacity: 100 Ah
- Connected Load: 20 Amps
- Efficiency: 90% (0.9 for Lithium-Ion)
Results:
- C-Rate: 20A / 100Ah = 0.2C
- Ideal Runtime: 100Ah / 20A = 5 Hours
- Real Runtime: 5 Hours × 0.9 = 4.5 Hours
Why Efficiency Matters?
Batteries are rarely 100% efficient. Lead-acid batteries suffer heavily from the Peukert effect, where discharging at high currents significantly reduces usable capacity (efficiency can drop to 0.5 or 0.6). Lithium-based batteries are much more efficient, typically retaining 90-95% of their rated capacity even at higher discharge rates.