🔧 Battery Performance Metrics & Operational Parameters
Understanding key battery performance metrics is essential for evaluating electrochemical behavior, efficiency, and long-term durability. These parameters serve as the foundation for battery modeling, simulation studies, and real-world diagnostics.
⚡ 1. Capacity (Q)
Capacity represents the total charge a battery can store and is measured in ampere-hours (Ah). It indicates how much energy is available for external loads.
- Q_nominal is the manufacturer-rated capacity under standard conditions.
- Capacity decreases over time due to aging, side reactions, and lithium loss.
- The temperature at which the rated capacity is achieved is called the nominal temperature.
🔋 2. State of Charge (SoC) & Depth of Discharge (DoD)
These complementary metrics quantify the usable charge within a battery.
State of Charge (SoC):
$$ \text{SoC} = \left( \frac{Q_{\text{remaining}}}{Q_{\text{full}}} \right) \times 100 $$
Depth of Discharge (DoD):
$$ \text{DoD} = 100 - \text{SoC} $$
Higher DoD levels accelerate battery degradation and shorten lifespan.
🔌 3. Open Circuit Voltage (OCV) & Terminal Voltage
- OCV is the voltage measured when the battery is at rest (no current).
- It varies with SoC and temperature.
- Terminal Voltage is the actual voltage under load and reflects internal losses due to resistance.
🧱 4. Internal Resistance (R_int)
Represents the opposition to current flow within the battery, comprised of:
- Electronic resistance (electrodes, current collectors)
- Ionic resistance (electrolyte, separator)
High internal resistance leads to voltage drops, reduced power output, and increased heat generation.
🔄 5. C-Rate
C-rate defines how quickly a battery is charged or discharged relative to its nominal capacity.
Charge current:
$$ I_{\text{charge}} = C_{\text{rate}} \times Q_{\text{nominal}} $$
- 1C → Full charge/discharge in 1 hour
- 2C → Full in 30 minutes
- 0.5C → Full in 2 hours
Higher C-rates generate more heat and reduce cycle life.
🌡️ 6. Temperature Effects
Temperature significantly influences:
- Capacity
- Voltage behavior
- Internal resistance
- Aging rate
Optimal performance is usually within 20°C to 40°C.
Extreme cold reduces capacity; extreme heat accelerates degradation or thermal runaway.
🔁 7. Round-Trip Efficiency (η)
Defines energy efficiency during one full charge–discharge cycle:
$$ \eta = \left( \frac{\text{Energy}{\text{out}}}{\text{Energy}{\text{in}}} \right) \times 100\% $$
- Typically ranges from 90% to 95% for Li-ion batteries.
🧪 8. Aging and Degradation
Main aging indicators include:
- Capacity fade: Decrease in total usable energy.
- Internal resistance growth: Causes voltage drop and thermal loss.
- Cycle life: Number of full cycles until capacity falls below 80% of initial.
✅ Summary
Battery performance metrics are essential for:
- Accurate simulation and diagnostics
- Real-time battery management
- Predicting aging and performance over time
Understanding and monitoring these parameters ensures safety, efficiency, and longevity across applications like EVs, portable devices, and grid energy storage.