Fundamentals of Battery Performance: Core Parameters and Their Interactions

Battery capacity (mAh): a measure of battery power

1. Definition and Essence

Battery capacity is expressed in milliampere-hours (mAh), which is the product of current (milliamperes, mA) and time (hours, h). For example, a 1000mAh battery means:

Discharging at 1000mA (1A) can last for 1 hour;

Discharging at 500mA can last for 2 hours.

Essentially: mAh measures the total amount of charge a battery can store, without reference to voltage, similar to the "water capacity" of a bucket.

2. Common Misconception: High mAh ≠ Long Battery Life

Misconception: Thinking that a 5000mAh battery will definitely last longer than a 3000mAh battery.

The truth: Battery life is determined by energy (Wh), not just capacity.

 

Energy density (Wh/kg): the core indicator of portability

1. Definition and Significance

Energy density refers to the amount of energy stored per unit weight of a battery (Wh/kg), and is a key parameter for measuring a battery's "slimming ability":

Volumetric energy density (Wh/L): affects device thickness (such as mobile phone batteries);

Mass energy density (Wh/kg): determines the lightweighting of equipment (such as the range of electric vehicles).

2. Comparison of energy density across different technology routes.

Battery Tybe	Mass Energy Density（Wh/kg）	Typical Applications
Laed-acid Battery	50-70	Electric vehicle starding battery
Lithium Iron Phosphate Battery	140-200	Energy storage power stations, commercial vehicles
Ternary Lithium Battery	250-350	Electric vehicles, high-end mobile phones
Solid Lithium Battery	350-500(under development)	Next generation electric vehicles and drones

3. The Double-Edged Sword of Energy Density

Advantages: When the energy density of ternary lithium batteries reaches 300Wh/kg, the range of electric vehicles can exceed 600km;

Challenge: For every 10% increase in energy density, the risk of thermal runaway increases by 15%, requiring a more complex temperature control system.

 

Charge and discharge curves: the "electrocardiogram" of battery performance

1. The electrochemical code behind the curve

The charge and discharge curve reflects the law of battery voltage changing with power, with typical characteristics:

Charging stage:

Constant current charging (voltage rises rapidly);

Constant voltage charging (current gradually decreases, voltage plateaus).

Discharge stage:

The voltage first drops rapidly, enters a stable plateau period, and finally drops sharply to the cut-off voltage.

 

2. Key Parameter Analysis

Voltage platform: The range in which the voltage remains stable during discharge. The higher and longer the platform, the better the battery performance.

For example: the discharge platform of lithium iron phosphate battery is 3.2V, and that of ternary lithium battery is 3.7V, the latter has higher energy.

Polarization phenomenon: The voltage drops faster during high current discharge (e.g., the voltage drops 0.5V lower during 10C discharge than during 1C discharge) due to increased internal resistance loss.

3. Relationship between the curve and usage scenarios

Electric vehicle acceleration: requires high current discharge (5-10C), and requires a low steepness curve platform (small voltage fluctuation);

Energy storage peak regulation: small current discharge for a long time (below 0.5C), platform stability is more important.

 

Cycle life: A timer for battery durability

1. Definition and Standards

Cycle life refers to the number of complete cycles from full charge to empty (DOD=100%) and then full charge, until the capacity decays to 80% of the rated value.

Typical data:

Ternary lithium battery: 1000 cycles (DOD=100%);

Lithium iron phosphate battery: 3000 cycles (DOD=100%);

Lead-acid battery: 500 cycles (DOD=80%).

2. The "Four Killers" Affecting Cycle Life

Overcharge and overdischarge: charging over 4.3V or discharging below 2.5V will cause permanent damage to the electrode structure;

High temperature environment: storage at 60℃ for 1 month, cycle life is shortened by 50%;

High current charging and discharging: 10C fast charging reduces the number of cycles by 30% compared to 0.5C fast charging;

Long-term storage with full charge: When a lithium battery is stored with full charge for one month, the capacity decreases by 5%.

3. The Golden Rule for Prolonging Life

Shallow charge and discharge: Keep the SOC between 20% and 80% for daily use (e.g. charge your phone when the battery is at 20%).

Avoid high temperatures: Avoid direct sunlight when charging your electric vehicle. When the temperature inside the car exceeds 60°C in summer, the battery life will decline rapidly.

Regular deep charging and discharging: Complete a full charge to empty discharge every 3 months and calibrate the BMS power display.

 

The "linkage effect" of core parameters

1. The trade-off between energy density and cycle life

Ternary lithium batteries have high energy density but short cycle life, making them suitable for electric vehicles that require longer driving range.

Lithium iron phosphate batteries have a long cycle life but low energy density, making them more suitable for energy storage power stations (which require frequent charging and discharging).

2. The Interaction Between Capacity and Charge/Discharge Curves

High-capacity batteries (such as 5000mAh) usually have greater internal resistance, and the voltage platform drops more significantly during high-current discharge;

At the same capacity, batteries with a higher voltage platform (such as 3.7V vs 3.2V) have higher energy, but may be accompanied by higher polarization losses.