Lithium-Ion Battery Characteristics - Part 1

Introduction

Lithium battery technology has become the standard power source for most portable electronic devices, including laptops, cell phones, and tablets. Its widespread adoption is due to its high energy density and long lifespan. However, when applied to electric bikes and other transportation systems, battery performance can vary significantly based on power draw, which directly impacts range and longevity.

Many electric bike battery manufacturers often mislead consumers with exaggerated range claims, leading to disappointment when real-world performance falls short. This article will explore ride scenarios, analyze their impact on battery life and range, and discuss necessary industry improvements to provide more accurate expectations for consumers.

Lithium-Ion Battery Characteristics

To understand electric bike battery performance, let’s examine the Panasonic NCR18650PF lithium cell. This cylindrical cell measures approximately 18mm in diameter and 65mm in length, making it significantly different from conventional alkaline D or C batteries.

Falco electric bike battery packs—Li1, Li3, and Li7—utilize these cells in different configurations to achieve the desired voltage and capacity.

Battery Pack Configurations

Falco’s Li1 and Li3 battery packs each contain 40 cells, while the Li7 pack contains 52 cells. These are arranged as follows:

• Li1 & Li3: 10S4P configuration (10 cells in series, 4 parallel groups), producing a 36V, 11.6Ah pack.

• Li7: 13S4P configuration (13 cells in series, 4 parallel groups), resulting in a 48V, 11.6Ah pack.

Cell Specifications

Each NCR18650PF cell has:

• Capacity: 2750mAh (2.75Ah)

• Nominal Voltage: 3.6V

• Full Charge Voltage: 4.2V

• Standard Charge Current: 1.4A

• Charge Time: ~4 hours

When connected in series, the voltage increases, but the current capacity remains the same. The true measure of battery capacity is in watt-hours (Wh), not milliamp-hours (mAh).

• A single cell has ~10Wh of energy storage.

• A 40-cell (10S4P) pack has ~400Wh.

• A 52-cell (13S4P) pack has ~520Wh.

Charging Efficiency

Battery efficiency during charging is crucial. If we supply 1.4A for 4 hours (totaling 5.6Ah), the actual stored capacity is only 2.7Ah. This suggests a charging efficiency of about 48%, with the remaining energy lost as heat. If we assume a constant 4.2V charging voltage, the input energy would be 23.52Wh, while the usable stored energy remains around 10Wh per cell, giving an overall charge efficiency of ~42% (which can be slightly higher in real-world scenarios).

Charging Characteristics

During charging, a battery follows a specific pattern:

1. Constant Current Phase: Initially, the charger supplies a steady 2A current.

2. Voltage Rise: As the cell charges, voltage increases to 4.2V.

3. Current Reduction: When voltage reaches 4.2V, the current decreases.

4. Charge Termination: Charging stops once the current drops to 55mA and the cell stabilizes.

A battery typically reaches 80% charge in about 80 minutes, with another 50 minutes required for stabilization and balancing.

Discharge Characteristics by Temperature

Temperature significantly affects lithium battery capacity:

• At 25°C (77°F): The cell delivers 2800mAh.

• At 40°C (104°F): Capacity slightly increases to 2900mAh.

• At -10°C (14°F): Capacity drops to 2450mAh.

• At -20°C (-4°F): The cell becomes non-functional.

Thus, colder temperatures reduce battery performance by about 11%, whereas warmer conditions may improve it by 5%. A well-designed battery pack, with four cells in parallel, can safely deliver up to 11A of continuous current while maintaining stable performance.

Discharge Characteristics by Rate

The rate at which a battery is discharged also affects performance:

• At 0.2C (0.55A): Voltage drop is minimal.

• At 2C (5.5A): Voltage drops by ~0.3V, indicating an internal impedance of ~60mΩ per cell.

• For a 10S4P pack: Impedance is ~0.15Ω.

• For a 13S4P pack: Impedance is ~0.195Ω.

Even at higher discharge rates (2C), the overall battery capacity remains nearly unchanged, which is beneficial for high-power motors. This means that electric bike batteries can sustain 22A of continuous discharge without significant loss of capacity.

Battery Life Expectancy

Battery longevity is influenced by usage and maintenance:

• Panasonic guarantees ~500 full charge cycles at 1C discharge (2.75A) before capacity drops to 2100mAh (a 22% reduction).

• Daily full discharges (5 days/week) will result in a 2-year battery lifespan.

• Partial discharges (~50%) can extend battery life to 5 years.

• Batteries also age over time, even when not in use.

Conclusion

Lithium battery performance is affected by temperature, discharge rate, and charging efficiency. The electric bike industry must set realistic range expectations by accounting for these variables rather than relying on idealized laboratory conditions. Providing transparent specifications will help consumers make informed choices and avoid dissatisfaction with real-world battery performance.

References

1. Tesla Roadster Battery System

2. Panasonic NCR18650PF Datasheet

3. Panasonic Battery Specifications