The 21700 lithium-ion cell rapidly gained traction in the electric vehicle market due to its high energy density and low cost, soon becoming a strong contender to replace the widely used 18650 cell. Studies show that 21700 cells deliver up to 35% higher energy density compared with 18650 cells using the same material system.
While the 18650 cell has been extensively studied, published research on the 21700 format remains limited. Market testing also shows substantial performance variations among different 21700 cell models, highlighting the importance of careful model selection.
Most previous studies compare different formats, comparing 18650 vs. 21700 energy, capacity, energy density, internal resistance, thermal properties, and cost. Thomas Waldmann further compared electrochemical, thermal, and geometric properties between classic 18650 and new 21700 cells.
However, direct comparisons between cells of the same format but different models remain scarce.
Given the abundant research on the thermodynamic characteristics of 18650 cells, a deeper investigation into the differences and similarities between different 21700 models is both necessary and valuable.
This study conducts a multi-scale comparative analysis of two different 21700 lithium-ion battery models with identical specifications. Using thermocouple sensors, we measured surface temperature under different discharge rates and ambient temperatures, supplemented by voltage and capacity analysis to obtain a comprehensive thermodynamic profile.
- Experimental Setup 1.1 Battery Samples
Two models of 21700 lithium-ion batteries produced by the same manufacturer were selected to ensure consistent production technology and quality control. Their basic parameters are shown in Table 1.
Table 1. Basic parameters of the lithium-ion cells.
1.2 Experimental Instruments
The experimental setup includes:
21700 lithium-ion cells
Battery charger
330-00A temperature-controlled chamber
Three K-type thermocouples
RS20K-C thermocouple transmitter
TEC-80K high-power electronic load
Custom battery test fixture
Battery internal resistance tester
Two USB-to-485 data acquisition units
Laptop for data processing
The temperature chamber simulates controlled ambient environments.
The RS20K-C transmitter collects battery surface temperature.
The TEC-80K load provides constant-current discharge at various C-rates while recording voltage and current.
Table 2. Specifications of instruments used in the experiments.
1.3 Experimental Method
To ensure sample consistency, internal resistance screening was first performed.
Fifty units of Model A and Model B were tested (resistance measurement diagram: Figure 1).
Cells were then capacity-tested, and five units from each model with similar capacity and internal resistance were selected for full thermodynamic analysis (Figure 2).
Experimental procedure
Charge each cell to 4.2 V and rest for 1 hour.
Place the cell in the climatic chamber and rest another hour.
Begin discharge testing once the cell temperature is within ±1 °C of the set value.
Perform discharge tests at 0.5C–2C and 25 °C–40 °C ambient conditions.
- Results and Discussion 2.1 Internal Resistance Comparison
Internal resistance data for 50 units of each model are shown in Figure 3.
Model B cells exhibit consistently lower resistance than Model A.
Average internal resistance:
Model A: 15.3 mΩ
Model B: 6.6 mΩ
Average difference: 8.7 mΩ.
2.2 Voltage Drop at Different Ambient Temperatures
The 0.5C discharge voltage curves for two sample models (40T and 50G) at various temperatures are shown in Figure 4.
Key findings:
At the beginning (DOD = 0–0.1), voltage drops rapidly as the cell transitions from float-charge to open-circuit voltage.
Mid-stage discharge (DOD = 0.1–0.8) remains stable.
End-stage discharge (DOD = 0.8–1.0) drops sharply to cutoff.
Higher ambient temperature results in higher discharge voltage due to increased electrolyte ionic mobility.
At all temperatures, 50G voltage > 40T voltage, indicating higher output power.
2.3 Capacity Under Different Discharge Rates
At 25 °C, capacity at 0.5C–2C is shown in Figure 5:
Increasing discharge rate reduces capacity in both models.
50G capacity declines more sharply at high C-rates (1C–2C).
Therefore, 40T offers better capacity stability across discharge rates.
2.4 Capacity Under Different Ambient Temperatures
Capacity at 0.5C and different temperatures is shown in Figure 6.
Findings:
Capacity increases with temperature for both models.
50G exhibits greater temperature-based capacity gain, achieving higher capacity at 30–40 °C compared with 40T.
2.5 Surface Temperature Rise Under Different Conditions
Surface temperature profiles under 1C discharge at various temperatures are shown in Figure 7.
Temperature rise follows a pattern:
Rapid rise (initial internal resistance heat)
Slower rise (stable mid-discharge)
Accelerated rise (temperature-induced resistance increase)
Across all conditions, 40T temperature rise > 50G, indicating that 50G has better thermal uniformity.
Temperature rise under different C-rates at 25 °C is shown in Figure 8:
Higher discharge current produces faster and higher temperature rise.
Maximum temperatures:
40T: 35.3 °C → 66.2 °C (0.5C → 2C)
50G: 32.3 °C → 58.1 °C
At 1.5C, peak temperature difference reaches 10.5 °C (50G cooler).
Thus, 50G consistently runs cooler than 40T at all discharge rates.
- Conclusions
This study provides a detailed thermodynamic and electrical comparison of two different 21700 lithium-ion cell models with identical specifications.
Key conclusions:
Internal Resistance
Model A exhibits 8.7 mΩ higher average resistance than Model B.
Voltage & Power Output
At identical conditions, 50G cells maintain higher discharge voltage, indicating higher output power.
Capacity Performance
At increasing C-rates, 40T shows better capacity stability.
At higher temperatures, 50G delivers greater capacity increase, performing better in warm environments.
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