Configuration of battery cells
Battery cells are configured in series or parallel, depending on the total voltage and capacity required. For example, series connection increases the voltage of the battery pack, while parallel connection increases the capacity of the battery pack.
LiFePO4 Battery Selection
When configuring a battery pack, it is critical to select batteries with similar performance characteristics, including voltage, capacity, and internal resistance. Using batteries with comparable characteristics improves the balance and consistency of the battery pack.
- Ensure that the batteries are from the same production batch or have similar manufacturing parameters to minimize performance differences due to manufacturing variations.
- The battery should meet the design requirements, such as nominal voltage, capacity, and maximum charge/discharge current. For example, a common nominal voltage is 3.2V, and the capacity is usually around 100Ah.
Capacity Matching
The batteries within a battery pack should have similar capacities to ensure that the overall battery pack capacity meets the expected energy storage and release needs. Charge/discharge testing is often used to evaluate and match battery capacity. Charge/discharge testing can determine the actual capacity of each battery so that matching can be performed. The tolerance range for capacity matching is usually defined at design time to ensure performance consistency. The typical tolerance is ±5%. For a 100Ah battery, the acceptable range is 95-105Ah.
Voltage Matching
The cells in a battery pack should have similar voltage characteristics to ensure balanced voltage in the pack during charge/discharge and to prevent overloading of the cells. Voltage matching is to ensure balanced voltage distribution between cells, with a common error of ±0.05V. The BMS (Battery Management System) can monitor and adjust/balance the voltage of each cell as needed.
Internal Resistance Matching
The cells should have similar internal resistance characteristics to ensure balanced current distribution during charge/discharge and to prevent problems such as overheating. Resistance matching ensures balanced current distribution and minimizes power loss and temperature rise. Resistance is usually tested by AC impedance spectroscopy or constant current discharge test. Batteries are sorted and matched based on these results. The tolerance of resistance matching is generally ±5%.
Temperature Control
The temperature must be controlled during configuration to ensure that the cells are within the appropriate temperature range, as temperature can significantly affect the performance and life of the battery. Configuration should be performed within the appropriate temperature range, typically 15-30°C. Use temperature sensors to monitor the ambient temperature and take necessary cooling measures. The BMS can monitor the voltage, capacity, and temperature of each cell and perform balanced charge/discharge as needed.
Balanced Charge/Discharge
Balanced charge/discharge ensures consistent battery state and is typically achieved through a BMS, which monitors and controls the charge/discharge of each cell.
1. During charging, the BMS can adjust the charge current or use active balancing techniques to maintain a consistent state of charge for each cell.
2. During discharge, the BMS can monitor the discharge voltage and capacity of each cell and adjust the load or use active balancing to maintain consistency.
Correctly matching LiFePO4 cells is critical to building a high-performance, safe DIY battery pack. Carefully following the recommended requirements for cell selection, capacity, voltage, resistance, temperature, and charge/discharge ensures that your custom LiFePO4 battery pack will operate optimally for many years. With careful matching, even a novice builder can successfully build a DIY battery pack to meet their specific energy storage needs.
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