There is an important distinction between a rechargeable battery cell and a rechargeable battery: the latter is typically a battery pack composed of multiple battery cells.
Battery cells have fixed voltage ranges that depend solely on their chemistry. For instance:
- A Li-ion battery cell operates between 3.0V (empty) and 4.2V (full).
- A LiFePO₄ (lithium iron phosphate) battery cell ranges from 2.5V (empty) to 3.65V (full).
In contrast, batteries (or battery packs) come in a wide variety of voltages and capacities. This is achieved by combining individual battery cells through configurations that connect them in series or in parallel.
Overview
Batteries are categorized by their voltages and capacities:
- Voltage: Increased by connecting battery cells in series (a configuration called a “string”).
- Capacity: Increased by connecting multiple strings in parallel.
For example, consider a traditional lead-acid car battery with a nominal voltage of 12V. Many modern batteries replace lead-acid with lithium-based chemistries due to their superior energy density and other benefits. To create a lithium-based 12V battery pack, you would typically connect four lithium cells in series (denoted as 4S).
Chemistry and Voltage
Depending on the chemistry used, the voltage of a 4S lithium battery pack approximates 12V:
| Chemistry | Empty | Full | Nominal |
|---|---|---|---|
| Li-ion | 12.0V | 16.8V | 14.8V |
| LiPo | 12.8V | 16.8V | 14.8V |
| LiFePO₄ | 10.0V | 14.6V | 12.8V |
Lithium-based batteries maintain a relatively stable voltage plateau for most of their state of charge. This makes their nominal voltage—the average voltage during normal operation—similar to that of lead-acid batteries. However, when fully charged, lithium batteries can exhibit significantly higher voltages.
Battery Management System (BMS)
Most battery packs include a built-in BMS to safeguard against common hazards:
- Overcharging: Prevents exceeding the safe voltage range of the cells.
- Over-discharging: Protects against excessive voltage drops that can damage the battery.
- Short circuits: Limits current to prevent overheating or fires.
However, certain specialized battery packs, such as those used in RC models and drones, may lack a built-in BMS. Instead, they include a secondary balancer plug that provides access to each individual cell. This enables external chargers and balancers to monitor and manage the cells independently, offering greater control for advanced users.
Key Points on Battery Pack Construction
- Series Connections: Combine cells to increase voltage. For example, a 4S configuration adds the voltage of four cells together.
- Parallel Connections: Combine strings of cells to increase capacity, sharing the load across multiple cells.
- Mixed Configurations: For higher capacity and voltage, cells are often arranged in both series and parallel, such as 4S2P (four cells in series, with two parallel strings).
Practical Considerations
When building or selecting a battery pack:
- Ensure Compatibility: Confirm the voltage range aligns with the device’s requirements.
- Use Appropriate Charging Equipment: If the pack lacks a BMS, use a charger designed for the battery’s chemistry with balancing capabilities.
- Prioritize Safety: Overvoltage and undervoltage conditions can damage cells or pose safety risks, particularly in unprotected battery packs.
By understanding the principles of battery pack design, you can select or construct packs that meet your specific needs while ensuring safety and performance.
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(content created Dec 08, 2024)