Some battery packs come unprotected, meaning they have no built-in BMS (Battery Management System). Instead, these batteries provide direct physical access to each individual cell through a separate balancer plug:
This design shifts the tasks typically handled by a built-in BMS to the devices the battery is connected to, saving cost, weight, and space while offering much more control.
For example, specialized chargers can monitor and speed-charge such battery packs. However, the downside is significant: if used improperly, unprotected battery packs can be damaged or even create a serious fire hazard.
Overview
The most common legit type of unprotected batteries are used in RC and modeling, powering high-performance applications like drones, cars, and planes. In skilled hands, RC enthusiasts counteract the lack of BMS protection by using appropriate chargers and devices that include undervoltage protection.
Unprotected batteries, however, pose risks when used by unsuspecting individuals. For instance, common 18650 cells typically come with a built-in BMS to protect against hazards like short circuits or overloading. But recycled 18650 cells, harvested from old electric car battery packs or similar applications, may lack such protection. These cells were originally used in large battery packs with centralized protection. When sold individually by untrusted sources, they may look identical to protected cells but can become dangerous in projects like electronic devices or e-cigarettes. A simple short circuit can cause these batteries to explode.
Why No Built-In BMS?
There are several legitimate reasons why some battery packs lack a built-in BMS:
- High Performance: RC applications and high-power tools like drills require batteries that can deliver extremely high discharge currents (e.g., 20C or higher). A built-in BMS capable of handling such currents would require larger and more expensive components, adding weight, cost, and complexity to the battery pack.
- Weight and Size: In RC applications like drones and airplanes, weight is critical for performance. Adding a BMS increases the size and weight of the battery pack, potentially reducing efficiency or flight time.
- More Control: RC enthusiasts often prefer external balance chargers, which allow them to monitor and charge cells individually. This flexibility enables better performance and longer battery lifespan, which a built-in BMS might restrict.
- Cost: Batteries without a built-in BMS are cheaper to produce and purchase. Many RC users already own advanced chargers capable of balancing cells and ensuring safe operation, so including a BMS would add unnecessary cost.
- Resilience: RC batteries often endure extreme conditions, such as rapid discharges or physical impacts from crashes. A built-in BMS could fail under such stress, making it an unreliable component.
Regardless, a battery must be protected. If you choose not to use a BMS inside the battery pack, it becomes your responsibility to ensure that protection is handled by the devices connected to the battery.
To facilitate this, unprotected battery packs with multiple cells must expose each cell individually via a balancer plug. This plug typically includes one common ground and one additional pin per cell.
Your Responsibilities
If you decide to use a battery without a built-in BMS, it is your responsibility to protect it. Failure to do so can result in damage or even severe hazards. With the appropriate technical knowledge, these responsibilities are manageable:
- Charging: Unprotected batteries provide physical access to each individual cell through the balancer plug. Specialized chargers can monitor each cell and ensure safe, high-speed charging. Chargers with balancer capabilities can equalize the charge levels of all cells, improving overall health and capacity. With normal batteries, such tasks are handled by an internal BMS, offering less transparency and control.
- Discharging: Over-discharging is the most critical hazard during operation, as it can severely damage a battery. Since unprotected batteries cannot prevent over-discharge themselves, it is your responsibility to ensure that the device being powered shuts off when the battery voltage drops below safe thresholds.
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(content created Dec 08, 2024)