The Battery Management System (BMS) of a lithium battery: protections, passive or active balancing, connected BMS
The BMS (Battery Management System) is the electronic circuit board that monitors the cells of a lithium battery. It's most often noticed when it cuts power. However, it does much more: it measures, protects, balances, and, in newer models, it provides information about the battery's status. This guide opens the door to understanding its functions.
The essentials
- The Battery Management System (BMS) measures the voltage of each cell group, the current, and the temperatures.
- It cuts off either the charging or discharging process by using transistors when a limit is reached.
- It balances the cells to ensure they maintain the same charge level, either by dissipating excess energy (passively) or by transferring it (actively).
- A connected BMS (Battery Management System) allows you to read the status of each individual cell from a phone or controller.
What the BMS measures:
- The voltage of each series of cells: This is the most important measurement, as the weakest group of cells determines the limits of the entire battery.
- The current, both during charging and discharging, is typically measured using a resistor.
- Temperatures are monitored using probes placed between the cells and on the power components themselves.
Protective gear
| Protection | Triggered by | Effect |
|---|---|---|
| Overload | A group operating above its maximum voltage | Disconnect the power |
| Deep discharge | A group operating at its minimum voltage | Cut the discharge |
| Overcurrent during discharge | Current exceeding the limit, for a period longer than a certain time | Cut the discharge |
| Short circuit | Extreme current | Cut in a matter of microseconds |
| Overcurrent during operation | Excessive charging current, including during electric braking | Disconnect the power |
| Temperature | The battery temperature is too high or too low to charge | Interrupts the charging or discharging process |
The power cutoff is achieved through power transistors located on the negative side of the battery. Some Battery Management Systems (BMS) have a single output for both charging and discharging (a common port), while others have two separate outputs: the charging port can then have a significantly lower current limit. Cutoffs during acceleration are detailed in our guide on batteries and BMS that cause acceleration cutoffs, and the effect on braking is explained in the guide on regenerative braking.
Why balance?
No cell is perfectly identical to its neighbor. Over time, slight differences in capacity and self-discharge cause the cell groups to diverge: one group reaches full charge before the others, while another group depletes first. Without correction, the battery loses usable capacity (charging stops at the first full group, and discharging starts with the first empty group), and the extreme groups wear out more quickly.
Passive balancing
The most common method. At the end of the charging cycle, the Battery Management System (BMS) connects a small resistor to the cells with the highest voltage to dissipate their excess energy as heat, until they reach the same voltage as the other cells. This method is simple and reliable, but slow: the balancing current is low. It only corrects small voltage differences, and only when the battery is regularly charged to its full capacity.
Active balancing
The BMS (Battery Management System) transfers energy from the most charged battery packs to the least charged ones, using capacitor or inductor circuits. It wastes less energy, operates with higher currents, and can intervene at any time, not just at the end of the charging cycle. It is more complex and more expensive. It is typically found in larger battery packs and can be used in conjunction with existing BMS systems.
Workshop tip
A passively balanced battery should be charged to 100% from time to time, and left charging for a while even after the "full" indicator light comes on: it is during this time that the balancing process takes place. A battery that is always charged to 80% will rarely achieve proper balancing.
The connected Battery Management System (BMS)
Modern Battery Management Systems (BMS), often referred to as "smart" BMS, communicate via Bluetooth or a wired connection. They provide information such as the voltage of each cell group, the current, temperatures, the number of charge cycles, and any recorded errors. This is a valuable diagnostic tool: any cell group that deviates from the others is immediately apparent.
Connected to the controller, a BMS (Battery Management System) also allows for further optimization: the controller can adjust its power output based on the actual state of the battery, rather than discovering it only when the BMS cuts off power. Our RageLink controllers, for example, will connect to the battery's BMS.
Frequently Asked Questions
My battery is losing range; could this be a balancing issue?
Yes, it's possible. With a BMS connected, compare the voltage of the battery packs when they are fully charged. A significant difference indicates an imbalance or a worn-out cell. Several full charges may correct the issue if the difference is small.
Is an active Battery Management System (BMS) always the best option?
Not necessarily. For a battery pack with well-matched cells and that is regularly charged to its full capacity, passive balancing is often sufficient. Active balancing becomes more useful for larger battery packs and cells that exhibit significant variations.
Is it possible to replace the Battery Management System (BMS) of a battery?
Yes, but this is a delicate procedure involving charged cells and should be entrusted to a specialist. The new Battery Management System (BMS) must be compatible with the number of cells in series, the battery chemistry, and the current ratings.