BMS for lithium-ion battery
BMS for lithium-ion batteries plays the main role in lithium battery function. As we all know LiFePO4 batteries pack a lot of power and value into a small package. The chemistry of these batteries is the main part of their superior performance. But all reputable commercial LiFePO4 batteries also include another important factor along with the battery cells themselves: a carefully designed electronic battery management system (BMS). A well-designed battery management system protects, this 100% test guarantees higher production ability, higher battery capacity, and reduced warranty commitments. Excellent BMS can also detect the humidity of the LiFePO4 battery pack to avoid accidents.
All our LiFePO4 batteries include an internal or external BMS. Let’s have a look at how a BMS protects and optimizes the operation of a LiFePO4 battery.
1. Over-current and Short Circuit Protection
Every battery has a maximum specified current for safe operation. If a load is applied to the battery which draws a higher current, it can cause overheating of the battery. When it’s important to use the battery in a way to keep the current draw below the maximum specification, the BMS again acts as a backstop against over-current conditions and disconnects the battery from the operation.
These protection concepts can be best understood with an example using one of Spaceflight Power’s batteries, its 12.8 volt 9 amp-hour battery with a maximum continuous discharge current of 20 amps. The first level of Over-Current Discharge Protection occurs when the battery discharges between 20 and 30 amps with a time delay of 10 seconds. To release the protection, the load needs to be removed for 15 seconds. The second level of protection occurs when the battery discharge between 25 and 35 amps for 3 seconds. The protection is again released upon removal of the load for 15 seconds. The third level of protection occurs when the battery is discharged between 40 and 50 amps for 31 milliseconds. Like the other releases, the protection is released upon the removal of load for 15 seconds. The multiple levels of protection allow the battery to discharge at a high rate to handle surge demands without damaging the battery.
A short circuit of the battery is the most serious form of the over-current condition. It usually occurs when the electrodes are accidentally connected with a piece of metal. The BMS for lithium-ion batteries contains a balanced circuit and additional circuitry that controls the parameters of the battery by protecting against over-discharging and over-discharging. The battery shuts down within 200-600 microseconds of an external short circuit, then resumes normal operation if the short circuit condition is removed.
2. Over and Under Voltage
LiFePO4 battery cells operate safely over a range of voltages, typically from 2.0V to 4.2V. Some lithium chemistries result in cells that are highly sensitive to over-voltage, but LiFePO4 cells are more tolerant. Still, significant over-voltage for a prolonged period during it is charging can cause to plate of metallic lithium on the battery’s anode which permanently degrades performance. As well as the cathode material may oxidize, become unstable, and produce carbon dioxide which may cause a buildup of pressure in the cell.
Under-voltage during battery discharge is also a concern when discharging a LiFePO4 cell below approximately 2.0V resulting in a breakdown of the electrode materials. Lithium batteries have a recommended minimum operational voltage. For example, the minimum recommended voltage is 11V. The BMS for lithium-ion battery acts as a failsafe to disconnect the battery from the circuit if any cell drops below 2.0V.
3. Lithium Balance Circuit
LiFePO4 batteries have a major difference from lead-acid batteries when it comes to balancing the voltage in each cell during its charging. Owing to small differences in producing or operating conditions, each cell in a battery charges at a slightly different rate. In a lead-acid battery, when one cell charges faster and reaches its full voltage, the typical low end of charge current, along with the overcharge-return, will guarantee the other cells get fully charged. In a sense, the cells in a lead-acid battery are self-equalizing while it is charging.
This is not the case with LiFePO4 batteries. When a LiFePO4 cell is fully charged, its voltage begins to rise further which may lead to electrode damage. In lithium batteries, as soon as the lowest voltage cell hits the discharge voltage cut-off, it will shut down the entire battery. This may mean that some cells have unused energy. Likewise, if the cells aren’t balanced when it is charging, charging will be interrupted as soon as the cell with the highest voltage reaches the cut-off voltage, and not all the cells will be fully charged.
Continually charging and discharging an imbalanced battery will lower the battery’s capacity over time. That’s some cells will be fully charged, and others will not, resulting in a battery that may never reach 100% State of Charge.
A well-designed BMS battery will ensure each cell safely and fully charges before the entire charging process is complete.
4. Over Temperature
Unlike lead-acid or lithium cobalt oxide batteries, LiFePO4 batteries operate efficiently and safely at temperatures up to 60℃ or more. But at higher operating and storage temperatures, as with all batteries, the electrode materials will begin to degrade. A LiFePO4 battery with BMS uses embedded thermistors to actively monitor the temperature during operation, and it will disconnect the battery from the circuit at a specified temperature.
Conclusion
LiFePO4 batteries are made up of more than just individual cells connected together. They also include a battery management system (BMS) which, while not usually visible to the end user, makes sure each cell in the battery remains within safe limits. Therefore, the LiFePO4 battery pack must be equipped with a targeted LiFePO4 battery management system BMS to effectively monitor, protect, balance, and fault alarm the battery pack, thereby improving the efficiency and service life of the entire LiFePO4 battery. BMS for lithium-ion batteries is like the brain of human beings, without BMS the battery cannot function properly.
