No one needs to be convinced of the advantages of batteries based on lithium technology. Their ability to store much larger amounts of energy than previously used nickel or lead batteries of similar dimensions has led to the massive development of Li-Ion, LIFePO4, or Li-Pol cells.
Additionally, lithium batteries are many times lighter than their predecessors.
Li-Ion and Li-Pol batteries are effectively displacing nickel cells in smaller devices (electronics, tools, bicycles, scooters), and Li-FePO4 batteries are becoming a replacement for lead batteries in energy storage, or batteries powering larger electric motors of various applications.
However, it is important to realize that all lithium cells require operation under strict conditions such as:
- Maximum charging voltage
- Minimum discharge voltage
- Maximum charging current
- Maximum discharge current
- Specified temperature range (for discharging and charging)
In the case of lithium cells, exceeding the parameters may not only shorten the life of the battery, but also lead to its destruction or even very violent ignition.
Although successive generations of batteries are being designed so that the risk of ignition-explosion is reduced as much as possible, it is nevertheless real in extreme cases. Therefore, as a rule, it is considered to use lithium batteries only with an electronic protection system – BMS. BMS – is an abbreviation for “Battery Management System”.
BMS’s functions:
- Separate monitoring of the parameters of each cell in the package
- Protection against exceeding the maximum charging voltage
- Protection against exceeding the minimum discharge voltage
- Short circuit protection
- Protection against exceeding the maximum charging current (optional)
- Protection against exceeding the maximum discharge current (optional)
- Protection against exceeding the permissible temperature range during charging and discharging (for BMS with NTC temperature probe)
BMS with cell balancing function also allows for equalization of voltages between cells in the pack (charging weaker cells at the expense of stronger ones). Differences in cell voltages and capacities are perfectly normal, due to manufacturing inaccuracies, and the balancer function allows better use of battery energy and extends battery life.
Due to different voltage ranges (min-max) for different types of cells (Li-Ion, LIFePO4, Li-Pol) the BMS must be structurally adapted to the given type of cell. In some BMS models the type and number of connected cells is set by the manufacturer, in others it can be selected during BMS programming before starting operation.
Monitoring each cell in the pack separately, the BMS will disconnect the charging system when the strongest cell in the pack reaches the maximum voltage for that cell type, and will disconnect the receiver on discharge when the weakest cell reaches the minimum voltage. Therefore, critical to the proper operation of the package is the selection of targets with the most similar parameters (voltage, capacitance, internal resistance). All targets must also be identically charged before bundling.
In the case of brand-new cells from reputable manufacturers, it is most often sufficient for them to be of the same type, from the same production batch and not previously discharged separately. With used cells, detailed selection is required.
BMS SMART allows monitoring of all battery parameters (including single cells) via a mobile application, additional screen, or PC. In advanced systems, they can also transmit information to external power supply system components, e.g. inverter via CAN/RS485 connection.
Data exchange takes place via Bluetooth, cable and increasingly via WiFi. In BMS SMART it is also possible to change security parameters. However, it should be noted that such actions require reliable knowledge of lithium cells. Entering incorrect values for a given cell type may be potentially dangerous.
BMS, regardless of its type and construction, is not a system that replaces the selection of appropriate pack charging and discharging parameters. These parameters should be selected by selecting the appropriate charger or inverter and receiver devices.
The BMS is a necessary, but additional protection in case of failure or overloading of connected devices.
In order to maintain the correct operation of the lithium battery, the protection and design of the BMS system allow for a short-term load with a current much higher than the nominal BMS current. This allows, for example, starting electrical devices. However, longer operation at such currents leads to permanent damage to the BMS not covered by the warranty.
For example, for a BMS 10S 40A Li-Ion, the current that triggers the overcurrent protection during discharge is approx. 120A. This protects the cells sufficiently, but continuous operation of the BMS with a current of more than 40A must lead to its failure. The situation is similar when charging.
The above comments provide a basic principle for selecting a BMS. In addition to the appropriate type and number of cells, the BMS is selected for the planned charging and discharging current, not for the cell capacity.
