How to select BMS ?

A BMS system can be made very sophisticated for optimum use of the battery but is subject to various considerations, including the following:

1.Cost of the Product:

The cost associated with the BMS should not overwhelm the cost of the product. For instance, the BMS implemented in a cell phone should not so sophisticated such that it contributes to the majority of the production cost.

2. Product Requirement:

The BMS should be designed as per the product requirement. Certain applications use batteries in sensitive environment and call for high level BMS system. For instance, in Electric Vehicles harbour large battery packs, inadequate BMS can lead to overheating of batteries which in turn cause the batteries to blast leading to fatal accidents. Hence sophisticated BMS needs to be implemented in such applications.

3. Types of Batteries:

Certain batteries are more sensitive than others and require better monitoring.

A generalized battery management system can be represented using following block diagram:

Figure 1. BMS block diagram.

The above orange channels show the communication channel while the black arrows denote electrical connections.

The data acquisition unit and the control unit together form the BMS.

The data acquisition unit is the monitoring unit that records real time data from the battery like the charging and discharging current, SoC of the battery, etc. depending on the sophistication of the BMS and the parameters required to be monitored. The data is then given to the control which uses it as reference data to generate control signals for the charger and converter units.

The converter, which is a DC-AC or DC-DC unit depending on the load, is used to interface the load with the battery thus controlling the discharge rate while the charging unit (AC-DC or DC-DC) is required to charge the battery.

The charging unit can also directly power the load in most applications like portable electronics.

The main BMS components are as follows:

A. The Charger

The Charger can also be referred to as the power module. The power module is generally an AC-DC converter which can be plugged into the mains supply. When the battery is running low, the charger can be used to power the device directly while simultaneously charging the battery. And supply the load bypassing the battery when fully charged to preserve the charge for later use when mains supply is not available.

The BMS must control two parameters for the charger. First the energy conversion process and the charging parameters of the battery. The charging current and voltage of the battery is continuously measured and fed back to the controller of the BMS. Which in turn generates the control signal to control the output of the charger. For instance, in simple converter it generates the duty cycle signal to control the switching of the power devices (MOSFET or IGBT) to adjust the output.

B. The Battery

Main function of the battery is to store energy. The BMS should be designed specifically for the target battery type:

  • Electronic Safety Switch for Li-ion batteries.
    • Li-ion batteries are very sensitive and require a safety switch to be integrated with the battery. Operating Li-ion cells outside the safe operating region can be hazardous. For li-ion batteries the safe operating region is defined by the operating voltage, charging/discharging current and the operating temperature. Operating the cell outside safe region high voltages can cause the cell to cause fire or explosion and at low voltages it might cause irreversible losses to the overall capacity.
    • Maintaining the battery at high voltage increases the battery capacity of the battery but reduces the cycle life considerably. So, the allowable maximum charging voltage is a trade of between cycle life and capacity. Higher the operating current more is the operating temperature of the battery. Depending upon the operating conditions and application of the cell, the limit for the maximum current should be decided. Both the max voltage and max current limits should be within the safe area of operation. Once the limits are set, the electronic safety switch can be controlled using a controller.
    • The electronic safety switch is generally a MOSFET in series with the battery. The controller senses the voltage of the battery and the current and generates isolation signal whenever any set limit is exceeded.
  • Charge Balancing
    • A single Li-ion cell can produce voltage around 3.6V and provide around 6A of max current. This is too small for most of the applications i.e., electric vehicles. For this purpose, number of cells are connected in series and parallel to derive the required rating. Adding cells in series increase the voltage while adding cells in parallel increases the current rating of the battery.
    • Two identical cells used for same application show difference in their performance. This is due to the variations occurred during manufacturing, difference in impedances, unequal heat distribution to the packaging style, etc. These variations become even more severe with ageing effect.
    • Due to this difference in SoC levels is observed across different cells during charging and discharging processes. There are many supervisory ICs available commercially. These ICs supervise individual cell voltages and interrupt charge or discharge currents when one of the voltage set limit is exceeded. A charge balancing circuit equally distributes the charge applied to or drawn from the battery between the individual cells by transferring charge from highest SoC cell to lowest SoC cell.
  • Smart Battery
    • Smart battery is a battery with a microchip installed on it. This chip implements the functions mentioned above with certain other functions like communication with the host and calculate and predict Real-time battery information. Smart battery comes with features such as, self-monitoring, charge control, fault identification, protection and communication with host to store data. Once you have the first two features in the battery it is relatively inexpensive to add the additional features.

C.  The DC-DC Converter

The DC-DC converter is used to interface the battery to the load. The battery rating may not be the same as the load demands. The DC-DC converter is in charge of providing appropriate voltage and current as per the load demands. This ensures efficient use of the battery and monitors the discharge current.

The converter is controlled by a microcontroller. The controller senses the battery parameters to be controlled and load requirements and generates appropriate control signals to adjust the converter output.

The above is a general description of a typical Battery Management System.

In summary

It is very important to match a specific BMS to your battery cells type and the target storage application if the battery cell life are to be prolonged. At Everspring, we have been assisting our partners to development and support such setup for large format Lithium Ion cells since 2001.

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