Choose right Battery Management System for your Battery
Battery is made of combination of more than one cell to operate in a specific voltage range. Each cell in a battery has similar behavior to another cell but not exactly same. This problem creates a requirement for a battery Management system in battery. Different functions of BMS are:
- Cell Overcharge protection
- Cell Under charge protection
- Battery Over/Under temperature protection
- Cell balancing
- Data logging
- SOC/SOH estimation
- Battery Parallel operation
- Battery power control and many more…
Today we are talking about the Cell Balancing feature of a BMS. What is cell balancing?
Cells are connected in series in a battery. In charging and discharging the cells in series have different voltage which creates problems in fully discharging and charging of battery. I.e.
As shown in block diagram when one cell voltage reaches 4.2 voltage in charging, BMS disable charging. Hence other cells remain uncharged which results in lower capacity of battery. This also affects the battery life as the battery may not go into the CV (Constant Voltage Charging ) stage. So, the cell whose voltage reaches 4.2 voltage earlier will degrade faster which leads to more dis-balancing, hence decreasing the capacity and life cycle of the battery.
This can be solved if somehow BMS keep all cells balanced. So, there are different ways to keep the cells balanced. Two common ways to balance cells are :
- Passive Cell Balancing
- Active Cell Balancing
Passive Cell Balancing :-
In Passive cell balancing BMS discharge the cells which have higher voltage than lowest voltage cell. In this topology BMS discharges the cell through a register dissipating the energy as heat. Due to heat generation cells can be discharged with small current from 20mA to 400mA only. The working of passive cell balancing is shown in block diagram :-
As shown in the simulated circuit a battery pack of 4 cells is charging with 16.8 volt. A passive balancing circuit is implemented to balance the circuit. The cell with the highest voltage is discharging with 150 mA current. The current is limited by a register of value 14 Ohms. The energy is dissipated as heat at the register. Simulated circuits can be tested at https://falstad.com/circuit by opening this config file. Passive cell balancing can take upto 100 charge/discharge cycles to balance a 50mV unbalanced battery pack. If the battery pack is of higher capacity then passive cell balancing has a negligible balancing effect. Hence in large battery packs of higher capacity passive cell balancing means no balancing.
Active Cell balancing :-
In active cell balancing to balance the cells energy is not dissipated as heat but transferred from one cell to others. There are different topologies which can be used to balance the cells by transferring energy. Working of active cell balancing is shown in diagram :-
As shown in block diagram energy from one cell to pack or from pack to cells can be transferred to balance the cell. In this topology the energy is transferred between the cells rather than dissipating energy by heat. Active cell balancing can balance cells with more efficiency and less time.
The active balancing circuit is complex and more expensive. Battery manufacturers are always confused when to select passive cell balancing and when to choose active cell balancing BMS.
Passive cell balancing can be useful if fulfill following criteria:-
- Battery capacity is less than 10 Ah
- Cells are perfectly binned w.r.t IR, capacity and voltage
- Cells are thermally stable
- Battery is to be operated according to cell manufacturer recommendation
- Battery is charged in temperature controlled environment
Active cell balancing can be useful in all applications. Active cell balancing is recommended if the battery pack is of higher capacity.
Note:- If a battery pack fulfills the passive cell balancing BMS criteria, it is recommended to use passive cell balancing BMS to reduce the cost of the battery.
If you liked this, click the💚 below so other people will see this here on Medium.
To get Battery Management system for your battery please visit https://electrifuel.com
