The BMS’s(Battery Management System) function is to control and operate the battery safely and efficiently. One of these functions include cell balancing. A LiFePo4 battery pack consists of a number of individual 3.2V cells connected in series. In most cases a cell manufacturer will send a cell shipment charged up to 40% SoC(State of Charge) according to shipping regulation.
Even though the average charge capacity of a cell shipment is at 40% SoC, the BMS will monitor each cell down to the millivolt(mV) and will indicate the cells to be at different SoC. The difference in SoC between each cell will be calculated with the entire pack SoC then lowered down to the lowest cell voltage reducing the available usable capacity of the battery pack. This is why effective cell balancing is very important.
The BMS will stop accepting charging currents once the pack reaches its maximum operating cell voltages and will also stop discharging once the cell reaches the minimum operating low voltage. Charging and discharging unbalanced cells could cause damage to individual cells as a lower cell will be discharged below the acceptable low cell voltage. The same issue applies for charging as the high SoC cells will be charged above the acceptable high cell voltage.
The in-house designed and developed advanced AI Solar MD BMS-EX version can balance 7x cells at the same time and also allows you to change balancing methods from top to bottom balancing.
Active and Passive cell balancing are the two techniques used when balancing a battery pack.
Passive cell balancing consists of using resistors that convert energy into heat to discharge each cell, which will be found on a BMS. This is a cost effective method though the pack is limited by the lowest cell SoC. It can also be considered as inefficient as the available energy is spent as heat.
Active cell balancing is done by discharging a high SoC cell’s energy to the lowest SoC cell by using a DC-DC Converter or capacitors. Benefits include reducing energy waste and the SoC is calculated according to the average of pack voltage though the system design is more complex and increases electronic equipment cost.
Top and bottom balancing are the parameters chosen by a battery manufacturer to balance a battery pack. With bottom balancing the pack can be discharged without over discharging the weaker cell. The BMS protects the weaker cell from overcharging as the max cell voltage will have been determined by the BMS. Top balancing allows for charging a pack voltage without the risk of the weaker cell, though the BMS will stop the pack from discharging below the weaker cell discharge voltage.
Simply put, top balancing allows you to operate around the max voltages of the cell and bottom balancing allows you to operate near minimum cell voltages. Operating the battery within these safe parameters will increase battery life and efficiency.