Basic Concepts in Battery Technologies
Lithium cells, which allow the production of battery packs at the desired voltage by connecting them in series, are not exactly identical in practice, although it is assumed that they go through the same processes when produced in the factory.
On the other hand, differences occur in the anode/cathode and electrolyte structures of series-connected cells over time due to reasons such as the busing structure of the battery pack in which they are used and the cells being exposed to different thermal stresses due to their position in the box. Even if it is thought that the same current flows through them during charging and discharging, it causes differences between the charge levels (SoC) of the cells and therefore the equality between the cell voltages.
Although different characteristics are seen in different Lithium battery chemistries, the chemistry with the most intense cell voltage imbalances is LiFePO4 – LFP. Large capacity Energy Storage Systems (BESS) and Electric Buses etc. LFP cells produced with high Ah capacity have higher chemical internal resistance than other Lithium Metal Oxide (NMC, LCO, LCA, etc.) chemistries. This causes additional energy losses resulting from internal resistance differences at high currents to create serious differences between cell voltages over time.
In the LFP battery pack we have, eventually 50-100 mV differences between cells become visible. This causes the charging to be cut off early during charging due to the cells remaining at a higher voltage and therefore higher SoC level. On the other hand, due to cells with low SoC levels, during discharge, these cells will enter SoC = 0% levels such as 2.5V earlier than others, causing the discharge to be stopped early.
OVP (Overvoltage protection) and UVP (undervoltage protection), which are the basic functions of BMSs, are activated early due to unbalanced cells for the reasons mentioned above. In this case, the usable energy (i.e. Ah capacity) of the battery pack will be lower than in a balanced battery pack.
Re-establishing the voltage balance between the cells of the battery pack, where there are differences between the SoC and therefore the open circuit voltages between the cells for the reasons explained above, is called "Cell Balancing". Technically, balancing is done in two ways. Passive and active balancing.
In this technique, there is a semiconductor or relay switch and a resistor connected in series to the +/- terminals of each cell in the BMS circuit. BMS software analyzes the voltage measurements and connects the switches corresponding to the cells whose voltage is above the average, allowing current to flow through the resistor. In this way, the energy of the cell is "burned" at V2/R power for a certain period of time. As the energy decreases, the voltage of the cell also decreases. The main problem in passive balancing circuits, which electronically involve only turning a switch on and off, is the dissipation of the heat generated on the resistor. When fast balancing is desired, the balancing current must be high, in which case the resistance is selected small. For example, if 1 A balancing current is desired to flow, it is necessary to balance a 3.2V LFP cell with a 3.2 Ohm resistor. Since it means a heat output of I2R, this balancing process will produce 3.2W of heat. This creates a situation that is difficult to disperse on the circuit board and requires the connection of cooling materials. Especially in order to apply this kind of balancing to 3-5 cells at the same time, very serious cooling structures must be created.
Due to these thermal problems, it is not practical to flow balancing current above 100-200 mA in passive balancing circuits. If the cell being tried to balance is 200-300Ah, these small currents can extend the balancing time considerably. It is not possible to wait for such a long time for balancing for BESSs or electric forklifts and buses that need to be charged and discharged frequently.
2. Active Balancing:
Although there are many different active balancing circuit topologies in the literature, three basic methods are used.
Intercellular charge transfer
Charge transfer from cell to pack
Charge transfer from Packet to Cell
Due to the complexity and high costs of electronic circuits, package < -> It is not appropriate to use methods based on intercell charge transfer except in special cases. In this context, methods based on charge transfer between cells in the package stand out as more applicable solutions.
Charge transfer between cells with capacitive elements:
It is based on the principle of alternatively connecting capacitors to neighboring cells in a certain period. In this method, which is called "Flying Capacitor" or "Capacitor charge shuttle" in the literature, it is necessary to use capacitors with very low internal resistance (ESR). This requires the use of appropriate semiconductor switches (MOSFETs) as it causes instantaneous high currents to flow during switching.
Charge transfer between cells with inductive elements:
In this structure, the load is transferred through the inductor (coil) with a buck/boost circuit between neighboring cells.
Active Balance in BATKON BMS:
BATKON team worked on many balancing methods during the 10-year lithium BMS design process and designed and implemented electronic circuits and embedded software that perform these functions.
At this point, it has converged to a Lithium BMS design that can use both Passive and Active (Inductive) charge transfer methods. In the Modular BMS design project codenamed SANDY carried out by BATKON, Active and Passive balance modules that can be added to the BMS main board as Daugher-Boards have been designed. While balancing is possible with currents up to 200 mA in passive balance modules, charge transfer of up to 1.5A between neighboring cells is possible in the active balance module.
BESS, E-Forklift, AGV, Solar Battery etc. where 200-300Ah cells are used. With the Active balancing module developed by Batkon, it is possible to balance the LFP cells efficiently, using a current as high as 1.5A and producing very low heat.