Balancer
The term English Balancer (in German about balancing controller ) denotes an electronic circuit, usually part of a battery management system is. It is intended to ensure the uniform electrical charge distribution of all similarly structured but slightly different galvanic cells in electrical terms due to manufacturing tolerances and aging within a battery pack. This achieves a compromise in terms of usable capacity and protection of individual cells from critical charge states.
Problem
To increase the nominal voltage, battery packs usually consist of several individual cells or cell blocks connected in series . Due to manufacturing and aging, there are fluctuations in the capacity and internal resistance of these cells. In practical use, without additional measures, this leads to the cells being charged and discharged differently, which leads to a critical total discharge when discharging or to overcharging when the end-of-charge voltage of individual cells is exceeded , although the total voltage is still within the nominal operating range . Depending on the type of battery, this can lead to irreversible damage to individual cells due to the decomposition of the electrolyte , so that the entire battery pack loses capacity. In addition, if the operating limits of individual cells are exceeded or not reached, protective circuits may be triggered and the battery pack deactivated.
Working methods
There are several different methods of balancing, which are referred to as passive and active balancing. If, due to its chemical structure, a battery can tolerate a certain deep discharge or overcharging without being damaged, this is sometimes referred to as natural passive balancing (this method is not, however, counted as balancing in the strict sense). With natural passive balancing, the excess energy of the already full cells is converted into heat directly in the battery cell or reduced by outgassing. In practice, natural passive balancing can only be used with overcharge-proof lead and nickel-cadmium batteries . With all other types of accumulators, in particular the different types of lithium-ion accumulators , charge equalization in the form of a passive or active balancer circuit is necessary.
Passive balancing
The frequently used and technically simpler method of a passive balancer only works in the area of the charging end when the cells of a battery pack are almost fully charged. In the case of cells that have already reached the end-of-charge voltage, the balancer switches an additional resistor parallel to the cell, whereby the voltage of this cell is limited to the end-of-charge voltage. This cell is then only slightly further charged or even slightly discharged, while the cells in the series connection which have not yet reached the end-of-charge voltage continue to be supplied with the full charging current. The power of the parallel resistor must be adapted to the charging current, since the excess energy leads to heating of the resistor.
Passive balancing of cells in the area of the end of discharge is also possible, but is rarely used in practice. Those cells with more charge are then additionally discharged to a greater extent via the parallel resistor than cells with less residual capacity. However, a common end of charge cannot be achieved in this way. Passive balancing in a partially charged state is also of no practical importance, since the state of charge of the individual cells can only be precisely determined via the cell voltage in the area of the fully charged or almost empty state . Even cells with the same open-circuit voltage can have very different states of charge when partially charged. Only devices that use low compensating currents to compensate for the 12 V block voltages in lead-acid batteries (PowerCheq) have become more widespread (e.g. in the CityEl ).
Active balancing
In active balancers, the balancer circuit realizes a charge transfer from neighboring cells to one another and transfers the energy from cells with a higher charge to cells with a lower charge.
In principle, the circuit represents several switching regulators specially optimized for the application , which work per cell and actively transfer energy from one cell with a higher capacity to an adjacent cell with a lower capacity. This process can take place during the loading process; As a rule, as with passive balancers, it is used in the area of the loading end. As is usual with switching regulators, additional energy storage devices are required to transfer the energy, and these are switched between the individual cells. Capacitors are used for smaller outputs, coils for higher outputs .
The simplified circuit diagram opposite shows part of an active balancer with two stages. Energy can only be transferred in one direction from the cell with the index n to the cell n − 1 below. For this purpose, the power transistor FET n is first switched on (the control circuit required for this is omitted for the sake of simplicity) and the coil L n is charged by the cell Cell n up to a certain limit current. This circuit is marked with a red circle (1). Then the FET n opens . Since the current continues to flow through a coil, a second circuit is formed, marked with a blue circle (2), which charges the cell Cell n-1 via the diode D n − 1 . This process is repeated until the charge of the upper cell n is equal to that of cell n − 1 .
This principle can be continued across a chain of cells of any length. At the lower end of the chain, a galvanically isolating DC voltage converter ( not shown here) is attached, which can take energy from the lowest cell Cell 1 and feeds the cell at the upper end of the potential potential-free. Any charge distribution can be actively balanced through this loop across all cells.
The advantage of active balancing is the significantly higher degree of efficiency , as excess energy is only converted to a small degree into heat. Active balancing is therefore primarily used for larger outputs, such as traction batteries in the field of electromobility or battery storage power plants . The disadvantage is the higher circuit complexity with the necessary control and the associated higher costs.
Practical execution
Balancers are used as part of battery management systems in notebook computers, camcorders , cordless tools and in the traction batteries of electric vehicles. In most consumer applications, the balancer and battery cells are combined in a common module. The balancer is then no longer visible from the outside. On larger battery cells, special passive balancer modules can also be placed directly on the cells. They work independently of each other and limit the end-of-charge voltage of the respective cell by specifically discharging them above the end-of-charge voltage via heating resistors.
In RC model making , on the other hand, the battery pack and balancer are usually used separately from one another, or the balancer is integrated in the external charger. In this sector, balancers are also offered in the form of printed circuit boards , sometimes also in the form of kits for self-construction. For operation, it is necessary that all necessary partial voltages are brought out to the battery pack with which each cell can be treated individually.
When starting up for the first time (in the case of finished battery packs with an integrated battery management system, this is done by the manufacturer), it is often necessary to roughly balance the charge quantity of the cells. This is usually done by specifically discharging the cells with the highest voltage level or by charging all cells in parallel until the end of charging before they are connected in series. The balancing system then takes on minor corrections in the subsequent normal operation.
Connection systems in model making
Since the manufacturers have not agreed on a uniform standard for their balancer connections, there are now a large number of connector systems for balancers in the field of model making. The most widespread is a single-row socket strip, which contains total plus and total minus on the outside. In between there are taps between the cells for individual measurement of each cell voltage and equalizing charge / discharge.
| Connector system | Hardware on the battery side | Manufacturer |
|---|---|---|
| EHR (EH)
2.5 mm |
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| XHP (XH)
2.5 mm |
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| FTP (TP)
2 mm |
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| PQ |
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| MPX |
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| Schulze Electronics | double row socket |
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Web links
Individual evidence
- ↑ a b Stanislav Arendarik: Active Cell Balancing in battery packs. NXP, company publication, 2012, accessed on October 9, 2016 .
- ↑ ElWeb: Powercharge Battery Optimizer , accessed June 28, 2013
- ↑ Active Cell Balancing Methods for Li-Ion Battery. (No longer available online.) Atmel, company publication, archived from the original on October 9, 2016 ; accessed on October 9, 2016 .
- ↑ JST EH connector 2.5 mm specification as PDF file accessed on December 23, 2018
- ↑ JST XH connector 2.5 mm specification as PDF file, accessed December 23, 2018