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<aside> đź’ˇ Passive balancing results in all battery cells having a similar SoC by simply dissipating excess charge in a bleed resistor; it does not, however, extend system run time. Active cell balancing is a more complex balancing technique that redistributes charge between battery cells during the charge and discharge cycles, thereby increasing system run time by increasing the total useable charge in the battery stack, decreasing charge time compared with passive balancing, and decreasing heat generated while balancing. Generally for low power application (RESS and small C&I ESS Application) the Passive Balancing BMS is good with its low cost, small size and very simple control mechanism.
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The cell imbalance falls into two major categories, they are internal and external sources. Internal sources include manufacturing variance in charge storage volume, variations in internal impedance and differences in self-discharge rate. While the external sources are mainly caused by some multi-rank pack protection ICs, which drain charge unequally from the different series ranks in the pack. In addition the thermal difference across the pack results in different self discharge rates of the cells.
The balancing topologies can categories as passive and active balancing as shown in Fig. 1. The passive balancing methods removing the excess charge from the fully charged cell(s) through passive, resistor, element until the charge matches those of the lower cells in the pack or charge reference. The resistor element will be either in fixed mode or switched according the system. The active cell balancing methods remove charge from higher energy cell(s) and deliver it to lower energy cell(s). It has different topologies according to the active element used for storing the energy such as capacitor and/or inductive component as well as controlling switches or converters.
Fig 1 Passive and Active cell balancing topologies.
Shunting resistor cell balancing methods are the most straightforward equalisation concept. They are based on removing the excess energy from the higher voltage cell(s) by bypassing the current of the highest cell(s) and wait to until the lower voltage cell(s) to be in the same level. The shunting resistor methods can be categorised into two sub-categories as shown in Fig. 2.
Fig 2 Shunting resistor a) fixed resistor and b) control shunting resistor
The main drawback in these methods the excess energy from the higher cells is dissipated as heat, and if applied during discharge will shorten the battery’s run time.
<aside> đź’ˇ That is the reason why UZ battery choose to balance the cell at charging stage.
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Capacitive cell balancing, also known as “Charge Shuttling” equalisation, utilize basically an external energy storage devices, capacitor(s) for shuttling the energy between the pack cells so as to the balancing. The capacitor shuttling can be categorized into three shuttling topologies; the basic switched capacitor, single switched capacitor and double-tiered capacitor topologies see Fig. 1.
3.1.1 Switched Capacitor
The switched capacitor is shown in Fig. 3. As illustrated it requires n-1 capacitors and 2n switches to balance n cells. Its control strategy is simple because it has only two states. In addition, it does not need intelligent control and it can work in both recharging and discharging operation.
The disadvantage of the switched capacitor topology is relatively long equalisation time.
Fig 3 Switching capacitor cell balancing topology.