Achieving cell balancing for 4LR44 battery

　　4LR44 battery first appeared in 1991 and have become the technology of choice for many applications, including satellites, ground vehicles and model aircraft, as well as laptops and mobile phones. This is mainly due to the outstanding energy density of 4LR44 battery, which is the ratio of stored energy to weight.

　　Voltage drop

　　4LR44 battery are designed to provide a voltage of approximately 3.0 to +4.3V. It is important to always keep the voltage of a lithium-ion battery within its design boundaries, otherwise the battery will suffer irreparable damage. If the battery's voltage drops below 3.0V, it will enter a deep discharge state, and once it enters this state, it will take hours or even days to recover.

　　In fact, deep discharge may cause the battery to short circuit, and once the battery short circuits, it cannot recover. Overcharging to a voltage higher than 4.3V can be even worse, as doing so can destroy the battery, causing overheating or other catastrophic consequences. In a simple application using only one lithium-ion battery, the electronic control circuit must protect the battery, disconnect the load when the battery voltage drops below 3.2V, and ensure that the voltage is below 4.2V during charging.

　　Lithium-ion battery structure

　　4LR44 battery are composed of 2 or more cells connected in series. In this structure, the battery voltage is equal to the sum of the individual battery voltages. For example, a 96V battery is obtained by connecting 24 4LR44 battery in series. After adding the load, the current of the load is provided by 24 batteries connected in series. If the battery is being charged, the charger needs to provide charging current to the battery pack connected in series. In both cases, the discharge and charge currents are the same for all cells.

　　Over its lifetime, a battery may be charged and discharged hundreds or even thousands of times. At this point, individual batteries may age differently. Some batteries will become somewhat mismatched (or more so) than others. If this condition is not corrected, one or more batteries may be undercharged or overcharged, both of which can lead to battery failure.

　　balance

　　The way to improve this situation is called balancing. Balancing is the process of forcing all cells to have the same voltage. This is achieved through a balanced circuit.

　　The Aeroflex balancing circuit uses a shared bus whose voltage is equal to the average voltage of all cells. The balancing circuit charges those cells that exceed the shared bus voltage and injects power into the cells with lower voltages. This is accomplished through high-efficiency bilateral DC/AC converters.

　　The size of the balancing current is proportional to the voltage difference, that is, as the battery gets closer and closer to the ideal balance state, the balancing current will approach 0. Figure 1 shows a balancing circuit for 5 cells, where the 1Ω resistor determines the transfer ratio, which is the ratio of unbalanced voltage to balanced current.

　　For batteries with larger capacities (usually 20Ah or higher), balancing currents up to 1A are required to minimize the time to equilibrium. For these applications, the bilateral DC/AC converter circuit operates at frequencies around 100kHz via planar transformers.

　　battery electronics unit

　　Each circuit is a forward converter with a resonant reset signal, and the switching frequency is controlled by a phase-locked loop to provide precise low-loss switching and high efficiency. Figure 2 shows a battery electronics unit (BEU) implemented using this circuit topology for a 24-cell lithium-ion battery satellite application. This BEU also provides a single cell voltage monitoring function. The accuracy of individual cell voltage is 10mV. This data is measured using 12-bit A/D and serial data telemetry technology.

　　For smaller batteries, it is more appropriate to use a circuit with a smaller balance current but a higher transfer rate. Figure 3 shows a low current cell balancer, Aeroflex’s 8645-13 module. This is a 6" x 2.3" circuit card used to balance 13 cells. This cell balancer is embedded inside the battery package and provides no monitoring functionality.

　　Balanced Advantage

　　By using cell balancing, system engineers can select a larger battery capacity based on the application because balancing enables the battery to achieve a higher state of charge (SOC). Without using the cell balancing function, a conservative design cannot get the SOC close to 100%.

　　The batteries are connected in series and the charging current for all batteries is equal. The charger monitors the total battery voltage and continues charging until it reaches a preset voltage, usually 4.2V per cell.

　　For example, a 10cell battery may need to be charged to 42V to reach 100% SOC. Without balancing between cells, there is no guarantee that each cell voltage is exactly equal to 4.2V. For example, one of the batteries may be charged to 4.4V and may become overcharged or damaged. Therefore, the SOC of unbalanced cells must be well below 100% to ensure that one or more cells are not overcharged.

　　On the other hand, in a properly balanced battery, where all cells have voltages very close to the average battery voltage, it is possible to charge the battery's SOC to close to 100% using a charging circuit that measures the total battery voltage.

　　Therefore, in applications without cell balancing, the SOC of the battery is usually in the range of 20% to 80%, and the utilization rate is only 60%. If balancing measures are added, the SOC range may be 5% to 95%, and the utilization rate increases to 90%. Therefore, the battery balancing system allows the battery size to be reduced significantly to achieve the same output capacity. This reduces the overall weight significantly, making it a good deal even when factoring in the weight of the counterbalancer.

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