The passive balancing hardware is implemented with a bypass resistor and a switch across every cell in the pack. The balancing resistor is typically used in one of two ways (Figure 2, below). It can be used to steer charging current around the cell so that batteries with a lower state of charge (SOC) can charge at a higher rate and remain charging without risk of overcharging and damaging cells with a high SOC. Optionally the resistor can be used to bleed excess charge from batteries with higher charge states to equalize them with batteries having a lower SOC.
Figure 2: Two options for passive cell balancing. Resistor value determines the primary function.
The primary hardware design concern is to determine the appropriate balancing current, which is set by the value of the bypass resistor. The required balancing current largely depends on the capacity of the cell, the amount of time that can be allowed for balancing, the expected amount of imbalance, and how the resistor will be used. If used to bypass the charger current, several amps will be shunted. If the balancing resistor is used to bleed excess charge, the resistors will be sized to meet the desired balancing time.
The passive balancing is only capable of correcting SOC imbalance stemming from pack loading due to the battery monitoring circuitry, and cell self discharge and internal resistance effects. If constantly monitored, these sources should only create small amounts of imbalance on a day-to-day basis. The BMS system for this lab evaluation has a balancing resistor of 33 ohms that sets the balancing current to roughly 100mA, a large balancing current for small batteries, but one that allows balancing operations to take a shorter amount of time.
The control program for the BMS hardware was written to both monitor battery status and manage battery imbalance. The system’s passive balancing feature can be turned on and off to determine the impact balancing has on a battery pack. Lab tests were run on two identical battery packs manufactured by Turnigy over many charge/discharge cycles. For comparison, only one battery pack was monitored to ensure that each individual cell voltage remained in normal operating range. The second battery pack was monitored and received periodic passive balancing.
Both battery packs used for this experiment consisted of six series lithium polymer batteries with a total capacity of 2.2 AHr. The individual cells have a max terminal voltage of 4.2V and a minimum terminal voltage of 3V. To simulate real-time use and to accelerate aging, both battery packs were continually charged and discharged under the supervision of the BMS. The discharge cycle was a fixed rate of two to three times battery capacity (4.4 to 6.6A), while the batteries were charged at a constant current of one to two times battery capacity (2.2 to 4.4A).
The basic monitoring system was set up to monitor the individual cell voltages for under- and overvoltage conditions, as well as any overcurrent faults. During discharge, any cell in the stack reaching the undervoltage limit of 3.005V would end the discharge cycle. During the charge cycle, if any cell in the pack reached the overvoltage condition of 4.19V cell charging was terminated. Each of the battery packs was charged and discharged repeatedly for 100 cycles to accelerate aging.
Part 2 of this feature details the control algorithm and presents the results of lab tests.
Cuyler Latorraca is an applications engineer at Linear Technology.
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