Balance hybrid/EV battery packs to extend operational lifetime: Pt. 1 – Hardware and strategy
With the growing popularity of using batteries for power sources there is an equal demand for maximizing their useful lifetime. Battery imbalance, a mismatch in the state of charge of the individual cells that make up a pack, is a problem in large lithium battery packs that is created by variations in the manufacturing process, operating conditions and battery aging.
Imbalance can reduce a battery pack’s total capacity and potentially damage the pack. Imbalance prevents batteries from tracking from the charged state to the discharged state and if not closely monitored can cause batteries to be overcharged or over-discharged, which will permanently damage the cells.
The batteries used in hybrid electric vehicle and electric vehicle battery packs are sorted by the battery manufacturer for capacity and internal resistance to reduce cell-to-cell variances in a given lot shipped to a customer. The vehicle battery packs are then built with carefully selected batteries to improve the total cell-to-cell matching in the pack. This should theoretically prevent large amounts of imbalance from developing in the battery pack, but despite this, the general consensus is that when building a large battery pack, both battery monitoring and battery balancing is required to maintain a high battery capacity for the lifetime of the battery pack.
As a first step to understanding the importance of balancing, two basic battery management strategies will be evaluated using two identical battery packs. Testing will explore how the total capacity of the battery pack is affected over the life of the battery.
To evaluate the strategies, a battery monitoring system (BMS) was designed. The BMS consists of three pieces: Monitoring hardware, balancing hardware, and controller. The BMS used in the testing is capable of monitoring cell voltages and battery load current, balancing cells, and is able to control the batteries’ connection to the load and battery charger.
A simple battery monitor and balancing system is shown in Figure 1, below. The BMS hardware design was built around the highly integrated LTC6803-1 multicell battery monitoring IC. The LTC6803-1 is capable of measuring up to 12 cells per IC and allows for a serial daisy chain that can connect multiple ICs, enabling a system to monitor over 100 batteries with one serial port.
Figure 1: Simplified schematic of a six-cell BMS system. An LTC6803 measures cell voltages and controls external cell discharge transistors. An LT1999 measures both charging and discharging currents to the battery pack.
When designing a battery monitoring system, certain specifications should get special consideration: First is the cell voltage accuracy—critical when trying to determine individual cell state of charge and one of the limiting factors in how close to the operational limit a cell can be operated. The LTC6803 has a resolution of 1.5mV and an accuracy of 4.3mV. This will allow the controller to make accurate decisions about the battery state, regardless of the battery chemistry used.
Second, a major source of imbalance in battery stacks is due to variation in the supply and standby current of the battery monitoring circuitry itself. The standby current is particularly important in automotive applications, as most vehicles spend the majority of the time turned off with the BMS in standby mode. The LTC6803 has just 12 µA of standby current; the range of current is specified from 6 µA to 18 µA, guaranteeing worst case a 12 µA imbalance between packs in a large cell stack—less than a 10 mAhr imbalance per month.
There are 2 ADC inputs that can be used to monitor battery temperature or other sensor data. The design shown in Figure 1 uses the Vtemp1 input for measuring battery current. Current is measured using an LT1999, a high voltage bi-directional current sense amplifier. The LT1999 has an input range of -5 to 80V and in this case is set up to monitor ±10 amps on the high side of the battery pack. The two GPIO pins that are available on the LTC6803 are used to control an active load and a charger. This allows for the LTC6803 to disconnect the batteries from the charger or load when the end of a charge or discharge point has been reached.
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.
Cuyler Latorraca is an applications engineer at Linear Technology.