Balance hybrid/EV battery packs to extend operational lifetime: Pt. 1—Hardware and strategy

Introduction
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.

Monitoring hardware
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.