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Design fuel gauging for multicell Li-ion battery pack (Part 2)

Posted: 07 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:design fuel gauging? multicell Li-ion battery?

By Sihua Wen
Texas Instruments Inc.

This is the conclusion of the two-part article that describes how the fuel gauging technology, the Impedance Track from TI, tackles these challenges in battery design and presents a brief example of a three-series, two-parallel battery pack solution.

Operation principle of impedance track gauging
As shown in Figure 3, the Impedance Track fuel gauge ICs allow for accurate measurement of the following key parameters:

? Open circuit voltage (OCV) of a battery when it is in relaxation mode

? Battery impedance: OCV-battery voltage under load/average load current, measured during discharge only

? PassedCharge: integrated charge or coulombs during battery discharge or charge

? QMAX: Maximum battery chemical capacity

? State-of-charge (SOC) at any moment, defined as SOC=QD/QMAX, where QD is the PassedCharge counted from the full discharge state

? Remaining capacity (RM)

? Full charge capacity (FCC), the amount of charge passed from the full charge state to the Termination Voltage

SOCBecause of a strong correlation between SOC and OCV for particular Li-ion battery chemistry, the SOC can be estimated from the OCV of the battery. The OCV I measured when the cells are in relaxation mode, which is defined as the state of the battery when its current is below a small threshold (such as 10mA) and when the cell voltage is stabilized. The SOC is then determined using the predefined OCV-SOC relationship. This serves to mark an initial battery state for subsequent discharge or charge cycles, and is done when the system is in low-power mode, such as shutdown.

ImpedanceAs shown in Figure 3, when the portable device is in normal operation, the load current shapes the discharge curve of the battery and leads to a deviation from the OCV characteristics. When a load is applied, the impedance of each cell is measured by finding the difference between the measured voltage under load and the OCV specific to the cell chemistry at the present SOC. This difference, divided by the applied load current, yields the impedance R. Additionally, impedance is correlated with the temperature at the time of measurement to fit in a model that accounts for temperature effects.

RMWith the impedance information, the remaining capacity (RM) is calculated using a voltage simulation implemented in the firmware. The simulation starts from the present SOCFINAL and calculates future voltage profile under the same load with a four percent SOC increment, consecutively. Once the future voltage profile is obtained, the impedance track algorithm determines the value of SOC that corresponds to the system termination voltage, SOCFINAL The remaining capacity may then be calculated using the following formula:

FCCThis is defined to represent the actual usable capacity of a fully-charged battery under a specific load. It can be calculated using the following formula, where QSTART is the initial capacity of the battery:

From time to time, the chemical capacity of the battery (QMAX) needs to be updated to account for the aging effects. This update happens less frequently than that of the impedance because of the much smaller changing rate of QMAX. The method is to take two OCV values before and after a charge (can also be a discharge) period. These two OCV values are first converted to SOC values using the OCV-SOC characteristics. Then the following equation leads to the new QMAX.

The above equation can be easily derived from the definition of SOC. Apparently, the algorithm does not require a complete discharge cycle to learn the battery chemical capacity. However, accuracy of the calculated QMAX is ensured only with relatively high PassedCharge and accurate SOC values.

Design impedance track fuel gauge
Impedance Track technology relieves designers from the burden of learning extensive knowledge of battery chemistry. Moreover, this new gauging technology does not require cycling of every production pack. Once the typical characteristics of the battery for a specific model are learned, the same configuration can be applied to all production packs without the lengthy cycling time, thanks to the learning capability of the algorithm.

Figure 3. OCV characteristics and battery discharge curve under load.

As an example, the design of a multicell fuel gauge solution using the bq20z90 is presented. It is assumed that the application is a three-series, two-parallel pack, with a capacity of 2,200=mAh for each of the Panasonic CGR18650C cells. The fast charging current is 4A, and maximum discharging current is 4A. The termination voltage is 3V per cell. At both charge and discharge, the maximum allowable temperature is 60C. The application operates at a constant power load most of the time. The pack is removable and does not require pre-charge. An independent, secondary voltage protector is needed to blow the fuse, if any of the cells goes above 4.45V.

Hardware design example
The application requires a chipset of three ICs: a) the bq20z90 fuel gauge IC; b) the bq29330 analog front end (AFE) IC; and c) the secondary voltage protector IC bq29412 that has an activation voltage of 4.45V. Figure 4 shows a functional diagram of the circuit. The AFE powers the fuel gauge directly from its 2.5V, 16mA low-dropout regulator (LDO). The AFE derives power from the battery voltage or the charger voltage. The main functions of the AFE are to condition the cell voltages for the 16bit voltage ADC in the fuel gauge and to provide hardware-level over current protection features.

The fuel gauge IC, running gauging and protection firmware, is the master of the chipset. The AFE is configured by the fuel gauge IC for how it should respond to handle fuel-gauging situations. These include when and which cell voltage information to provide to the fuel gauge IC, and what overload and short-circuit threshold and delay value should be used.

While the fuel gauge and its associated AFE provide over-voltage protection, the sampled nature of voltage monitoring limits the response time of this protection system. The bq29412 provides a fast-response, real-time and independent over-voltage monitor that operates in conjunction with the fuel gauge and the AFE. When any of the cells exceeds a voltage of 4.45V, after a capacitor-programmed delay, the bq29412 outputs a triggering signal to blow the fuse.

Figure 4. Fuel gauge circuit functional block diagram.

Configure the fuel gauge
The first decision to be made when configuring an Impedance Track fuel gauge is to determine the chemistry ID of the cells, and to select the corresponding firmware file. The cell used in this design is the Panasonic CGR18650C, belonging to the LiCoO2/graphitized carbon chemistry. It is identified by a chemical ID of 0100 according to the convention set by the Impedance Track algorithm. Available from the bq20z90 web folder, the correct firmware for this chemistry (bq20z90_v110_ID0100_default.senc) can be downloaded. Once the firmware is loaded into the chip, all types of fuel gauging configurations can be done by customizing the data flash. One convenient way of doing this is to use the bqEVSW evaluation software.

The numerous data flash constants, grouped into twelve classes, govern the operation of the fuel gauge. For example, the "1st Level Safety" class protects the battery from some less severe voltage, current and temperature conditions. Triggering these protections only temporarily disables the charging or discharging function. Then the battery pack can recover when the conditions go away. The maximum temperature of 60C for charge and discharge allowed in the application can be configured in this class. To avoid verbosity, Figure 5 lists most of the applications and operations that can be configured in each of the classes.

Figure 5. Configuration of battery data flash classes.

Selectable features can be found in the configuration class with three configuration registers: A, B and C. Note that because the example application is a removable pack, the NR bit should be zero to disable non-removable in configuration Register B, and it is necessary to implement a System_Present signal on the pack connector. With the data flash configuration in place, the pack should function normally with the target cell configuration.

In production, the Impedance Track fuel gauging only requires a typical pack to be cycled to learn the typical values of the impedance and chemical capacity, which modifies the "Ra Table" data flash class and the chemical capacity QMAX in "Gas Gauging" class. Then the same data flash configuration file can be used for all production packs.

To view Part 1, click here.

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