Rechargeable Batteries and Their Management (part 2)

10. Battery Management

Two decades ago, battery management meant a reliable, fast, and safe charging methodology for a battery bank, together with the monitoring facilities to detect the discharge condition of the battery pack. With new battery technologies emerging, demands from the cost-sensitive portable product market, and the medium-power-range products such as uninterruptible power supplies (UPS) and telecom power units, the attributes of a modern battery management system must include:

• Battery charging methods and charge control.

• End of discharge determination.

• Gas gauging.

• Monitoring battery health issues.

• Communication with the host system or power management subsystems.

The following sections discuss concepts and techniques related to managing nickel-based, sealed lead-acid, and Li-ion battery systems. A discussion on rechargeable alkaline batteries is beyond the depth of this section. Kuribayashi (1993), Levy (1995), and Ivad and Kordesch (1997) provide some details on managing rechargeable alkaline cells.

10.1 Charging Systems

While four major battery families can accept either a standard (16-24 hour) or fast (2-4 hour) charge, the discussion here is limited to charging methods.

Slower charging schemes tend to be found in simpler, price sensitive applications, which do not need (or cannot afford) much beyond a charger and a low battery indicator.

The objective of fast charging a battery is to cram as much energy as it takes to bring the battery back to fully charged state in the shortest possible time without damaging the battery or permanently affecting its long-term performance. Since current is proportional to energy divided by time, the charging current should be as high as the battery systems reasonably will allow. For the constant-current cells (NiCd and NiMH), a 1 C charge rate typically will return more than 90% of the battery's usable discharge capacity within the first hour of charging. The constant-potential cells (lead-acid and lithium-ion) are a bit slower to reach the 90% mark but generally can be completely recharged within 5 hours.

Fast charging has compelling benefits but places certain demands on the battery system. A properly performed fast charge, coordinated to the specifications of a battery rated for such charging, will deliver a long cycle life. The high charging rates involved, however, cause rapid electrochemical reactions within the cells of the battery. After the battery goes into overcharge, these reactions cause a sharp increase in internal cell pressure and temperature.

Uncontrolled high-rate overcharge quickly causes irreversible battery damage. Therefore, as the battery approaches a full charge, the charging current must be reduced to a lower "top-off" level, or curtailed entirely.

10.1.1 Charge Termination Methods

If a rapid charge is applied to a battery pack, it is necessary to select a reliable method to terminate charging at the fully charged position. Two practical approaches for charge termination are temperature termination and voltage termination.

10.1.1.1 Temperature Termination Method

Temperature is the main cause of failure in a rechargeable cell, so it makes sense to monitor the cell temperature to determine when to shut off the charge to a battery. Three methods of charge termination, based on temperature, are common: maximum temperature cutoff, temperature difference, and temperature slope. The maximum temperature cutoff system is the easiest and cheapest to implement but the least reliable. Using a bimetallic thermal switch or a positive temperature coefficient thermistor, a simple, low-cost circuit can shut down a charging current at an appropriate temperature.

The temperature difference (DT) method measures the difference in ambient and cell temperatures to compensate for a cool environment. The DT method requires monitoring two temperature sensors, one for the battery temperature and one for the ambient temperature. This method may be unsuitable if the difference between cell and ambient temperature is very large.

The DT method can become unreliable with a quickly changing ambient temperature unless an equal thermal mass is attached to the ambient sensor. This means that the DT method is suitable for a primary charge termination at lower charge rates, up to C/5, if the ambient temperature is not going to change often.

The DT method also provides an excellent backup charge termination scheme.

The temperature slope (dT/dt) method, a more sophisticated temperature termination scheme, measures the change in temperature over time. This method uses the slope of the battery temperature curve and, therefore, is less dependent on changes in ambient temperatures or in large differences between ambient and battery temperatures. Accurately adjusted to a particular pack, and with careful attention paid to the type and placement of the temperature sensor, the dT/dt method works very well. This method is suitable for charge rates up to 1 C and provides an excellent backup method.

10.1.1.2 Voltage Termination Methods

Four commonly available voltage termination methods are maximum voltage, negative delta voltage, zero slope, and inflection point. The maximum voltage (Vmax) method senses the increase in battery voltage as the battery approaches full charge. However, this is accurate only on a highly individualized basis. It is necessary to know the exact value at the voltage peak, otherwise the batteries may be over- or undercharged.

FIG. 17 Termination methods based on changes in voltage and voltage slope

Temperature compensation also is required because of the negative temperature coefficient of battery voltage. The maximum voltage will increase if the batteries are cold, causing an undercharge because the charging voltage will reach the maximum voltage trip point early. If the batteries are hot, the maximum voltage may never be reached and the batteries will be cooked. Therefore, the Vmax method generally is not recommended for fast charge rates.

The negative delta voltage (--A V) is the most popular fast charge termination scheme. It relies on the characteristic drop in cell voltage that occurs when a battery enters overcharge, as shown in FIG. 17. With most NiCd cells, the voltage drop is a very consistent indicator and the -A V method is fine for charge rates up to 1 C. An inherent problem with this method is that the batteries must be driven into the overcharge region to cause the voltage decrease. Pressures and temperatures rise very rapidly at fast charge rates beyond 1 C. In cyclic applications, the battery must be able to endure that continual abuse.

Another concern is that cells like the NiMH types do not always have the characteristic decrease in voltage, compared to NiCd cells, as shown in FIG. 10. This creates a problem of forward compatibility when moving from NiCd to NiMH cells. Most manufacturers of NiMH cells do not advocate the -AV method of charge termination.

The zero slope method monitors the point where the slope of the battery voltage reaches 0. This method is reliable for rapid charge rates up to 4 C and is less susceptible to noise on the voltage sense lines. However, a few types of batteries, such as the NiCd button cells, may have a voltage slope that never quite reaches 0. Therefore, the zero slope method is better suited as a backup method.

In the inflection point (dV/dt) method the system monitors the change in voltage over time and is the most sensitive indicator for preventing overcharge.

The inflection point method relies on the changes in the voltage slope shown in FIG. 17, which occur during charging and are an excellent primary termination method for up to 4 C charge rates.

The change in the voltage slope is an extremely reliable and repeatable indication of charge. It does not rely on a decrease in voltage, which may not always occur. Instead, this method looks for the flattening of the voltage profile as the battery reaches full charge. By monitoring the relative change in the steepness of the voltage slope, this method avoids having to use absolute numbers.

10.1.2 NiCd and NiMH Fast Charge Methods

Nickel-based batteries, such as the NiCd and NiMH types, have become the most popular choice for portable products. Although it is not correct to consider the NiCd and NiMH electrochemistry or charging regimens as interchangeable, they are similar enough to discuss together.

There is no one best way to fast charge a NiCd or NiMH battery. Variables introduced by the allowable cost and size of the application, the choice of charge termination method(s), and the specific battery vendor's recommendations all influence the final choice of charging technique. FIG. 18(a) shows the voltage, pressure, and temperature characteristics of a NiCd cell being charged at a 1 C rate. FIG. 18(b) shows similar data for NiMH cell.

These curves illustrate the need for a reliable termination of the high-current portion of the charge cycle, and assist in understanding the various fast charge termination methods outlined in Table 5. For both types of electrochemistry, the ideal fast charge termination point is at 100-110% of the returned charge.

The charging current is then reduced to the top-off value for 1-2 hours, to bring the cell to a state of slight overcharging.

FIG. 18 Charging indications for nickel-based batteries at a 1 C charge rate: (a) cell voltage, temperature, and pressure for a typical NiCd cell at a 1 C rate charging, (b) NiMH voltage and temperature characteristics

This compensates for the inefficiencies of the charging process (e.g., heat generation). If the specific application will have the battery on standby for more than several weeks or at high temperatures, the top-off charge is followed by a continuous, low-level "trickle" charge to counter the self-discharge characteristics of NiCd and NiMH cells.

Under certain conditions, particularly following intervals of storage, a NiMH battery may give an erroneous voltage peak as charging commences. For this reason, the charger should deliberately disable any voltage-based charge termination technique for the first 5 minutes of the charging interval.

See Table 4 for representative charging recommendations. The most appropriate method should be selected in consultation with the manufacturer. Table 5 summarizes the fast charge termination methods for NiCd and NiMH cells.

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TABLE 4 Representative charging recommendations for different batteries.

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TABLE 5 Fast charge termination methods for NiCd and NiMH batteries

Charging Technique | Description

Negative delta voltage (- A V)

Zero A V

Voltage slope (dV/dt)

Inflection point cutoff (d2V/dt 2)

Absolute temperature cutoff

Incremental temperature cutoff (A TCO)

Delta temperature/delta time (AT/At)

Looks for the downward slope in cell voltage, which a cell exhibits (>>30-50 mV for NiCd, 5-15 mV for NiMH) on entering overcharging.

Very common in NiCd applications due to its simplicity and reliability Waits for the time when the voltage of the cell under charge stops rising and is "at the top of the curve" prior to the downslope seen in over-charging. Sometimes preferred over -AV for NiMH, due to relatively small downward voltage slope of NiMH Looks for an increasing slope in cell voltage (positive dV/dt), which occurs somewhat before the cell reaches 100% returned charge (prior to the zero AV point)

As a cell approaches full charge, the rate of its voltage rise begins to level off. This method looks for a 0 or, more commonly, slightly negative value of the second derivative of cell voltage with respect to time Uses the cell's case temperature (which will undergo a rapid rise as the cell enters high-rate overcharging) to determine when to terminate high-rate charging. A good backup method, it is too susceptible to variations in ambient temperature to make a reliable primary cutoff technique Uses a specified increase in the cell's case temperature, relative to the ambient temperature, to determine when to terminate high-rate charging.

It is a popular, relatively inexpensive and reliable cutoff method Uses the rate of increase of a cell's case temperature to determine the point at which to terminate the high-rate charge. This technique is inexpensive and reliable once the cell and its housing have been properly characterized

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10.1.3 Sealed Lead-Acid Batteries

Unlike nickel-based batteries, sealed lead-acid batteries are charged using a "constant-potential" (CP) regimen. CP charging employs a voltage source with a deliberately imposed current limit (a current-limited voltage regulator). A significantly discharged battery undergoing CP charging initially will attempt to draw a high current from the charger. The current-limiting function of the CP regimen keeps the peak charging current within the battery's ratings.

Following the current-limited phase of the charging profile, a sealed lead-acid battery exposed to a constant voltage will exhibit a tapering current profile, as shown in FIG. 19. When the returned charge reaches 110-115% of the rated capacity, allowing a dischargeable capacity of 100% of nominal, the charge cycle is complete.

FIG. 19 Typical current and capacity returned vs. charge time for CP charging.

The specifics of fast charging sealed lead-acid batteries are more vendor dependent than those of NiCd or NiMH units. The information in Table 4 uses data from GS Battery Inc. The primary termination method, current cutoff, looks at the absolute value of the average charging current flowing into the battery.

When that current drops below 0.01 C, the battery is fully charged. If it will be in standby for a month or more, a trickle current of 0.002 C should be maintained. The backup termination method, according to the vendor's recommendations, should be a 180 minute time out on the charging cycle.

To satisfy more stringent charge control recommendations, where the battery temperature, voltage, and current need to be sampled, many dedicated charge controller ICs are available. The bq2031 lead-acid fast charge IC from Benchmarq Microelectronics Inc. and UC3906 (sealed lead-acid charger) from Unitrode Integrated Circuits are examples.

The UC3906 battery charger controller contains all the necessary circuitry to optimally control the charge and hold the cycle for sealed lead-acid batteries.

These integrated circuits monitor and control both the output voltage and current of the charger through three separate charge states: a high-current bulk-charge state, a controlled overcharge, and a precision float charge or standby state.

FIG. 20 is a block diagram and one implementation of the UC3906 in a dual-step current charger. Sacariesen and Parvereshi (1995) and Unitrode Application Note U-104 provide details about charge control in sealed lead-acid batteries using the UC3906.

10.1.4 Lithium-ion Chargers

Li-ion batteries require a constant potential charging regimen, very similar to that used for lead-acid batteries. Typical recommendations for Li-ion fast charging are listed in Table 4. As with lead-acid batteries, a Li-ion cell under charge will reduce its current draw as it approaches full charge.

If the cell vendor's recommendation for charging voltage (generally 4.20 V 4-50 mV at 23°C) is followed, the cells will be able to completely recharge from any "normal" level of discharge within 5 hours. At the end of that time, the charging voltage should be removed. Trickle current is not recommended.

If the voltage on a Li-ion cell falls below 1.0 V, recharging should not be attempted. If the voltage is between 1.0 V and the manufacturer's nominal minimum voltage (typically, 2.5-2.7 V), it may be possible to salvage the cell by charging it with a 0.1 C current limit until the voltage across the cell reaches the nominal minimum, followed by a fast charge.

Due to the special characteristics of Li-ion batteries, most manufacturers incorporate custom circuits into their battery packs to monitor the voltage across each cell within the battery and provide protection against overcharging, battery reversal, and other major faults. These circuits are not to be confused with charging circuits. For example, the MC 33347 protection circuit is such a monolithic IC from Motorola.

FIG. 20 The UC3906 and its implementation: (a)block diagram, (b)implementation in a dual-step current charger. (Unitrode Inc.)

10.2 End-of-Discharge Determination

Determination of the point at which a battery has delivered all of its usefully dischargeable energy is important to the longevity of the cells that form that battery. Discharging a single cell too far often will cause irreversible physical damage inside the cell.

If multiple cells are placed in series, unavoidable imbalances in their capacities can cause the phenomenon known as cell reversal, in which the higher-capacity cells force a backward current through the lowest-capacity cell. Knowing the end-of-discharge (EOD) point provides a "zero capacity" reference for coulometric gas gauging.

The actual determination of the EOD point typically is done by monitoring cell voltage. For the most accurate determination of the EOD when the load is varying, correction factors for the load current and the battery's state of charge should be applied, especially to lead-acid and Li-ion batteries. The essentially flat discharge profiles of NiCd and NiMH make these corrections a matter of user discretion for most load profiles. Table 6 shows voltages commonly used to indicate the end-of-discharge point for the four battery types.

10.3 Gas Gauging

The gas gauging or fuel gauging concept discussed here does not refer to the gases that may evolve by the battery reactions but rather to the concept of the battery as a fuel tank powering the product. Gas gauging, therefore, involves a real-time determination of a battery's state of charge, relative to the battery's nominal capacity when fully charged.

It is possible to make an inexpensive and moderately useful state of charge measurement from a simple voltage reading, if the battery being used has a sloping voltage profile. Hence, Li-ion batteries and, to a lesser extent, sealed lead-acid batteries, should be amenable to such an approach. In practice, the results are less than optimal: Cell voltages depend on loading, internal impedance, cell temperature, and other variables.

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TABLE 6 Typical end-of-discharge voltages

Cell Type EOD Voltage (V) Comments Lead acid 1.35-1.9 (1.8 typical) Dependent on loading, state of charge, cell construction, and manufacturer NiCd 0.9 Essentially constant NiMH 0.9 Essentially constant within recommended range of discharge rates Li-ion 2.50-2.70 Dependent on manufacturer, loading, and state of charge

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This reduces the attractiveness of the battery voltage method of gas gauging; and that it is unsuitable for NiCd or NiMH batteries, due to their essentially flat voltage profile, makes it commercially untenable. A clever and effective alternative is the "coulometric" method.

Coulometric gas gauging, as its name implies, meters the actual charge (current x time) going into and out of the battery. By integrating the difference of current in and current out, it is possible to determine the charge status of the battery at any given time. There are real-world details, of course, that must be observed in the actual implementation of such a gas gauge; some of the most important of these are:

1. It is necessary to have an accurate starting point for the integrator, corresponding to a known state of charge in the battery. This problem often is resolved by zeroing the integrator when the battery reaches its EOD voltage.

2. It is necessary to compensate for the temperature. The actual capacity of lead-acid batteries increases with temperature, that of nickel-based batteries decreases as battery temperature rises.

3. Appropriate conversion factors should be applied for the particular charge regimen and discharge profile used. Under conditions of highly variable battery loading, dynamic compensation may be advisable.

10.4 Battery Health

With battery health defined as a battery's actual capacity relative to its rated capacity, the health of the battery can be determined and maintained in three steps:

1. Discharge the battery to the EOD point, preferably into a known load.

2. Execute a complete charge cycle, while gas gauging the battery.

3. Compare the battery's measured capacity to its rated capacity.

This sequence will simultaneously "condition" the battery (e.g., overcome the so-called memory effect of capacity of NiCd batteries) and indicate the capacity of the battery after conditioning. This information can be used to ascertain whether the battery is in good shape or approaching the end of its useful life.

11. The System Management Bus, Smart Battery Data Specifications, and Related Standards

In the mid 1990s, new industry standards were proposed to standardize the battery and power management subsystems within portable products. The following standards were proposed:

1. System Management Bus (SMB) Specification.

2. Smart Battery Data Specification.

3. Smart Battery Charger Specification.

4. Smart Battery Selector Specification.

These specifications form the Smart Battery Systems Specification, which presents a solution for many of the issues related to batteries used in portable equipment such as laptop computer systems, cellular telephones, and video cameras. Fundamental to the system is the concept that the battery contains all the necessary components to determine the battery's state of charge, predict the time to full and empty charging, specify the charging voltage and current, and determine when the battery is fully charged or fully discharged.

A typical smart battery system is shown in FIG. 21(a). It consists of an AC/DC converter (unregulated), power switch, system power supply, smart battery charger, and smart battery selector, all of which communicate with the system host and the system elements themselves via the SMB. In this case, smart battery A powers the system while smart battery B is getting conditioned or charged.

FIG. 21 The smart battery system and an SMB module: (a) a typical multiple smart battery system, (b)the bq219XL module, (c)the bq219XL connections. (Benchmarq Microelectronics, Inc.)

The system management bus is a two-wire interface through which simple power-related chips can communicate with the rest of the system. It uses I2C as its backbone (Paret & Fenger, 1997). A system using an SMB passes messages to and from devices instead of tripping individual control lines. Removing the individual control lines reduces the pin count. Accepting messages ensures future expandability. With a system management bus, a device can provide manufacturer information, tell the system its mode or part number, save its state in case operation is suspended, report different types of errors, accept control parameters, and return its status. The SMB may share the same host device and physical bus, as long as an appropriate electrical bridge is provided between the respective devices.

Benchmarq et al. (1995a) provides details related to communication protocols available for use by devices on an SMB. Using gas gauge ICs such as the bq2092 or bq2040, Benchmarq provides a complete smart battery module such as its bq219XL ( FIG. 21(b)). Designed for battery pack integration, the bq219XL combines the bq2092 or bq2040 gas gauge IC with a serial EEPROM on a small printed circuit board. This smart battery module provides a complete solution for the design of intelligent battery packs using the SMB protocol and supports the smart battery data (SBD) commands in the SMB/SBD specifications. The board includes all the necessary components to accurately monitor battery capacity and communicate critical battery parameters to the host system or battery charger.

The bq219XL also includes four LEDs coupled with a push-button switch to activate the LEDs to show remaining battery capacity in 25% increments.

Contacts are provided on the bq219XL for direct connection to the battery stack (B +, B-) and the two-wire interface (C, D). For further details on the operation of the gas gauge and communication interface refer to the bq2092 or bq2040 data sheets. A tutorial on the bq2040 for battery system designers is provided in Benchmarq (1998a).

For mobile communication products, the full-function SMB architecture is too costly and hence the SBS implementer's forum is attempting to develop a specification that meets the needs of the market by eliminating many advanced features (Heacock, 1998). A complete description of SBS standards and implementation is beyond the scope of this introductory section; for further details, see Benchmarq et al. (1995a, 1995b, 1996a, 1996b), Dunstan (1995), and Heacock (1998).

12. Semiconductor Components for Battery Management

In practical systems, rechargeable batteries could be charged using a variety of components, from simple voltage regulator ICs to microprocessor-based systems (Kerridge, 1993). Specially designed battery-management ICs provide fine control and a useful indication of a battery's charge condition both on and off charge.

Battery voltage, charge/discharge current, and cell case temperature supply the clues to information about battery-management ICs. However, each clue is the result of a chain of events far removed from the chemical reaction necessary to monitor and control. Further complexities arise because the three properties interact. Further, instantaneous measurements of any of the three attributes are virtually useless; all battery-management ICs include some sort of time to add meaning to the data. Table 7 shows some representative battery-management ICs from Benchmarq, Unitrode, and Dallas Semiconductor.

FIG. 22 A microcontroller-based universal battery monitor

FIG. 23 The bq2010: (block diagram, (b) typical implementation.

There are many means of providing a complete universal battery management system. Years ago, most companies involved in the design of chargers for battery banks had design resources dedicated internally to provide such solutions. Today, with the rapid changes in rechargeable battery chemistry and the associated charge/discharge management requirements, many companies prefer to use standardized semiconductor components dedicated to this function. Two possible approaches involve using a customized microcontroller or an ASIC dedicated to battery management. FIG. 22 is the block diagram of a microcontroller-based universal battery monitor.

Using ASIC solutions, such as bq2010 from Benchmarq Microelectronics, the standby current necessary for battery monitoring circuits can be minimized while providing many gas gauge functions. A block diagram of the bq2010 forms FIG. 23(a). Specialized component manufacturers recently introduced many gas gauge ICs for multi-chemistry (NiCd/NiMH/Li-ion) environments (Freeman, 1995; Benchmarq, 1997). VLSI components such as the bq2040 with an SMB interface are intended for battery-pack or in-system installation to maintain an accurate record of available battery charge. In addition to supporting the SMB protocol, it supports smart battery data specification, rev. 1.0 (Dunstan, 1995; see FIG. 24). Another example is Linear Technology Corporations' LTC ® 1325 battery management IC, which provides a complete system that can accommodate all plausible types of electrochemistry and their concomitant charging needs and charge termination algorithms, with few or no hardware changes for different battery types.

FIG. 24 The implementation of a universal battery monitor using the bq2040. (Benchmarq Microelectronics, Inc.)

FIG. 25 Li-ion pack safety. (Benchmarq Microelectronics, Inc.)

The two critical aspects of battery management for Li-ion cells are charge control and pack safety protection. The fundamental purpose of the safety circuitry is to protect the Li-ion cells from abusive conditions. FIG. 25 illustrates the basic concept of a protection circuit using the bq2058. The control IC monitors each series potential in the pack for overvoltage (charge), undervoltage (discharge), and overcurrent conditions. The three thresholds are set to match the requirements of the manufacturer and Li-ion battery type. For further details on this circuit, the bq2058 data sheet is suggested. In designing Li-ion battery management systems, temperature effects must be carefully considered (Fundaro, 1998).

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