Virtually All battery charger designs depend on the associated battery. Widely used rechargeable battery-based systems include:
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Nickel cadmium (NiCd) finds use where long life, high discharge rate and economical price are important.
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Nickel-metal hydride (NiMH) has a higher energy density than NiCd, but at the expense of reduced cycle life.
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Lithium-ion (li-ion) is used where high-energy density and light weight are of prime importance.
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Lithium-ion polymer (Li-polymer) has chemistry similar to the li-ion in terms of energy density.
The charge and discharge capacity of a secondary battery is in terms of “C,” given as ampere-hours (Ah). The actual battery capacity depends on the C-rate and temperature. Most portable batteries are rated at 1C. A discharge of 1C draws a current equal to the rated capacity. For example, a battery rated at 1000 mAh provides 1000 mA for 1 hr if discharged at 1C rate. The same battery discharged at 0.5C provides 500 mA for 2 hr.
Performance and longevity of rechargeable batteries mainly depends on the quality of the chargers. One type of charger (used only for NiCd) applies a fixed charge rate of about 0.1C. A faster charger can take 3 hr to 6 hr with a charge rate of about 0.3C.
In addition, a charger for NiMH batteries also could accommodate NiCds but not vice versa, because a NiCd charger could overcharge a NiMH battery. Lithium-based chargers require tighter charge algorithms and voltages. Avoid a charge rate over 1C for lithium battery packs, because high currents can affect the lithium. With the majority of lithium packs, a charge above 1C is not possible, because the protection circuit limits the amount of current the battery can accept.
Precise full charge detection of nickel-based batteries uses special ICs that monitor battery voltage and terminate the charge when a certain voltage signature occurs. A drop in voltage signifies the battery has reached full charge, known as negative delta V (NDV).
After full charge, you can trickle charge a NiCd battery to compensate for its self-discharge characteristics. The trickle charge for a NiCd battery ranges between 0.05C and 0.1C. To reduce memory effects, there is a trend toward lower trickle charge currents.
NiMH battery chargers now use a combination of NDV, voltage plateau, rate-of-temperature-increase (dT/dt), temperature threshold and timeout timers. The charger uses whatever comes first to terminate the fast-charge. NiMH batteries that use NDV or the thermal cut-off control tend to deliver higher capacities than those charged by less aggressive methods.
Li-ion chargers use a voltage-limiting device. However, li-ion batteries have a higher voltage per cell, tighter voltage tolerance and the absence of trickle or float charge when reaching full charge. Charge time for li-ion batteries charged at a 1C initial current is approximately 3 hr. Full charge occurs after reaching the upper voltage threshold, and the current drops and levels off at about 3% of the nominal charge current. Increasing li-ion charge current has little effect on shortening the charge time. Although it reaches the voltage peak faster with higher current, the topping charge will take longer. Because li-ion batteries cannot absorb overcharge, these batteries should not use a trickle charge. Overcharging can cause the cell to overheat.
Li-ion batteries have good cold- and hot-temperature charging performance. Some cells allow charging at 1C from 0°C to 45°C. Most li-ion cells prefer a lower charge current when the temperature gets down to 5°C or colder. Avoid charging below 0°C.
Several charger ICs for li-ion batteries provide the capability to charge the battery via a USB port or an ac adapter. For USB operation, the user can plug the USB cable into a desktop or laptop computer and use the 5-V output to charge the battery pack in a cell phone or PDA.
An example of a standalone battery charger with fixed charging characteristics is the Texas Instrument's (Dallas) bq24150 (see the Figure), a compact, flexible, high-efficiency, USB-friendly switch-mode charge management IC for single-cell li-ion and Li-polymer batteries. The I2C interface allows the programming of the charge parameters and also reports charge status to the host. It integrates a synchronous PWM controller, power MOSFETs, input current sensing, high-accuracy current and voltage regulation, and charge termination into a small WCSP package.
The bq24150 charges the battery in three phases: conditioning, constant current and constant voltage. The input current is automatically limited to the value set by the host. Charge is terminated based on a user-selectable minimum current level. During normal operation, the bq24150 automatically restarts the charge cycle if the battery voltage falls below an internal threshold and automatically enters sleep mode or high impedance mode when the input supply is removed.
SMART BATTERY CHARGER SPECIFICATION
The Smart Battery Charger Specification was introduced in 1998 by battery and IC manufacturers in order to provide a system solution for rechargeable batteries used in portable equipment. Revision 1.1 of the specification states, “The electrical characteristics of the Smart Battery Charger feature charging characteristics that are controlled by the battery itself, in contrast to a charger with fixed charging characteristics that will work with only one cell type. The Smart Battery/Smart Battery Charger combination provides distinct advantages in system safety, performance and cost.”
The battery itself contains the charging characteristics and safety limits, which allow chemistry independence and charging algorithms for the specific cell type. This reduces the system cost and complexity, because the charger only has to provide the charging voltage and current specified by the smart battery.
The specification explains that it “defines the data set that is used to communicate with the Smart Battery Charger. It is not designed to limit innovation amongst battery or charger manufacturers, but rather provide a consistent set of information to communicate with any particular battery charger.”
Communication with the charger is via the System Management Bus specification (SMBus). The SMBus is a specific implementation of the I2C bus that provides data protocols, device addresses and additional electrical requirements that are designed to physically transport commands and information between the smart battery, system most, smart battery charger and other smart devices.
SMBus is a 2-wire interface through which simple, power-related chips can communicate with the rest of the system. It uses I2C as its backbone.
A system employing the SMBus passes messages to and from devices as opposed to tripping individual control lines. Removing the individual control lines reduces pin count. Accepting messages ensures future expandability.
With SMBus, a device can provide manufacturer information, communicate to the system what its model/part number is, save its state for a suspend event, report different types of errors, accept control parameters and return its status.
Two types of smart battery chargers defined include Level 2 and Level 3. All smart battery chargers communicate with the smart battery using the SMBus; the two types differ in their SMBus communication mode and in whether they modify the charging algorithm of the smart battery. Level 3 smart battery chargers are supersets of Level 2 chargers and as such support all Level 2 charger commands.
The SBS simplifies the design of a closed loop battery-charge system because all the required safety features are accounted for within the battery. This results in lower NRE (non-recurring expense) costs and allows robust systems.
